D-jet or K-jet injection? If D-jet, check the ECU. My 74 450SL had similar symptoms. Turned out the ECU was bad--1 pin was losing voltage. The car would crank, but not start.

 

PS--top silver, bottom blue.

 

Thank you rcrmonte3, she's a K Jet. Good point about the ECU, we'll inspect it too.

Also, appreciate your advice re ballast - does the wiring look correct in my yoke? The reason I ask is because when the original blue one was replaced - I've no idea if it was a straight swap, or did someone get creative and rewire cabling too... The joys of 40 year old cars ?

 

I really can't tell on the wiring, but you have both of them wired together and I wonder if that is part of the problem. Aren't they supposed to be on different circuits?
I'm sure someone here can set me straight and tell you what's right. Mine appear to be on separate circuits.

 

No wonder your car runs poorly, you have the wrong combination of Ballast Resistors.

Without walking you through every step of the wiring diagram, there are basically two things that happen with the ballast resistors.

In normal operation, the 0,6 and 0,4 resistors sum to 1,0 ohms and the purpose is to limit the flow of current through the coil so it doesn't overheat or burn up. The ignition control unit in a '79 is a solid-state reluctor-triggered switch, but the quality of the silicon transistors they had in those days was not sufficient to run a high-amperage load through it, not like later days.

So there are two resistors in series between the battery and the coil. During normal operation, both are in the circuit. But when the starter solenoid is engaged, one of them (the 0,4 ohm one, first in the series) is bypassed by full +12V straight from the coil. Then current comes straight from the starter, and goes to the 0,6 ohm one, where it is reduced to the correct amperage for the coil. (this all changed in '82, I will post how and why shortly)

Of course the reason for this is that when the engine is cranking, the starter creates such a voltage drop on the battery that a full +12V is NOT present at the first ballast resistor, to be reduced further to coil current. The starter pulls the voltage down to around nine volts, so bypassing the first resistor to allow a full +12V to go to the SECOND resistor, and from there to the coil, is a good idea, and essential during cold OAT temperature conditions.

OK that is background. In your specific case, you have two 0,6 ohm resistors, which means that you are reducing the current to your ignition coil too much, during normal operation. When cranking its the same as the factory config, but that might not be the only problem, we'll have to see.

With the engine not running, on my '78 I measure +11.8V before the first resistor; +8.8 before the second, and only +4.8 before the third.

Those are BIG voltage drops caused by these resistors-- if you have one that is too high a resistance value, you will not get sufficient coil current to light the fire reliably.

Change the first 0,6 out for a 0,4 and report back here. I attach a photo to show you how its supposed to be.

Please post a photo of your coil also, want to see if you have the right one.

 

Thanks rcrmonte3, appreciated.

Hello 304065, first of all thanks for the crash course on this vital part. That makes complete sense, I hope that the incorrect set up is the bad boy! I've looked at the diagram you provided and on the surface mine appear to be wired correctly, however could you post a pic please showing how your resistors are wired on the left hand side please?

Below some pics of my coil, looks like it's the original blue one. I've ordered a new Bosch 0221122001 plus Bosch resistors A0001581345 & A0001582045. Unfortunately it will be late next week / weekend before I've had a chance to fit them as I'm travelling until Thursday, Sept 13.

 

Glad I could help in my little way. It's good that 304065 could tell you about the proper wiring, as I'm more into the mechanicals and don't know a lot about wiring. Guess I need to learn more about it. Now that mine runs again, I can concentrate on all the other stuff it needs.

 

I hear you rcrmonte3! I'm what you can call an aspiring enthusiast, or a pseudo-mechanic - learning as I go! ;-) I hope that replacing those couple bits & pieces will resolve the issue, though while searching the forum for similar threads I've come across a couple of horror stories... Fingers and toes crossed I'll get away unscathed!

 

That's the right coil all right, made in December, 1978!

I tried to get the best possible photo of my original wiring for you.

