As I had posted before, I have a hard start problem on my 85 190E, and looking through the archives saw that the overvoltage relay fuse is a common failure problem. So, I checked mine and sure enough it was blown. Easy fix! Or so I thought. When I tried to start it with a new fuse, it would not start at all unless I held the pedal to the floor, and then it just barely held an idle, like it was running on 2 cylinders. Hmmm. So back to the archives, and see it could be a possible fuel pump relay problem and then proceded to jumped the relay. No difference, relay works fine. Jumped the OVR, again no
change. I used circuits #30 and #87 for the jump. So, I pulled the fuse out and it starts right up, runs like it did before. So, how can the car not start WITH the fuse in, and start without it? Exactly what is the fuse activating to cause this? Any thoughts are welcome!

Steve

I can't give you a whole ton of info on this, here's what I know, hopefully SteveB or Benzmac can help more:

On your 85, probably about all the OVP is being used to power up is the engine control module. If the fuse is blown (or missing) the engine runs without the engine control module in sort of a "limp home" mode called Fixed Operating Mode, or "FOM". There is no Lambda control, it uses the same mixture setting regardless of operating conditions, like engine coolant temp or ambient air temp or the like, I don't even think it uses the airflow meter in FOM. So you're not really getting an ideal mixture like this, so poor performance, fuel mileage, poor starting (especially when cold). I am unsure why it fails to start or run when the control module is powered up. My initial reaction would be possibly the air flow plate (base mixture setting) is way way off, either rich or lean, but in FOM it will at least run, if not the greatest.

Gilly

That's pretty much what I was thinking. When I connect the fuse, all available sensor information is then sent to the ECU like it's supposed to, but one of the sensor's is relaying a really bad signal to the ECU and the computer is just acting accordingly. So, I need to find what component is defective or not working properly and fix it. Unfortunately, that's what I was afraid of, which means a I am going to have to make a trip to the dealer....

Steve

I was thinking more along the lines of this:
Fuse in OVP blew for whatever reason (such as someone tried to jump it and it spiked the fuse, or someone momentarily crossed the jumper cables (wrong way), so for whatever reason the fuse blew. Technician came along, owner complains of driveabilty problem "ever since car was jump started", and tech, not thinking of actually testing something, starts screwing with the air flow sensor plate/mixture adjustment, which of course didn't fix anything since it's in FOM. Tech gives up on fixing it, now if you actually DO fix the car (by replacing the blown fuse) it runs so bad it won't even start because the mixture adjustment is wrong.
If you know where this adjustment is made, just out of curiousity is the little ball bearing gone that plugs off the top of the adjustment tower? That would sort of verify what I'm saying may have happened, as the tech probably left it all unsealed.

Gilly

I think you probably hit it on the head. Can you give me an idea exactly where the adjustment tower is? Is it next to the airflow sensor? I am new to this engine and am used to working on Audi's(much worse) and don't have a shop manual yet. Thanks again.

Steve

Yes, by the air flow sensor, it's a sort of an aluminum finger that sticks up, it lines up with that hole in the air cleaner, actually it's the reason for the hole in the air cleaner.
Look on top of that, can you see a hex shape in the bottom of the hollow part or is there like a steel BB shoved in the top of it?
The BB is there as an anti-tamper blocking thingie, if it's missing it may be a fair bet that someone screwed with that adjuster screw and that's why it's not starting other than in FOM, the mixture is either too rich or too lean.
To do a rough adjustment to get it to at least run, look at the top of the airflow sensor plate, and also the bore of the air intake around this plate. You'll notice the bore has a taper to it, then it reaches a point where it's not tapered anymore, just like a very thin edge. The top of the airflow sensor plate should be right where that thin edge is, +- about 1mm. If it's way high or way below that edge, try adjsuting that screw in there to achieve that position and try it again with the fuse. Then you'll have to check the on/off ratio using a volt/ohm meter with duty cycle feature. At that point I'd need to consult with one of the other guys who've done this recently to make sure you set it correctly, the settings vary a little from year to year and I'm not certain I'd tell you the correct setting. (PS If you mess this (rough adjustment)up, don't worry, if you pull the fuse it'll still run)

Gilly

Hey, super information, thanks for all your help. I am going to dig into it tomorrow provided it's not raining here in New Joisey and check for the BB on the adjuster screw and then the sensor plate measurement. If the sensor plate height is off, it should be pretty obvious, as the air/fuel mix has got to be pretty far off.

Steve