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The Comprehensive Illustrated Djet (72-75 350-450SL) Thread

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The Comprehensive Illustrated Djet fuel injection system (72-75 350-450SL) Thread


IMPORTANT!!!! I didn't find this for the first month, this is CRITICAL

The Manual!!!!!

Model 107


I know that seems obvious, but in rebuilding my D-jet system a number of times during the process I'd spend countless hours chasing down complicated problems, when in fact, once my dist cap was fitted incorrectly, and in another I forgot to connect a wire on the 34th re-assembly, in another my fuel filter got re-clogged from running Techron through the system, wriggle those fuses, keep re-visiting the basics during the process when problems arise, sometimes they turn out to be something simple (this was from a complete restoration/rebuild from a car that sat for ten years) but don't get tunnel vision

Third, I found you can listen to each injector with a LONG screwdriver to hear if it's "firing" it's not a "test" but at least you can hear if it's working, and you can "listen" to other parts with an 18" length of Garden hose if you don't happen to have a stethoscope laying around

Every single 72-73 350-450 SL owner comes in and has the same 13 page thread with all of the EXACT same symptoms, it's like reading the same book over and over, if you are the proud new owner of a 72-73 350 or 450 SL with the D-jet Fuel injection system, we can see the future...your future...over time our own MBGraham has answered each of these threads, and had to answer the same problems, the same questions, over and over and over, this thread is my attempt to put all MBGrahams's information and Photo's into one thread so he doesn't have to answer them over and over, and we newcomers dont have to crawl through 300 threads to find each bit of information, this thread is to say thank You to Graham, and to help newcomers as much as this forum has helped me

another thing that is CRITICAL before you go any further is check all electrical connections, MB used copper in all their connections and in many cases the fuses and wire leads are in other metals, causing the copper to "sacrifice" creating a layer of corrosion between even good looking contacts, even if your fuses LOOK good, if you have an unsolvable problem, check the ground wires, remove, polish and put conducting oil, same with all connections and fuses, this also holds true for dash lights, no dash lights, fiddle with the rheostat every time you get in the car, after a few hours of fiddling your dash lights will resume normal function, and I mean HOURS, every electrical contact point in the entire car is prone to this phenomenon, I spent 100 hours trying to diagnose my fuel injection/fuel delivery system and it was a bad ground the entire time, fixed by wriggling it back and forth

check those connections and grounds before you throw parts at it, I have not had to replace a single part on my car so far other then rubber hoses, and when it was garaged in 2002 the repair estimate was $6000 in parts and labor, every single part was reparable or cleanable

This information is presented in no particular order, as a matter of fact the over-all diagnostics are further down in the thread, I kind of went with what came to my mind first ie; what is most common, I would like to repeat the photos and solutions are just about ALL MBGrahams unless otherwise noted, and when they are in my words, it's because I learned them from Graham, and MKNMike, and 1972350SL, and Dave and Rowdie Nobby and the ear guy and the red car guy and the other regulars, this thread is dedicated to all of them whether they contributed directly to this particular thread or not, since I read their problems and solutions, they helped me solve mine

I hope no one is offended by my use of their info, I tried to give all credit where credit is due, apologies if I missed anything, any mistakes in this thread are mine and mine alone, all information I have learned or posted has come directly from this forum

So, without further Ado:

The AAV (Auxiliary air valve) Diagnosing/surging idle, high idle, poor performance when hot

The first problem you are likely to encounter with your new purchase is a "surging" idle, when the idle runs high, then low fairly rapidly back and forth, slow fast slow fast, this is probably caused by a stuck AAV (auxiliary air valve) and or leaks from cracked rubber hoses, or less likely, leaks around the injectors themselves

it's a good idea to have hose clamps at the top and bottom of the little hoses on the injectors, one side will have clamps, one wont, adding the clamps saves heartaches, also if you pull the injectors, replace the caps and tips, they will be brittle, the injectors themselves are nearly bulletproof, but the seals and caps can be replaced.

A faulty AAV will as well as a leak will cause your idle to remain high when your car does reach operating temperature, as it doesn't close when it is supposed to, allowing a great deal of air in, causing the motor to "race", then something happens I don't remember what, a safety feature causing the RPM's to "fall", return to normal, air rushes in, it races, falls, races, falls, it sounds like a 3 year old going vroom vroom vroom in your car

To check if your AAV is functioning properly, first, does it surge and is your idle extraordinarily high? if so try turning down the idle screw while warming up before it reaches it's operating temperature of 175F, if it fails to reach (X) Rpm's it won't surge back and forth, after it reaches 175, pinch the hose on the right (facing the car) (I used needlenosed vice grips) if it dies, the AAV is stuck open and needs to be taken apart and rebuilt (next post)

