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1985 380sl
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Discussion Starter #1
It's a daily driver that had been running great. A few days ago I drove it ~20 minutes then stopped at a light and it died. Electric power supply seemed fine as exterior/interior lights, radio, antenna etc. worked but the engine wouldn’t turn over, so I called a tow truck and went home to start searching the forums.

After a bit of digging, and given that the fuel accumulator, distributor, alternator, and fuel pump have all been replaced within the past two years, it seemed as if the Fuel Pump Relay (FPR) and Overload Voltage Protector (OVP) were obvious culprits worth investigating. And since my good friend @cushjbc had recently suggested a GTG and had the same FPR/OVP theory, we set up a day to do some troubleshooting.

We started by using a multimeter to test for power at the fuel pump and determined there’s no power with ignition at 1 or 2 but there is power when ignition is at start. Does this mean the FPR is working correctly?

Next we tested OVP initially by turning ignition and heard OVP click with ignition at position 2. We then pulled OVP and tested for continuity, which was good. We noticed it rattled when shaken. Don’t know if that’s normal or relevant. Cush inspected the circuit diagram imprinted on the fuse (see attached photo) and felt pretty sure about what everything was on that diagram except the horizontal rectangle shown above and between pins 31 and 15, indicated with Z22. His theory is it’s a Zener diode, and being power-to-ground it’s probably a work device.
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In any case, he did some wiring magic and hooked up the OVP to a battery and ran a current through it to see if we could get a current to pin 87. There was ~1 second delay between when we switched power on at terminal 15 (applied 12 volts) and when we got 12 volts out at terminal 87. So perhaps the mystery device (horizontal rectangle on diagram) causes that delay? In any case, we did have success getting a current. But with him being a D-Jet guy and me being a nitwit we don’t know if this means it's actually working properly.

Next… When my car conked a few days ago the battery seemed strong but today it seemed a bit weak, and to rule out the possibility that the computer wasn’t getting enough voltage we jumped the battery to see if that would start the car but no happiness. The engine cranks but never turns over.

Also, Cush managed to find the FPR. My forum search found that the FPR is located in different locations based on model year, and mine was behind the glovebox. We pulled this for a photo op (see photos). It doesn’t look like a Mercedes part. This, too, rattles when shaken.
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Other items of note:
  • There is a smell of gas after trying to start. Not overpowering but noticeable.
  • Just for kicks we also replaced all the fuses in the fuse box that weren't brass.
Lastly, being the relentless sleuth that he is, after he left this afternoon Cush learned there are other tests that can be performed on the relays but we ran out of time today.

I’m seeking input on next steps. Since I don’t know jack about any of this stuff and am unable to perform any of these diagnostics on my own, I’m inclined to spend ~$50 apiece on a new FPR 81-85 MERCEDES 380SL FUEL PUMP RELAY 001 545 53 05 FULLY TESTED WARRANTY | eBay and OVP and plug them in and cross my fingers. If it doesn't solve the problem than I'll have extras in case I do need them someday.

Anyone else have ideas/suggestions? Thank you!
 

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1973 450 SL
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We started by using a multimeter to test for power at the fuel pump and determined there’s no power with ignition at 1 or 2 but there is power when ignition is at start. Does this mean the FPR is working correctly?
According to Service Manual article 07.3-001 page 23:

"The fuel pump is provided electric current by means of a relay. The fuel pump is therefore running only:
a) When the starter is operated
b) When the engine is running
This will prevent the delivery of fuel with the ignition switched on and the engine stopped (e.g. following an accident)"


I had a look at the ETM for Joser's car. The Fuel Pump relay has a battery bus input at pin 8, a start/run bus input at pin 9, and a ground at pin 11. The FP relay has a card that closes the solenoid connecting battery bus power at pin 8 to the fuel pump power lead at pin 7 under the two conditions covered above. The first condition (a) above occurs when starter power is applied at pin 12. The second condition (b) occurs if the TD signal from the ignition switching unit presents 80 RPM at pin 10.

Joe and I successfully tested the operation of condition (a). Tonight, I figured out how to test the presence of a TD signal at pin 10 but have no idea how to determine if the FP relay is functioning properly on condition (b).

The fuel pump relay has a transmission kickdown and shift lever position input at pin 6. I can't say I understand what that is about. The other magic on that card is an RPM limit that opens the relay at 5500 RPM (or 5300 RPM if the kickdown switch is closed - I think).

I wish we had a working spare FP relay.

