Misdiagnosed '94 SL320 - PeachParts Mercedes-Benz Forum

johneurostar · #1 11-23-2010, 02:30 PM

Hello everyone. I'm in the process of sorting out a w129 SL with the M104 engine and 85K. It was my late father's car who passed away recently.

The car had been sitting for some time and I found it in his garage with a dead battery, not surprisingly. After an overnight charge it started without issue, but after several minutes of running it developed an erratic/rough idle while in park. To my surprise it began to surge quite profoundly upon engaging drive and the car even began lurching forward as the engine's revs went up and down, up and down. When placed back in park the car resumed it's rough idle. Additionally, the car is running very, very rich. Extremely rich. An overload protection relay came to mind until it was quickly realized there was none.

The fuses in the base module were found intact, but it was revealed during close inspection that the engine wiring harness was deteriorating so it's been replaced. In the past that always cured weird problems, but this time my luck finally ran out; the problem still exists

So the memory bank began churning back to the days when my father and I worked together ago in our family-owned MB repair shop. However the years gone by have required me to engage in some serious memory recollection.

Eventually previous cases came to mind. There were many weird problems that were encountered with M104 electronics; namely W124 chassis cars. Most of the challenging cases, usually with non-descript scanner codes, were directly attributable to faulty harnesses or even electronic accelerators.

The codes I was able to retrieve were acquired using an Assenmacher tester with the 38pin "mushroom" plug. The codes are "12 fuse F1 or control module" as well as "Lambda 35 system operating at rich limit mixture too lean" error codes.

Last night my friend gave me several air flow sensors to plug in as testers without effect. In the old days I always kept several electronic accelerators which were used as test units to diagnose problem cars, but that luxury is gone.

My gut tells me to change the O2 sensors but before more parts get thrown into the vehicle, perhaps something has been overlooked?

Any ideas would be greatly appreciated!

mbdoc · #2 11-29-2010, 09:40 AM

Sounds like an air mass failure for sure.

olpos · #3 11-29-2010, 04:53 PM

Try just unplugging the 02 and seeing if the car runs better.Have you driven the car on a good ride yet,this may help.Sounds like the E.A. unit but that would be a last item because of cost
Rich

Gilly · #4 11-29-2010, 05:02 PM

Not sure about the rich mixture, but MAF sensor makes sense. I think this is a HFM car, if it is LH then maybe you are plugging in the wrong type of sensor (unless the connectors are different, not sure on that either)
The surging could be that the throttle motor needs to be adapted (relearned, initiallized, what ever you want to call it).
Sometimes simply turning the ignition to the run position and waiting a few minutes will let it set itself, other times/cars need to have it done with the shop computer.
Gilly

tinypanzer · #5 11-30-2010, 12:22 AM

Might want to try measuring system voltage with the thing idling and in park, and the E-brake on, and you're standing to the SIDE.

Perhaps the battery is so weak that it can't operate the electronics correctly. Dead batteries can cause no end of havoc, and the notion that just because it took a charge and started the car must mean that the battery is good is false. Test it to be sure.

jcyuhn · #6 11-30-2010, 11:06 AM

Agree, sounds like an air mass meter failure - but you generall get a code 4 with that as well. I had similar symptoms on a 124 car that had a dead AMM and aged O2 sensor. Replacing the AMM greatly improved the idle and driveability, but there was still a surge. Replacing the O2 completely cured the idle. The car had about 120K miles on it at the time.

Code 35 and lousy running also sound like a vacuum leak to me.

How old is the gasoline in the tank? I'd probably go for a fresh fill, some fuel system cleaner, and a long drive before making too many judgements on the running qualities of the engine.

johneurostar · #7 12-05-2010, 04:09 PM

Quote:

Originally Posted by olpos

Try just unplugging the 02 and seeing if the car runs better.Have you driven the car on a good ride yet,this may help.Sounds like the E.A. unit but that would be a last item because of cost
Rich

I know I'm carpet bombing this thread with posts but the car is right next to me on the lift running. Just unplugged both O2 sensors with only a minor change if any. Mileage within the last year has been very low; maybe about 30miles but stored with a full tank.

Once again thanks for the ideas.