If, when you squeeze the AAV hose shut, it doesn't affect the idle, and you are able to turn the idle screw all the way closed and it still idles, (doesn't die) you have an airleak either in the injectors or one of the hoses (more likely) as these get brittle and crack, if you are able to turn the idle screw all the way down without squeezing the hose leading to the AAV, the AAV is the culprit allowing all the air in as well


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Rebuilding The AAV:

If, when the engine is up to 175F and it dies or slows dramatically when you pinch the hose leading to the AAV, it's time to rebuild it, it's incredibly easy to remove, and quite easy to disassemble, I used a "scrench" which is a spark plug wrench for chainsaws, the 2 sizes were perfect, the large size to remove the first piece, the smaller for the second to avoid scoring the inside

What you do is place the AAV in a vice just closed enough to hold it, but open enough to allow the inside to drop through easily, I closed the vice until it was touching the two flanges on top of the AAV then backed off just a hair, tap tap with a hammer, and the top falls out, flip it over, put in small end of scrench, pipe or even a long spark plug socket, tap tap your AAV is disassembled

In inspecting the piston and cylinder, I notice that they both have wear marks in certain areas even although piston moves quite freely. Almost like it is slightly loose and cocks slightly as it moves. Or maybe they are just wear marks caused by dirt getting between piston and cylinder. May have to hone them or perhaps make a Teflon piston. Aluminum against aluminum is never good - causes galling.
clean the piston well, I used carb cleaner and steel wool IIRC, and some 2000 grit knife sharpening sanding belts, maybe 3000, very very fine, a little grease, check to see if it moves freely

then I actually pulled on the little pin coming out of the bulb until I got it moving freely, I had to disassemble and re-assemble it about 3 more times until I cleaned and loosened it enough to perform properly, don't make the same mistakes I did, to check it, use a torch, a gas stove or hot (above 150F water) then I put it under hot tap water which I would turn to cold in order to not break or freeze anything, I did this repeatedly, better to do this while it's apart, stress repeatedly, the pin came off in my hand, that was fine, I then re-assembled it, taking special note of the directions of the tubes, then brought it back inside and tested it again, stress repeatedly, to open and close looking through the aperture to see the piston open and close, you can't clean this or play with this too many times, as all told I think I had to remove it a total of about 6 times before it functioned properly, stress, don't make the same mistakes I did, while it's apart, BE THOROUGH

re-attach it to the car, start it up, get it to normal operating temperature, pinch the hose, if it dies or slows at all, rinse and repeat until it opens and closes correctly, I also "adjusted" it in place by tapping it down with a hammer, about a 16th or 32nd of an inch for the ultimate "fine tune"

Next if when you pinch the hose and it doesn't affect the idle speed, turn idle down all the way, if your car doesn't die, you have a leak somewhere, start with the easy stuff, replace the hoses connecting the AAV and the idle screw to the air intake


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fuel delivery (fuel pump) relays/fuses ECU (electronic Control Unit)

The Bottom 3 "related" attachments also have tests to perform on the ECU, which is located against the firewall up under the glovebox all the way forward and fairly high up, it looks EXACTLY like a stereo amp, and my stereo amp was actually zip tied to it in front of it, made locating it a bear, it's removed by reaching in and undoing a clip identical to the one on your distributor cap

When you turn the key to the "on" position, past ACC you should hear the fuel pump "whirr" for a second, it's under the passenger side rear wheel, if it doesn't "whirr" there are 3 problems you can have, faulty main relay, blown fuse, faulty pump, or just no electricity to the relay if the car has been sitting, my car had sat for ten years and I had to 'wriggle" and or clean -every- -single- -electrical- connection in the -entire- car, no exaggeration, every light, every fuse, every ground, that itself took days and actually happened as a mistake troubleshooting the fuel pump

1) if you try everything else, and no sound from the pump, remove the box around the fuel pump and connect a battery or a charger directly to the fuel pump, to clear mine it took 4 hours of reversing the polarity back and forth while sticking the "sucking" end into carb cleaner, I recommend having a cigarette dangling out of your mouth and having the carb cleaner and gas in a plastic cup so it spills a lot while you send sparks up from the connections, in case the cigarette doesn't cause you to explode, the sparks will, hillbilly mechanic at his best

If the pump is now pumping AND NOTE AND VERY IMPORTANT (my pump ran but wasn't pumping) AND is discharging a LOT of fuel, move to the relays, it took 4 hours and me re-assembling the fuel pump twice to get it working, although I suspect it didn't help to dis-assemble it, for 900 dollars it's worth being covered and gas and carb cleaner for a few hours

If, when you KNOW your pump works, but still no "click" time to check the relays located under the passenger side kick panel on the left by removing under the glove box and a small panel before the fuses that reaches the door, above the fuses is a bar more or less with some grounds and one relay, that is to your power windows, remove the screw closest to you and slide the bar down to reveal the relays

I relate to the "surrender". I just walked in from the garage after starting to work up a sweat. I crawled under there to check into the relays. Before I ended up messing with some other stuff, I did realize that the wires running to my relays are numbered. I did not read them all, but saw the bottom four in the back and figured they should be numbered like this:

1 2
3 4
5 6
7 8

( 9 is the one in front, and 1 is the FPR.)