I also had a look at the Overvoltage Protection Relay in the ETM. The "What's This" question posed above . . . it is a resistor in series with a Zener diode. This causes a voltage drop and a the 1" delay we discovered when testing it. I think his OVP relay is working fine.

I thought about bridging pin 8 and 7 with a switch. This eliminates all the logic of the FP relay allowing the fuel pump to run to see if we can get the car started. If this works, I think it means the FP relay is the culprit. Is this a bad idea?
 

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1985 380sl
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Discussion Starter #3
I'm the guy on the right, and I just know everything is going to magically work out great.
2618423
 

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'80 450SLC Afro RHD Ikonengold
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The rectangle is resistor and it is there to suppress destructive inductive forces when a coil is powered on and off. Some have diodes instead of resistor, others combinations.

Z22 is protection device and it is similar to zener but it is likely to be an avalanche diode or a simple surge suppressor diode. Either way, it is supposed to be inactive until breakdown or avalanche voltage is reached, thereafter conducting sufficient current to pop the fuse. There is a chance that a single brief voltage surge (in my guesstimate about 18-20V) just caused the fuse to pop and no damage to "zener" but the nature of these devices is to burn to short this creating a crowbar effect and making fuse indeed pops.

Delay... it should not be there, at least not from the drawing.

From what I read here, OVP should not cause no-start condition. FPR could but you could check if pump receives voltage whilst cranking . WUR is wired in parallel to pump, if you have it it is a quick check.
Just remember that FPR is actually FPcoR (co = cut off) and it needs tacho signal from the electronic ignition module to run.

Hope did not confuse you too much
 

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1985 380 SL
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The first diagnostic step in a no-start should be ignition, get a proper spark tester and see if you have a spark when cranking. You could also use an inductive timing light. You shouldn't assume something is ok just because it was replaced in the past.

When the car died did it jut stop like it was switched off or did it kind of slowly die?
 

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560SL '88 Suzuki GS1000E '78
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Hi, normally the OPR isn't necessary for starting. If you have trouble with idle, ABS or SRS, the issue may be related with the to that. But to be honest, at the moment I'm not completely sure if the fuel pump-relay is supported by the OVR as well. Anyway: the fuelpump must run and you need pressure in the fuelsystem. If you check the pressure, you will know more.
You can open both relays and check the cirquit boards for cracks ore loose parts
Martin

Edit: AlterSchinken has the best answer. Before start soldering and measuring pressure, you should check if the ignition is still working
 

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1989 W124 260E
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Did you say that the engine would not - crank turn over ?. Wont be any items other than battery alternator starter switch .Auto box inhibitor .Work the selector lever a few times and try again.Check fuses in the fuse box charge battery fully .
 

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1983 380SL
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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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Autohaus has the Hueco for less than $50.00. I don't trust PartsGeek. Unless they have improved they have lousy customer service. Won't answer the phone or respond to emails.
 

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1983 380SL
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Agreed...Autohaus or Pelican are better places to get your parts.

Parts-geek was the first thing that google returned on ICU.
 

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1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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It does sound like an electrical problem. Were the infamous green wire and connections checked?
 

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1973 450 SL
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The more I think about it, Joser's symptoms point to ignition. I'm not experienced in K-Jet or KE-Jet other than reading this forum from the D-Jet cheap seats. We had a look at FPR and OVP because many threads here point to that. I'll do some reading in section 15 of the Service Manual to learn how to test the TSZ 8 ignition system.
 

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1985 380sl
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Discussion Starter #13
Thanks for all the input, gang! I'm back at my job this morning (the one that pays me). Will circle back to my car later today with more detail.
 

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1985 380sl
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Discussion Starter #14
Thanks again for everyone’s input so far! Here’s an update.

First, I should reiterate that yesterday’s clever sleuthing only happened because @cushjbc generously lent his time and expertise working with me all day. And last night’s write-up was - for better or worse — what you get when a know-nothing like me translates what someone knowledgeable did into terms I can understand. Imagine watching a surgery being performed and then describing it to a panel of doctors to get their input. “He used a pointy thing and made a bleedy thing and massaged that squishy thing and something turned purple and what do you think?” That’s what you get from me.

That said, here’s the latest.

My mechanic suggested a quick way to rule out relays as the problem for the no-start. We charged the battery (which was mostly dead as of this morning) and then bypassed the relays altogether by running power from the battery directly to the FP, the theory being that if the car started then the relay(s) were in fact the culprit. But no-start, so it’s looking like relays are off the hook — unless we’re missing something, in which case I trust you’ll set me straight.