Connected to one terminal of the (blue, 0,4 ohm) resistor we have two wires, both 1.5mm^2 diameter, one in a black vinyl sheath. One of these is the power feed from the ignition and the other feeds power TO the TSZ box.

Then out the other terminal of the blue resistor we have a "jumper" red/yellow to the input terminal of the other (silver, 0,6 ohm) resistor. Also connected to the output terminal of the blue resistor is a red/violet wire which comes from the starter solenoid. This is what brings in full battery voltage when the starter is engaged. You can see why it connects on the "downstream" side of the blue resistor-- this battery voltage bypasses the first resistor entirely.

Then the "jumper" is connected to the input of the silver resistor. And finally, out the other side of the silver resistor, we have a black/red straight to the coil 15 terminal. At the coil 15, you can see that a smaller .75mm^2 wire splits off to the diagnostic plug.

Hope the photos help. If you are not sure, STOP, as connecting things wrong can fry the electronics, and could be a potential source of personal injury or property damage.

Good luck!

 

304065, THANK YOU so much for taking the time to write this up - I'm very grateful for your help with this! That's phenomenal that the coil is 40 years and possibly still works - I've come across a statement in one of the forums they ought to be replaced every 8 years...

I'm sure between the photos, diagram, and description you provided we can figure it out - on the surface the wiring looks the same so I'm hopeful the resistors just need to be swapped, yet I'll double click on the config and if I run into any head-scratchers I'll report here first - I'm risk-averse when it comes to this baby girl All parts are on the way so I've pencilled in Sat, Sep 15th to carry out this work.

Thanks again gentlemen - I'll be in touch!

 

Guys, just to let you know that I've reviewed the wiring and I'm happy to confirm it matches 100% 304065's config, so all I need to do is swap out the resistors and the coil. Also, the distributor cap and rotor arm arrived so I'm planning to replace them this weekend as well - I hope they will breathe new life into my Eleanor! I'll keep you posted.

 

So, replaced the ignition coil and the resistors - unfortunately no difference, the engine's cranking but still not starting. Next step is to replace the distributor cap, rotor arm and spark plugs tomorrow- though my gut tells me this won't make a difference. That said, I've a new suspect - battery. The current one is a Bosch S4 which I bought 3 years ago after I got the car, and I’ve driven ca. 2k miles since. At any rate, when the car failed two weeks ago it took over 24hrs for the battery to recharge, which I put down on trying to start the engine multiple times. The car’s been sitting in the garage since, and today the battery took over 8hrs to become fully charged again – depletion rate which indicates it’s not healthy, and even though to my untrained eye the engine’s spinning correctly it may not crank fast enough to create sufficient vacuum to start. So, ordered a new battery online – should be here Tuesday.

I took the opportunity to clean out all the gunk, grease and dirt – see before and after pics, or rather after and before

 

I may have missed it but did you try pouring a little gas down her throat or some starting fluid?

 

Also, what does the fuel unit on top of the engine look like? (under the air cleaner) Does it look like a huge spider? If not, you have a DJet. My 74 450SL has a DJet, so it's a good probability your 74 has the same.
You say there's no power to the cold start valve. Do you know why?
Have you checked voltage to all the pins on the ECU?
The ECU controls the engine end of the fuel system.
When my ECU died, the engine would crank until the battery died, but not start. A mechanic friend and I checked out the entire fuel and electrical systems which were all working properly. Last thing we checked was the ECU. There was 1 pin (sorry, I don't know which one) losing voltage, and that was causing the crank, but no start condition.
I may be wrong, but I firmly believe it's the ECU.

 

PS--I also had the intermittent stall and no restart, warm or cold.

 

It is a '79 so it does not have an ECU.

miko1aj. I think the '79 had a safety connection on the air housing on the right side between the fd and throttle plate.. It is blue/green color with two wires connected.