Originally Posted by rowdie
Section 07.4-040 of the on line manual which is linked in the EGv107 shows the #1 relay is the FP and #3 just below is the master relay for the ECU.

Cool. So we are making some progress here:

(1=)FPR ...................... (2=)ColdStartValve
(3=)MainInjection ........ (4=)AC-Starter
(5=)ChangeOverValve... (6=)
(7=).............................. (8=)

(10=)Windows (on the outer metal bracket)

I assume 6,7,8 must be:
-ElectricFan (Is that the cabin blower or Aux fan?)

There's an online manual that is just like the CD manual in the encyclopedia Germanica.
to finally check fuel delivery I disconnected the fuel line before the splitter and put it in a cup and turned the key (disconnect the wire going to coil) and checked in the cup for fuel, of course since you already drained the tank and put new gas in, then pump until the gas coming out doesn't smell like turpentine THEN replace your fuel filters, you may need to clean the fuel filter before your pump again, located next to the pump under rear tire

dont forget to check the fuses

If I could read German I could tell you what this says.

As best I can translate:

1. Automatic Antenna
2. Parking, Boot (Trunk)Light, Glove box light , Entrance lights, Warning Flasher
3. Radio
4. Flasher
5. Windshield Wiper, Discs washing pump (whatever that is), Signal horns, cigarette lighter (I think)
7. Heat-Chill Blower
8. Instrument panel (I think)
9. Additional fan - Air Conditioning
10. Can't read this one on the cover
11. Rear window heater
12. Automatic Gear ?? (Something to do with Transmission?)
13. Sidelight right, Tail light right,
14. Main Relay, Fuel Pump Relay
15. Nebulous Light, Nebulous Tail Light
16. Sidelight Links tail light left
17. Right high beams
18. Left High Beams
19. Dimmed Headlights Right
20. Dimmed Headlights Left
If all else fails, download these three "cheatsheets given to me by MBGraham and follow them in order, I had to remove the ECU to perform some of these tests, but that was a last resort, and ultimately only necessary to prove the ECU was fine

these saved my bacon, turned out MY problem was a faulty ground to my main relay, took me 3 twenty hour days to figure that out, which I did by "jumpering" the relay


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Distributor removal and repair, Fuel injection trigger points location, disassembling/re-assembling by Graham

I recently discovered that the centrifugal advance in my 350SL distributor was not working. Found this out by hooking up timing light and rotating distributor - timing mark that I had painted on balancer did not move!

I tried to find a used replacement but nothing worked out. So, I decided to dismantle and find out what was wrong.

Others may want to do this, so this is a method that should apply to any early V-8s. (Please feel free to comment!)

Firstly mark exactly where your rotor is pointing - rotate it back & forth and check and mark the range. Also mark exactly where the hold down bolt is in it's slot. (MB says to set engine at TDC on #1 so that rotor centreline is pointing to the TDC mark on the distributor base. They don't say anything about the cam positions, but if you look through oil filler cap, the two lobes should be pointing up. - I skipped doing this, but it IS a good idea)

To remove the distributor, first remove the cap and lay it back on engine with wires still attached. Then, use an extension (preferably magnetized) to remove the holddown/adjusting bolt on the left side. It has a hex socket (allen key) - I had to cut off an allen key and tape it into a small socket, then use a socket extension to reach the bolt). I stuck a small magnet near the end so the bolt would not fall into one of those unreachable spots! Once the screw is out, just lift the distributor out - You will need to disconnect the thin green wire and unplug the main cable.

Once out, put in your vice and check it out.

I first removed the trigger points. Once out, I cleaned them using a small piece of cardboard. But they looked in great shape for 30-somethings. I then set them aside.

Set the points to about 0.014", replace the distributor cover, crank her up and see if she goes!

Hook up your timing meter with vac advance plugged (used golf tee). Then vary rpm from idle to 3000 rpm and check that the mechanical advance is now working. Finally check dwell, readjust if need be and then do a final timing setting. I chose to set to 30 deg BTDC at 3000 rpm.

Please note that I am a first timer at this, so don't blame me if you screw up! I did get some advice from experts, but only by email. If others see errors or have comments please add so others can benefit.

Hope this helps someone!
ok...does your trigger points have the plastic covers over them? if so, these measurements will be hard, if not impossible to get.

Im going to give two sets measurements with the digital caliper, of all four trigger points.

The first will be me attempting to measure just the little nub that sticks up off the face of the rubbing block(where the shaft actually rides)
so this is what the rubbing block sticks out.

1. 1.92mm
2. 1.84mm
3. 1.87mm
4. 1.90mm

second set is actually pinching the rubbing block, and the spring metal in the caliper(im sure a much more accurate measurement.)

