'88 560SEL Mystery Stalls
 

Hi all,

Yes, it's been a very long time since I've posted here, but I really need some advice.

My 560SEL has now stalled on me three times in the last couple weeks, twice while driving, and once it refused to start after just being turned off minutes earlier.

The symptoms are strange but at least consistent. The two times it stalled while driving, it was on the same stretch of road, going up a moderate hill. Of course it won't stall there every time, that would be too convenient. Anyway, just driving along, and the engine shuts off instantly and completely. No stuttering, shuddering, anything, the motor just quits as if you'd pulled the key out. The car then refuses to restart for maybe 30 seconds, the engine will turn over fine but does not fire. Then it starts, and runs just fine as if nothing had ever happened.

There are so many things it *could* be, and I'm hoping y'all can help me sleuth this one out before it royally screws me over and leaves me stranded.

Thanks much,


-tp

Well, it did it again yesterday. This time it happened in my parking lot, so I was able to sit for a while an analyze symptoms. Here's a picture I snapped of the dash lights after the stall.

I did a cursory inspection of fuses, grounds, etc. Nothing obvious. Any suggestions greatly appreciated.

I'm still get used to the electrickery surrounding Mercedes petrol engines - from my reading it seems like the first suspect for these types of problems is the OVP relay (OVP = overload voltage protection). It seems as though changing out an OVP relay is the first thing any petrol Benz owner does before he has his breakfast.

I'm guessing it is near to the battery on your car (as it is on my W201).

I agree those can be a pain. I'll give it a shot, though I'm doubtful. Usually the OVP relay will cause the ABS to come on, and as you can see, just about everything *but* that light came on. Yes, I know the picture sucks, but I had to make it very poor quality to upload it here with the 72kb limit.

Maybe I'm taking the wrong approach in asking for non-specific advice.

Let me try this another way -

We all know that MBs of this era have solder joint problems in various modules, right? Well tonight, I'll take apart my OVP relay and inspect it, even though the symptoms don't exactly fit.

Can anyone help me list all the modules that could make the engine just stall completely? I'll go through them one by one if I have to. Some of them I even have spares for.

Thanks,

-tp

Fuel pump relay.

PaulM

Second on the fuel pump relay. Our 420SEL would do this and I found a big cold solder joint on the relay.

My list:
-Fuel Pump Relay
-OVP (maybe?)
-26 Pole connector near fuse box.

When it does stall, see if you can hear the fuel pumps. Does it restart immediately? or is there some time that must pass before it will start?

Thanks for the input. Yes, it does appear that a 30 second wait before it will restart is the pattern. It's stalled a total of 4 times now, so it's hard to be much more detailed than that I'm afraid. I can't say that I've noticed one way or the other about the fuel pump running, but if it stalls again I will be sure to check that. Thanks again.

Hi all!

Looks like the FPR was in all probability the correct guess.

I've found two cracked solder joints on it, both mounting the actual relay to the PCB. It's kind of hard to see in the picture, but it's cracked all the way around both of those joints. If you press the relay from behind, the cracks open up all the way.

THANKS SO MUCH for suggesting what should have been obvious to me. Sometimes it's hard to see clearly when you're in the middle of the problem.

CHEERS!

(edit) the cracked solder joints in the photo weren't the cause - those joints are for mechanical mounting only, the traces go nowhere. However, I did find a couple more bad joints on traces that DO go somewhere, so it's probably still the cause.

A big thanks for the help!


Just made the trip to and from NYC, and not one single stall along the way! The problem is definitely fixed now! Thanks again for all who replied!


Cheers,
-tp

I had a similar problem with my 300CE. Came to a destination and turned the car off. Came back and it wouldn't start. First thing I checked was the fuel pump relay. Wiggled and tapped it and the car started right up. Having a spare relay and OVP in the glove box isn't a bad idea, especially if you travel long distances or far away from home.

Having studied the W126 circuitry I suggest as follows.........
 

The Fuel Pump Relay FPR can be removed and in the socket it plugged into just short pin 7 to pin 8. Use a sturdy cu lead here since >10 Amps.

Run engine as normal but when parking unplug the short to not drain battery.

The Over Voltage Protect Relay OVP can similarly be removed and in the socket it plugged into short pin 30 to pin 87. Again same advise re unplug short after trip.

Key off please when doing these tests.

Does anybody see a problem with this advice. All you need are two short 4" leads with male banana jacks on each end. Keep them in your glove box and put a copy of these instructions with them.

[For the more curious: Shorting the OVP does indeed remove the zener diode voltage protection but that is a secondary issue if you are doing some controlled testing as being suggested here. Please no battery charging or other testing beyond normal driving while in this state. I have tested this fully on my own car and proved that normal engine and all other ops were preserved in this state. This allows one to eliminate both the OVP and FPR from the list of possibilities. ]

I second on the pump relay thing