 

rcrmonte3, rowdie - thanks a million for your response, I really appreciate you taking the time to share your expertise!

rcrmonte3 - my Eleanor is a 79, and she's 100% K-jet. When your ECU died, were you able to resurrect it or did you have to get another one?

rowdie - nope, haven't tested with the starting fluid yet. I thought I'd eliminate all the obvious points of failure around ignition before resorting to ether as the engine ran flawlessly those last few years other than the recent challenges with cold start. Thanks for the tip re safety connection, I'll have a look shortly and post some pics afterwards.

 

You might not have an ECU, but isn't there something that control the KJet? When my ECU died, it couldn't be resurrected for less than a rebuilt/remanufactured one, so I bought a reman. I'm thinking of buying another reman just in case....
Hope you get your electrical problem solved. That safety switch rowdie mentioned sounds like a good lead. I wonder if that is or is like a neutral safety switch?
I forget--are you getting spark to the plugs?

 

rcrmonte3, I’m doing a crash course on the K-Jet workings and as far as I can tell it’s a mechanical injection, unlike D-Jet which was electronic.

So, I’ve replaced the distributor cap and the rotor arm, and unfortunately still no joy… I was sort of anticipating that, so I’m not disappointed – but I will be if the new battery doesn’t do the trick!

Yes, getting spark to the plugs. An interesting development regarding the safety switch which controls the pump – when disconnected, I hear the pump running continuously as soon as I switch on the ignition, and I leave the ignition on for 15 seconds without cranking the pump keeps running. With the switch connected, the fuel pump doesn’t “buzz” at all when the ignition is on – and it never has, so as far as I can tell it’s an “expected” behaviour as it has been the same for over three years. In my CLK the pump kicks in for 4-5 seconds as soon as I switch on the ignition, and I thought it peculiar the SL didn’t but since the car was running spotless I didn’t pay attention to it afterwards. Any thoughts on this? Does pump kick in your SL when ignition is switched on?

Also, could you let me know what’s the CAA in your batteries? Mine is 680A, as recommended by Bosch and other online shops, but while researching this topic I found the US versions have 850A.

Attached some pics of the engine bay which I'm cleaning and detailing as I go...

 

Never had one with that safety switch so I don't know how it should behave. On all of mine D-Jet, K-Jet and KE-Jet the fuel pump(s) runs for a second or two when first turned on then shuts off.

 

All the K-Jets I've seen did not have priming run of the pump. Could the K-Jet you have be a "K-Jet with lambda".

Standard (RoTW) cars with K-Jet have simple pump relay that is driven when the baffle plate in the airflow meter deflects down. This is a safety as well in accident where engine stops.
K-Jet with Lambda and later have a relay that requires a pulse from the electronic ignition to run.

But then again, it could be the US version cars had K-Jet (no lambda) with an electronic FPR...

 

Interesting note about the battery. When you read that the other cars have an 850CCA capacity, group 49 battery, that's probably talking about cars that are LHD and so have the battery back in the trunk. That's what I have, a huge Bosch AGM Group 49, the thing could start a diesel tractor in Siberia with all that power.

Anyway, what raises my curiosity when you mention the battery is that it wasn't holding a charge, and took a long time to charge. The taking a long time can be a function of the charger-- sometimes AGM batteries require a higher charging voltage to get started-- but if it was old, replacing not a bad idea.

If your battery is draining, it's time for a parasite current drain test. I had the same problem when I inherited my 107- the battery was going flat after a couple of days. A parasite current drain test is very simple--you connect a DVOM in between the battery negative terminal and the ground strap, taking care to isolate it so nothing is touching the body-- and set the DVOM to the "amperes" scale. You can then observe, with the key and accessories off, how much current is draining, in the form of milliamps. You then open the fuse box cover and sequentially remove all the fuses until you see a zero current drain. The fuse that causes it to go to zero is the affected circuit. In my case, I removed ALL the fuses, but still saw a drain- the cause didn't reveal itself until I pulled the extra fuse for the automatic climate control (which is in its own black plastic fuse holder) and that was the cause, the Servo.