1. 8.81mm
2. 8.70mm
3. 8.77mm
4. 8.79mm

yep. im bored.

hope that helps a little, at least to determine if your rubbing blocks are worn down.

i dont know what the rubbing block on new points would be, does anyone?

here is the best i can do to show you were those measurements came from. the red being the first set, and the blue being the second set of numbers.

keeping in mind that as you look at the back fo the trigger point rubbing blocks, there is the spring steel that is dark in color, and a wedge of other metal that is lighter, i was careful not to measure that other metal, and have the caliper jaw resting on the spring steel.
I measured a new set. They were about 9.02mm using your second method.

My old set of points measured 8.5,8.75,8.81,8.73. Closing angles for those were in 130-139deg for 3 of them, but the worn set stayed open for 163deg, maybe more. This set caused all kinds of problems - rich running backfiring etc.

Seeing everyone like pics, here are some:
1. New and old style rubbing blocks
2. Old style points
3. New style points
4. old block
5. New block


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testing the injectors by Graham and 1972350SL, both on the car and with handheld DIY device

way is to remove the fuel rail one side at a time, leaving the electrical connectors, and fuel hoses on, put each injector into a small cup or something.

disable the primary circuit on the coil.

watch and have someone crank the engine, the injectors should fire in the correct firing order listed on your valve cover, note any differences. in the quality/quantity of fuel in the cups.

there is also a leak test, where you run constant 12V to the pump, and use the same cup method(Mercedes makes a special collection trough) and look for leaks.

limit is 2 drops per minute
This is what I did to test injectors one at a time. Used a garden sprayer to provide pressure (It is actually my brake pressure bleeder). 1/2 fill it with carb cleaner or something that might help clean the injectors. Pump it up to about 32psig while connected to an injector. Fire the injector using a battery. Note how much flow you get in some fixed time, say 10sec. Compare results for injectors.

You can pull the inlet screens and clean them. Maybe soak the injectors in Seafoam or Carb Cleaner and try test again.

While you have the injectors out, replace the pintle caps and the top hat seals.

PS: Having a spare injector plug helps.


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testing and adjusting the manifold pressure sensor with CO2 levels by Graham

I have the same car. The emission specs for the 117 engine according to the MB Technical Data Manual are:

Full load shifter position S 3,000 rpm = 2% - 5%
Upper partial load position S, 2,500 rpm 300 mm Hg vacuum 0.1% - 0.5%
Lower partial load 3rd gear shifter position S, 1,500 rpm 300 mm Hg vacuum = 0.2% - 0.5%
Idle neutral, idle speed, oil at operating temp = 0.5% - 2.0%

All % CO and would apply, I guess, to either dyno or road testing. I just used the idle and the full load for setting MPS & ECU.

Those are the North American specs. Euro are slightly different.

What type of meter do you have? Some of the old Wheatstone Bridge ones are apparently susceptible to moisture and kind of useless. If it is not one you can take in the car with, then you could try this:

First set timing so you have 5-8deg BTDC without vac and with A/C turned OFF. Check that you have good vacuum at idle (16-17" is normal). Also make sure your Throttle Position idle switch is functioning properly. Method is in engine manual (also HERE and in pdf attached)

- Warm the car by driving it for at least five miles.
- Disconnect the throttle position switch plug (so as to disable the idle circuit) and set the ECU knob in the middle of it's range which is 11 clicks from right or left.
- Adjust the 4mm MPS screw in VERY SMALL increments so that you get about 3.5 percent CO at 2500-3000rpm. (at least as a starting point) ADDED - One mechanic suggested previous, another suggested setting to about 1% at idle with TPS disconnected)
- Shut engine down and reconnect throttle plug.
- Start up again and set idle speed to about 750-800 rpm's.
- Use the ECU knob to set the idle co to about 1.5- 2.5 %CO. (rev enginel in between adjustments and taking new reading)
-Take car for a drive and then recheck CO at same conditions.
- (ADDED) If you have portable CO meter or AFR gauge, road test or check on dyno - I found I had to use brake to apply load and achieve the TDM conditions.

On my own car, I used this procedure. At first, I had a loan of an older CO meter - But it did not give consistent results. Since then, I have purchased a wideband Air/Fuel ratio gauge. (There are a number of manufacturers - mine is an AEM and is a permanent mount. Innovation makes portable units. Cost $200-$350 and well worth it.) This allows me to monitor AFR at all times. Better than watching TV :)

My car currently cruises at an AFR of 13-14 and idles at about 13.5. But I can set these wherever I want. Car seems to like to run on rich side. But for economy, it could be leaned out (but not too much or you could burn valves pistons etc!) But who cares about economy on and old SL ;)

Sorry this is so long winded. I have had my 350SL for 20 years, but with AFR gauge, have finally got somewhere this year!


Over-all Diagnostics with illustrations

Troubleshooting D Jetronic.

