ChamaRiver

My MB 500sel engine runs/idles ok at first, then starts to oscillate up and down. I connected a small 12v bulb to the hot wire on the Idle Speed Adjuster and to ground and the light stays on for a short while then starts dimming and brightening as the engine begins to idle fast and slow.

I have changed out 3-Electronic Idle Speed Control Units and 2-Relay Overvoltage Protectors, engine oscillates with one of the Idle Speed Control Units (bulb lights) and bulb does not light with the other two Idle Speed Control Units. Switching out the 2-Relays makes no differences.

When sitting in the car with engine running at idle (750rpm) , when the bulb goes out the engine speeds up the idle to about 1.2k-rpm and holds.

Another thing the car does is when driving is at around 40+ MPH to 50 MPH the engine speed keeps jumping up and down, and some times will actuall get to 60+ and slows down like it is not getting fire or gas. Then if I can get it pass jerking at around 60 it will all of a sudden take of smooth as silk. Any speeds when it is not jerking, it run perfect and accelerates ok. Once in a while if I jump on it from the start it will jerk up and down on the acceleration.

I checked the Idle Speed Adjuster and it is working ok, it seems like the power fluctuating is the problem, is there possibly something else effecting it? Engine temperature appears to also effect it when it warms up to about 90c.

Anybody have some advise on what to check out?

engine video; http://www.prestonimages.com/mb2.htm

Elektrische

My car has an electronic Idle Speed Control unit which is located under the carpet and a plastic cover where the front passenger would place his feet.

And things which influence that idle speed control on my car are...

Speedometer
Engine temperature
A/C Compressor
Throttle idle switch
Engine RPM
Starter Lockout

Perhaps your car is the same or similar?

Also they don't show the internals of these electronic gizmos, but I suppose it might increase engine idle if it detected the car's voltage was getting low?

Here is the factory service manual, there will be testing instructions for each of the above inputs somewhere in there...
http://www.w124performance.com/service/W126/Main.html

ChamaRiver

My 85 500sel (U.S.) is not laid out quit like yours, units are under hood. Thanks for the link though, I have a couple manuals, but that site has other information from manuals I do not have, so it will help in other areas.

ChamaRiver

May have found the problem, had to crimp down the connections on the Idle Speed Control Module plug due to loosing continuity to Idle Speed Control Adjuster. Will be driving the car into town today, so will know if this was it or not after test drive. Will update tonight.