He next did a quick visual check to see if there’s a spark by pulling distributor cap and holding it close to its fitting while I turned the ignition. There was a good spark so this is looking like the ignition coil may be OK, right?

He did notice the distributor cap housings and related wires are quite old (naturally) and also rather gunked up and corroded which could be a factor, plus the spark plugs are also quite old so those seems like something I might want to replace anyway.

Also Cush did a ton of research on the ignition system should this turn out to be the culprit.

So there’s the update.

Also...

@Djenka018 — Thanks for the detailed description of the fuse!

The first diagnostic step in a no-start should be ignition, get a proper spark tester and see if you have a spark when cranking. You could also use an inductive timing light. You shouldn't assume something is ok just because it was replaced in the past.

When the car died did it jut stop like it was switched off or did it kind of slowly die?
Today we did the visual spark test, seeing as my car is sitting in my garage with no diagnostic tools. Tomorrow we’re towing to my mechanic’s and this is on the diagnostic to-do list.

normally the OPR isn't necessary for starting... If you check the pressure, you will know more.

Edit: AlterSchinken has the best answer. Before start soldering and measuring pressure, you should check if the ignition is still working
Good to know, and officially on the diagnostic to-do list.

Did you say that the engine would not - crank turn over ?. Wont be any items other than battery alternator starter switch .Auto box inhibitor .Work the selector lever a few times and try again.Check fuses in the fuse box charge battery fully .
It cranks but doesn’t turn over. Cush and I did replace all fuses with brass fuses. OK, battery alternator start switch is on the diagnostic to-do list.

spray some starting fluid into the intake. If it fires, you have a fuel problem. If not you have a spark problem. You may need a new ignition control unit.
That sounds like a simple test that even I could perform, and it’s on the diagnostic to-do list.

Were the infamous green wire and connections checked?
Didn’t know about this! It’s now on the diagnostic to-do list.
 

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1985 380sl
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Discussion Starter #15
Well, I think we found a clue!
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Just kidding, this was the actual culprit causing the no-start: a cracked & cooked primary distributor cap. When my mechanic pulled the distributor cap and had me crank the ignition to see if there was a spark he noticed how burnt and corroded it was and thought it was too much of a coincidence and opted to pursue this as the the most likely cause.

When he pulled it, it broke and then you could really see how baked it was. Apparently, the cap had cracked enough that the spark no longer went to the spark plugs and instead went to the grounding plate (at the base of the two spring-loaded screws), so no current arrived where it was needed.

So.. to everyone who suspected an electric problem, you were right!

The distributor cap, ignition rotor, and plug wire set have all been replaced (the wires were as hard as a rock), and she's purring like a kitten again.

@cushjbc asked if the wire sleeves and wire numbering rings came with the replacement apparatus, and the answer is no, so now there's a new project... pulling the whole thing and restoring it to the original condition with those items.

Are we a bunch of fanatics or what?? This isn't the kind of thing that would likely make much of a difference during resale, but it sure gives us fanatics a thrill!

In any case, I really appreciate all the fast and insightful responses to my original SOS. This forum is awesome!! 🙏
 

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1973 450 SL
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I hate that we didn't find this; opting for the FP relay and OVP relay bunny trail. Learned some things about '85 KE-Jet along the way. Look forward to a parts scrounge at UP&P today with @joser85
 

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Discussion Starter #17
I hate that we didn't find this; opting for the FP relay and OVP relay bunny trail. Learned some things about '85 KE-Jet along the way. Look forward to a parts scrounge at UP&P today with @joser85
Given all the issues others have had with the relays, I still think it was a really good bet, and I enjoyed diving in and learning about these things. It would be nice to have a spare of each and keep them in an emergency kit for road trips. I plan to order one of each... unless we score them on our UP&P expedition today, WOOT!
 

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1985 380sl
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Discussion Starter #18
Follow up: We scored a couple OVP relays at UP&P Thursday (plus some other goodies) but didn't find any FP relays compatible with Cush 450 or my 380.

2619045
 

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Yes we did . . . and my car is much faster and jumps higher with my new(er) antenna switch and a new(er) rear window defrost switch that Joser suggested I buy but didn't until I couldn't stand it and went back on Friday.

I looked at 2 more MBs for a spare FP relay for Joser's car - no luck.
 

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Discussion Starter #20
Yes we did . . . and my car is much faster and jumps higher with my new(er) antenna switch and a new(er) rear window defrost switch that Joser suggested I buy but didn't until I couldn't stand it and went back on Friday.

I looked at 2 more MBs for a spare FP relay for Joser's car - no luck.
Ha! Now THAT's dedication!
 
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