No start and fuel pump not working

1)Test the fuse,relay,wiring and fuel pump.

No start and fuel pump working

1)Test the main relay and power from ECU.Ensure there
is a good ground from ECU terminal #11 to ground.
2)If the ignition system has a good spark,good power
supply,ground to the ECU and continuity at the trigger
points(cover this later) and injectors fail to pulse,
replace the ECU.

Starts when cold and dies when the key is released

1)Trigger points in distributor lack continuity.When
the engine starts,it is starting off of the cold start
injector and as you release the key,the cold start
injector shuts off.

Stalls when engine is cold

1)Inspect the auxillary air valve.When the engine is cold,remove the hose on the intake side of auxillary
air valve and let air to be drawn into it.If the engine speed increases greatly,replace the auxillary air valve.
2)Temperature sensor 2 is also critical to cold operation.

Misfire or unstable idle

1)Ensure that the ignition system is in good shape and also the intake is in good order.These need to be
confirmed first as they are more likely to cause this problem.
2)Inspect the circuits and sensors of temperature sensor 1 and 2 and continuity through both sides of trigger points.
3)Inspect the vacumm hose that goes between the pressure sensor and the intake manifold.It should have
no kinks and not collapse with vacumm present.
4)If all the above check out,do an injector flow test.

Misfire while driving

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Do a fuel pressure and volume test.
3)If the fuel pressure is constant,test temperature sensor 2,pressure sensor and throttle switch circuits.

Power is suffering

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Most likely this is due to low fuel pressure.Do a
fuel pressure-volume test and pay close attention for loss of pressure.
3)If correct,test the circuits of the pressure sensor and temperature sensor 2.If these meet specs,do an injector flow test.
4)If everything checks out,repeat this test.

Stalls when you decelerate

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Inspect the hoses and wiring for good contact.
3)Check your throttle stop adjustment and curb idle

Smokes and runs rough

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Do a fuel pressure and volume test.
3)Inspect the circuits of the temperature sensor 2,trigger points,pressure sensor and injectors.If they
check out fine,do a injector flow test.

Misfire under Load

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Do a fuel pressure-volume test.
3)Inspect the circuits of temperature sensor 2,
pressure sensor,trigger points and injectors.
4)If the above are good,do an injector flow test.

Trailer hitching

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Do a fuel pressure-volume test.
3)Inspect the circuits of temperature sensor 2,
pressure sensor,trigger points and injectors.
4)If the above are good,do an injector flow test.
5)Check the throttle switch.

Idle speed too high

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Check and adjust the throttle stop and curb idle speed.

Idle speed too low

1)Ensure that the ignition system is in good shape
and also the intake is in good order.
2)Check and adjust the throttle stop and curb idle


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Throttle body

This is from the Volvo Classic forum.

The link for the entire document is
VClassics Interactive - Archive

It's a pretty good overview of D-Jet.

Throttle Switch:
As our D-Jet cars approach thirty years of age, the Throttle Switch is a common source of trouble. Typical symptoms include hesitant acceleration and "bucking" at sustained speeds. Fortunately, it's very simple and can generally be repaired with careful cleaning of the internal contacts.

Under the cover is a circuit board with 22 segments, across which three wipers are swept as the throttle butterfly shaft moves. One segment and one wiper make contact and signal the ECU when the throttle is closed. Another segment and wiper make contact to signal wide open throttle. The middle wiper sweeps the remaining 20 segments as the throttle is opened. Each time it "climbs" a segment, it signals the ECU to fire all injectors one additional time. This is, essentially, what D-Jet uses in place of a carburetor's accelerator pump.

Check the adjustment of the switch on its shaft and the condition of its contacts by turning on the ignition (don't start the motor) and manually operating the throttle through its full range. You should be able to count exactly 20 clicks from the injectors as you slowly open the throttle


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cold start injector troubleshooting and location

I haven't run across anyone having troubles with this yet, and no "fixes" other then listen for it when you start the engine, if it doesn't work (if you don't hear/feel it fire) remove and treat like any other injector, I don't know what causes it to fire, I will forum crawl later


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more diagnostics

Testing the D Jetronic Components

Main relay

1)There should be 12 volts at pin # 24 of the ECU.

2)If not,check for 12 volts at pin # 24 of the relay.

3)If 12 volts is not present at the relay but it clicks,replace with a new relay.

4)If there is 12 volts at pin # 24 of the relay,repair
# 24 wire between relay and ECU.

5)If the relay does not click,inspect the wire from the main relay pin # 45 to ground.

6)If the ground is good and 12 volts at relay pin # 38
when the engine is cranked,replace the main relay.

Trigger points

1)With the engine not running and ECU harness disconnected from the ECU,check the resistance between
ECU harness 12 and 21,then from 12 to 22.One should show a low resistance and the other should show an open circuit.

2)Rotate the engine 360 degrees and check the resistance.They should have an opposite reading now.