Anyway, if the engine is cranking over strongly I kind of doubt you have battery voltage that is too low to power the ignition system, especially now that you have changed to the correct the ballast resistors. But it's worth measuring 1) battery voltage at the terminals with key off; and 2) at idle, and 3) at 3000 rpm. This will give you a good sense of the health of the charging system. Moot for now because you can't get it to run!

You changed rotor and cap-- are you sure the rotor has the right resistance. On the bottom of the rotor it will have an R and a number. The rotor for a 78 says R5, or 5,000 ohms resistance. The rotor for an 82 and later says R1, 1000 ohms resistance.

How do you know you have sparks?

 

Wow. My 89 420sel fuel pump ran for a second or two when I turned on the ignition. That car had a KJet type fuel injection. At least, it looked like KJet. My 74 450SL fuel pump also runs for a couple seconds when I turn on the key, but stops after that.
Is that safety switch rowdie mentioned getting voltage?
Do you have a Neutral Safety Switch--other than the 1 rowdie mentioned?
Did you pour some fuel or starting fluid down the KJet? What happened?
All I can think of is to check/recheck the entire electrical system related to the engine starting and running.
304065 looks to be on the right track.
Knowing as little about automotive electricity as I do, Why would the huge difference in resistance between the rotors make a difference? Other than that 1 lets more current thru. Does that make the spark hotter?

 

Rowdie – thanks for your response, appreciated.

304065 - thanks for the guidance on the parasite test, that will be my next step if the new battery doesn’t do the trick. I can’t tell you right now what resistance the rotor has as I’m away till tomorrow, but both rotor and cap are correct Bosch part numbers 0001583402 and 0001582531 respectively. Re sparks, I’ve removed a spark plug and my neighbour observed it while I cranked the engine, and he confirmed that it was sparking alright. Another reason why I think the battery is dodgy is because when I plugged in the same charger to my CLK (which had been parked for a few days at this point) it showed full charge after 15 mins – granted the battery's slightly smaller but still charged way faster than the SL. I guess I’ll find out tomorrow or Wednesday for sure, one way or another!

rcrmonte3 – thanks for your perspective, too. As far as I can see there’s only one safety switch, and my buddy electrician who was supposed to come out yesterday couldn’t make it - we’ve rescheduled for Sunday, Sept 30th. I haven't tested with the starting fluid yet, I’m a bit hesitant to do so until I've eliminated all the obvious points of failure around ignition – and the new battery is the last on my to-do list. If that fails too, all bets are off!

Last question that pops to my mind – could a faulty fuel accumulator or a warm up regulator prevent the engine from starting?

 

Gents, replaced the battery and while's she's cranking much faster - still won't start...

My next checkpoint is to connect the cold start valve to the battery and check if it's spraying fuel. If it does then at least I know it's mechanically sound, and the next step is to figure out why it's not getting juice when the engine's cranking. Any thoughts, tips, or advice what else to check?

Any thoughts / experience if a faulty fuel accumulator or a warm up regulator prevent the engine from starting?

 

Guys, given the ballast resistor issue has been resolved and yet my fair lady is still having issues starting up I've created a new thread https://www.benzworld.org/forums/r-...107-sl-slc-class/3007121-1979-450sl-cranking-but-not-starting.html#post17585075 with subject more in line with the current challenge.

I hope to see you there!

 

Hello Djenka018, thank you for sharing your experience - that's somewhat calming news that the K-Jets you've seen didn't prime the pump either because given my on-going issues I started to doubt if that set up was correct.

Mine is a 1979, the pump relay is right above the fuse board, top row on the left and I do think it's the simple pump relay - when I disconnected the safety switch on top of the engine, underneath the air filter, the pump would run continuously as soon as the ignition was switched on.

If you have any other thoughts or tips for me I'll really appreciate, also please note I started a new thread which deals with the main theme, i.e. engine cranks but won't start.
https://www.benzworld.org/forums/r-...107-sl-slc-class/3007121-1979-450sl-cranking-but-not-starting.html#post17585075

 

Mikolaj

see the response HERE re FPR functionality