Hopefully someone can help me diagnose a problem. This morning when I started up to go to work, I noticed very weird and delayed throttle response from my 1992 500E. I had to push the accelerator down much farther than usual, maybe 40-50% of the way, to get any response. I looked down and saw the tach was inoperative. The car was shifting very strangely, perhaps a limp-home mode? And as I got on the freeway, I found the cruise control was also inoperative. The speedometer and other gauges all are working fine. No ASR light, no check engine light, no warning lights of any kind! All fuses in the fuse box (in front of the brake master cyl.) appear fine. Is there a different fuse box anywhere? The owners manual makes no mention. Idle speed seems a tad high, it is mostly steady but will drop and then rev back up every 30-45 sec. it seems. Without a tach it is hard to say what it is actually idling at.
I had removed the air cleaner assembly last night to replace the temp sensor for the aux. fans, but it drove fine for the 30 miles (and several restarts) I drove afterwards last night. I wondered if I could have disturbed something under there, but don't know why it would have been a delayed reaction before the problems started.
Does anyone have any ideas? I do not know where to begin looking. I would much rather try and diagnose this myself if possible, rather than taking it to our local dealer, who sucks...
Oh, I also noticed the A/C was not engaging when I drove it to lunch (has worked perfectly until now).... and NOW the ASR light is on on the dash!!! Jeez...
I have heard this before. Sounds like you have engien wiring harness problem including the wiring to the throttle actuator. I have a 1993 300E and I am having the same problem. I already rewired my engine wire harness and i still have to fix wiring from my throttle actuator.
I'm thinking you may be experiencing the dread limp home mode. Try pulling any/all codes if you can. Try a hard, power-off reset afterward. The ASR or other critical system detects a failure and shuts down the car. The ASR and Check Engine lamps usually come on and remain steady when this occurs.
The effect immediately after the sensor service is good. Re-check all work, wires connectors etc. Maybe disable or swap the old sensor back in? You always ask - what was the last thing I did?
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Slit the cover on your wiring harness some where easily accessable and close to the engine, I checked mine about 3" behind the temp sensor on the thermostat housing. Look at the condition of the insulation on on the wires, if it is cracked, there is a high probability that your problem is the wiring harness as Raul21263 suggests. It is my understanding the most MB techs will not trouble shoot a car if the wiring harness has not been replaced. Depending on when any replacement was done, there is the possibility that the new harness would be covered with the same low quality insulation. My car had the harness replaced back in '98 (a major contributor to my buying it) and now I am dealing with the second harness failure for this car.
This is an expensive part ~$1000.00 US, but it is very easy, although time consuming to repair. If you can solder and heat shrink tubing, you can fix it. Take your time and replace one wire at a time Search this forum for details. The important part in repairing these is to replace ALL of the wires with No exceptions. Replacing the control modules that might get fried by a short that you did not bother to repair make the wiring harness look like pocket change
The speedometer and other gauges all are working fine. No ASR light, no check engine light, no warning lights of any kind!
If your upper wire harness is still in good condition, then most likely it’s the Throttle Body Actuator that needs to be replaced - new unit goes for around $1,200.
Btw, the frequent limp-home mode does not damage the engine what’s so ever, just a pain in the arse to deal with the restarts. If it is your wire harness as the culprit, by now the limp-mode would be just one of your “new” issues to deal with and you would be ware of "them" for sure by now. Any good Merc indy shop can do the job, good luck.
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