3)If not,perform the same test at the trigger points themselves.This will tell you if it a component or wiring fault.

Temperature sensor 1

1)With the engine not running and the ECU harness
disconnected from the ECU,connect an ohmmeter between
ECU harness pin # 1 and 13.At an ambient temperature
the reading should be 200 ohms.

2)If the resistance is well above 200 ohms,check resistance at the sensor itself to determine if it is
a component or wiring fault.

3)Check resistance of all the terminals to chassis ground.There should always be an open circuit.

Temperature sensor 2

1)With the engine not running and the ECU harness
disconnected from the ECU,check the resistance between harness terminal 23 and ground.The resistance should be 2000 ohms at ambient temperature.

2)If the engine is near operating temperature,the resistance should be below 1000 ohms.

Throttle switch

1)Test the throttle switch with the key on,engine off.

2)Slowly open the throttle,and as it opens the injectors should alternately click.You should hear
20 evenly spaced clicks.

3)If this is not met,place a 0.016 in.(0.4mm)feeler
gauge between the throttle stop and throttle stop screw.Connect a voltmeter to terminal 17 of the throttle switch.The voltmeter should show voltage while the ignition key is in the on position.Remove the feeler gauge and the voltmeter should show no voltage now.

4)If this fails,loosen the screws and rotate the switch until it meets these requirements.

5)If they can not be met and voltage is always present no matter where you position,replace the throttle switch.

6)If the voltmeter reads no voltage,ensure voltage is
being supplied to the switch.If so,replace the throttle switch.

7)If no clicks are heard while performing test 1 and 2,check wire numbers 20,17,14 and 9 for continuity end
for end from the ECU to throttle switch.

Pressure sensor

1)With the engine not running and the ECU harness
disconnected from ECU,check the resistance from ECU
harness terminal # 7 to 15.Should be 90 ohms and then check the resistance between # 8 and 10.Should be
350 ohms.If this is not met,test the sensor itself.

2)on occasion,the sensor resistance will check out,but will not hold vacumm.Using a hand held vacumm pump perform this test again (step #1)

Injector circuit

1)With the engine not running and the ECU harness
disconnected from ECU,check resistance of ECU harness
pins # 3,4,5,and 6.All should have less than 25 ohms.

2)If the resistance is greatly higher or lower than
25 ohms,test the injector itself.

3)If the injectors pass,repair the wiring harness.
Intermission (Music plays) with light reading and misc Info about the 72-73

I like the '73 because it is one of the two years of the W107 without catalytic converters ('72-'73) and has the std bumpers, before the gvmt mandated rubber-baby-buggy bumpers. Other notes:

>> Lifters are solid/mechanical- easy to adjust, if you buy the tool and are willing to pull the valve cover gasket once in a great while.

>> Mechanical seats adjusters- never power,

>> Early hybrid ignition- relay over electronic over std.coil. The relay is the old fashioned "points", which power a solid state transistor switched unit, that triggers the coil. Two notes: The correct points are Bosch chrome plated, but will look normal. If not chrome plated, they should be cleaned with a non-abrasive wipe to remove any oil film once in awhile. The reason is that there is not enough current passing through them to burn off any oil. Also, the coil is special and should be light blue, if the original and/or correct. If the transistor module is still there, it is a black box, about the size of small table radio (3" x 5" by 2").

>> Plugs- now that we do not have leaded gasoline, a modern set of platinum plugs may last longer. The old std. steel tipped plugs (use ONLY Bosch that have plated threads to avoid galling in the Al heads) used to be used up at 6k miles! Due to the lead mostly- wore out the plug. If not replaced, engine would not run or would miss under load!

>> Its a Rudolf Ulenhaut car (vaunted Chief Engr at MB), so should be tough as nails. The old 4.5L V8's were not Al blocks, but Ferrous. The heads are Al, with special sodium filled valves- race car quality, but the valve guide seals, if original, are solid plastic and wear a little. With miles on them they will allow a little oil to drip down on the tops of the cyls when engine is shut off. This is evident when you start up cold, and observe a puff of smoke out of the exhaust for about 2-3 seconds. They can be replaced by modern flexible rubber guides less prone to leaking.

>> The valve guides are replaceable, but the heads must be pulled and that is a dealer or shop job.
>> Brakes are easy to work on.

>> A/C had plastic control valve that was the dash mounted unit. It cracks and leaks, but there are metal after market replacements I think.

So it is an old car, but simple as they get, but yet the first modern MB SL. No more swing axles in the rear and no kingpins in the front. Big steering wheel, cuz MB thought you should be able to drive with the pwr stg out.

Oh yeah, and they are pre-galvanized bodies. Rust like crazy, if not de-rusted and kept well painted. Annual ritual, to keep the body tight, per the MB manual; it was and is illegal to drive a car in Germany that has holes of rust in the body- ticketed and black flagged if you will.

Pure trivia, the Ziebart Rustproofing company was started by a German soldier named Kurt Ziebart, who had been an American POW. ("I was riding a motorcycle with a sidecar and drove right into an American tank column. They said 'you are a prisoner' and I said 'ja, ja, ja'". Spent last two years of WWII in a POW camp. Then he immigrated to the US. He had been trained as an automotive body shop technician and was doing body work on American cars in the 1950's in the Detroit area. Lots of rusty cars then. So he decided to start under-coating cars for a fee. He was basically a local Detroit operation, when he sold out and the buyers took the franchise nationwide in the US. Kurt then bought (started? I don't know) a Mercedes dealer near Traverse City, MI, in a little town called Acme. Old Michigan MB owners remember Acme Motors. End of story.

What I know.
Odds and ends/stories of people actually troubleshooting their D-Jets, helpful to have a story, these are important because the actual problem solving and issues mis-diagnosed, and correctly diagnosed begin to tell a story, in one example the OP does everything under the sun to repair this, that and the other thing, when in all actuality, repairing the AAV first would have led to saving probably weeks and countless hours of frustration, myself I have spent 50-60 hours at a time in this restore chasing the wrong horses, when in actuality the "fix" was a 3 minute job, taking a half hour/hour to read these threads will in all probability save you thousands of dollars and countless hours of incredible frustration, like literally hundreds of hours of tearing your hair out as you rage at this recalcitrant over-engineered sadistic beast in your garage, when in fact, 5 or 10 well spent minutes will turn her back into the obedient love child you dreamed of when you bought her

or in other words, this is the "don't make the same mistakes we made" part of the thread

1972350SL's Story:

1972 350SL, only minor bugs left by 1972SL350

MKN's Story

Rolling Restore a Black Lipsticked 1973 450SL - Bringing a car out of storage by mknmike

The Proctologists story

1972 350SL fuel delivery/Ignition by The Proctologist

Backfire Issues

Engine Backfire Question? by DEJZU1

Djet Rebuild

About to start 20 years sitting D-jetronic on a 350 SLC ;) by Super_boss

Because it was so hard to find: Spark Plug Gap for the 72

Correct Spark Plug Gap for 350sl 1972 by diaao

Here is the thread with ALL the Djet location Photos

DJet Parts - Location Pics by MBGraham
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After I had my car running for a few hundred miles or so, it started "hitching" just like on one cylinder, would happen so fast I thought I imagined it, once I just happened to be looking down and saw my RPM's drop to zero and back up again so fast I thought it was my imagination...

later that day it started again, and swiftly got worse, started dying at lights, ran great, would start "hitching and missing up to 3 spark plugs not one, until it died

I happened to have my tools including my voltage tester, checked for fuel delivery, check, spark at plug, dist cap, no spark, leaving coil, nope, entering coil, nope, top resistor, nope, bottom resistor, check,

OK, so I had spark from a resistor TO a thing called "The Switchgear" but no spark coming out (I already had pulled the coil and walked 5 miles to a parts store in 90 degree heat, only to find it closed, I saw a mercedes mechanic/shop who recoiled in horror from the grease on my hands and had utter incomprehension as I asked about things like....points....)

so my first thought was it was the coil, then after further investigation, all signs pointed to the switchgear, power in, no power out

WAIT Graham said, check for the little green wire that is grounded on the outside of the dist cap, if frayed, dead ground, no spark

check, frayed wire

he also said "check your points"

since it sat for 10 years the dist cam had gotten dirty and maybe a bit rusty, which wore down the "bumpers" which caused the points not to open, which caused no spark to the top resistor

the switchgear was fine (located under wheelwell in front of drivers side tire, follow wires from bottom resistor next to coil. or look under the bumper, it looks about like an electricians box for a light fixture)

Removed the green wire, insulated it, re-attached it, took out the points, cleaned them up real nice, had trouble with gapping the points (setting the dwell with no dwell-o-meter or whatever)(a business card is about the right size) but no luck, tried books of matches, business cards (they'd been in my wallet for awhile and had swelled a -bit- fatter) so finally I tried a playing card from a travel kit, it was VERY thin (regular playing card .032 inches, this card about .013 inches, we were looking for about .014, with "play" as I tried to set it at exactly the high point of the cam, truthfully I set it with the card, bought a gapper thing, feeler gauge or whatever, ran worse, went back to the card, more flexible able to get a more accurate gap, re-checked with feeler, spot on, .014)

I learned there are 2 "Top Dead Centers" when putting your dist back on, one causes "great balls of fire" from your air intake, the other causes your car to run well, trick is open oil cap on valve cover and line up TDC with the high point of the cam showing

if your car "rumbles" and rocks back and forth and belches great gobs of black smoke, and runs rich, your gap is too wide, if it starts "missing" and "hitching" it's too narrow, find the sweet spot, took me about 8 times to learn the trick

You are going to learn some new words and new skills if you hang around this place. And probably purchase a bunch of tools. These things are from the time when it WAS possible to work on your own vehicle. Unlike the modern stuff where you have to have a obscenely expensive electronic diagnostic tool to open the hood.

Dwell (actually: Point Dwell): The length of time the breaker points are closed, expressed in degrees of the rotation of the engine.

If the points are adjusted properly, they are closed for 34 degrees of rotation of the engine for each cylinder. This allows the current to build up in the coil. The points are actually open when it's time for the plug to fire. (when the rotor points to the plug terminal on the inside of the cap) Think of it like pumping water(electricity) into a toilet tank(coil); Slow and easy in to fill up, then fast and hard out at the flush(fire the plugs). Only with electrons.

The length of time the points are closed determines the amount of 'water' in the tank; if the gap is too narrow/dwell too long, there is a lower amount of 'water' in the tank, and the spark is weak.

On an 8 cylinder engine, a 45 degree dwell will mean that the points are closed all the time. This is bad: no spark. If the gap is too large/dwell to small, there won't be enough 'water'(electricity) into the 'tank'(coil) to give a good 'flush'(spark). Not EXACTLY right, but close enough to give you an idea of how the system works.

Automotive ignition theory and plumbing in the same post. Only possible when discussing electronics. Whoda thunk it?:D

now after you fix all this, and have learned so much, replace immediately with a petronix set, and never deal with it again


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Timing, Vacuum Diagrams

First the link to the timing stats

then MBGrahams links to vacuum system

I posted the diagrams for the early 107 engine, door locks and heating system in the other thread. Question - Is there a way to upload the pictures to benzworld - my web space is getting kind of full!

Here they are again. Maybe someone with a manual could add the later models. I am trying to get ready for a trip South!

Engine vacuum retard:

Heater Vacuum controls:

Doing some work on dads new 72 350sl, the other day he started it up cold and it began surging, just like the old one did before we replaced the AAV.

Before we condemn the AAV, im going thru the vacuum system, and found that this guy has no vacuum lines going to it(has 2 ports)

this vacuum switch/thing sits on the rear of the coolant reservior

I vaguely remember this from the old SL, am i correct in thinking that it is supposed to have one tube coming off the small tube on the throttle body, then go to the distributor?

still working on getting the intermittent splashy idle on decel worked out, so looking at this as a possible issue.

what is this piece? what does it do? i figure its sposed to be there, so someone thought it was neccessary.
That is true. Swtich A/C on and you hear idle speed increase. The retard is only activated at idle (Throttle switch against stop) and only if coolant temperature is below 100C. On some cars transmission oil pressure also has an affect, but ISTR, not on the SL.

BTW - this is described in section 14 of manual - emission controls. Not the first place I looked :(
But there is a catch with this valve... actually "Switch-over" valve

It is not just: open|close (as I found recently), it is switchover (directional) valve

When active, it will supply vaccum to retard side of ignition.
When inactive, it will close vac side shut and open retard side to surrounding air (thus not affecting - reducing advance)
To check: when engine at 80*C, a/c on should result in rpm increase to ~850rpm.

I had hoses mixed up for the last 9 years and I cannot believe change. Actually, I can feel more power when accelerating from idle. Strangely, oil pressure now does not drop below 2.2bar even if engine temp goes to 100*C
Motor vehicle Text Engine Vehicle Auto part

Text Font Line Number

Text Font


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Vapor Lock

Common problem. For a start try adjusting the idle speed up so that is 700rpm when the engine is really hot . Sometimes this will keep engine turning over and get cooler fuel to the injectors. What is happening, is that the fuel is vaporizing either in the rails, or more likely as it passes through the injectors. The engine does not get enough fuel and even if it does start, it is very lean.

On my car, I adjusted idle mixture to rich side (4+%CO) and set idle so it is 1100rpm when I start in morning. Have had few problems, but we don't get 95F very often.

Another owner installed a bypass solenoid around teh fuel pressure regulator - it opens when the engine is turned off. On restart, you get fresh cooler fuel to rails. He said it was working in the Spring, but I need an update.

I have not updated the attachment, but it has some of the info I gathered.


Congrats on getting this going Procto. It will save a lot of time for askers and answerers ;)

Hopefully this thread will stay free of discussions - It would be best if those with questions or comments started a new thread.
Graham, will you use the top box /reply to add/correct/fill in stuff I left out? I am also going to send you and 1972350SL my PW so you can edit the original posts where needed

truth is, this is ALL you, I just put it all in one place, now we can just start giving new Djet owners restorers and resurrecters one link

I cant find anything about The ECU, you know of any threads to pillage for that? I only found out about it from my troubleshooting attachments you sent me
Hopefully this thread will stay free of discussions - It would be best if those with questions or comments started a new thread.

That can happen.

Dig's edit:
BenzWorld links cleaned up and thread immortalized.
Added 7/26/2011
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