Check engine light - on after key out!
 

This is for my W124, 300E - check engine light has always been on, with engine running. I'm guessing it is the O2 sensor, saving that project for another day.

However, the last few fays, the light STAYS ON even after I shut off the engine and TAKE THE KEY OUT:eek::eek::eek:

I replaced the overload protection relay in October of last year. Could it be defective already?

Or is the car possessed by some menehunes playing tricks on me?

Same thing happened to my '94 when the engine wiring harness failed.

I'm not driving it now because I'm afraid of getting stranded somewhere away from home. (light stays on, battery drains...possibly?)

Would it cause problems if I drive it while the part is getting ordered?

Could damage the engine computer , start a fire ( even with engine off ) damage to sensors. The bio degradable wiring issue extends to under dash and the rest of the car, under hood fails first due to heat.

Until the new part arrives, I'm gonna disconnect the battery to prevent further damage.

But I can't order the part yet because there is no number on the wiring harness.

if you don't have a alternator light,maybe voltage regulator.

Do you mean a light on the dash?
There is one that looks like a battery - does that represent the alternator?

Replaced the EUC, but the check engine light still stays on, even after the key is out.
The code says 1, which I understand to mean no faults.

Am I not clearing the reader correctly?

200K on the clock, the first thing I would do is check the electrical portion of the ignition switch, is there is a problem with that it could easily manifest itself as "electrical ghosting" like this.

And that could be pretty quickly and easily diagnosed or eliminated by swapping some other electrical switch into place of the original, even and unknown used switch is highly unlikely to have the exact same specific flaw so any change from the present state of that light illuminated would confirm the original switch as the source of the problem.

So BatteredBenz, is it as "simple" as changing out the existing ignition switch?

Like this ?:
Pelican Mercedes-Benz Tech Article - Replacing your Ignition Switch and Lock on your Mercedes Benz - 190E, W124, W126, W201, W123

Good idea, I will try this before the wiring harness.

I'm heading out and therefore haven't checked out that tech link, but essentially after removing the under dash panel you gain access to the rear of the entire ignition/lock assembly. From there the key is turned to Position 1, doing that turns the "key" at the rearmost part of the assembly, that allows the wiring harness plug to be disconnected from the electrical switch, three screws attach the electrical switch to the entire assembly, remove them and swap a replacement into position, one screw to hold it in place, reinstall the wiring connector and see what happens.

A quick way to test the Ig SW theory is just to unplug it.

Great point! That indeed would most easily isolate the switch from the equation, ruling it in or out as a potential root cause of the weirdness.

Quick update, had been rainy and today a nice day to be outside to follow up:


Connected a light pen from to neg cable to neg post to observe when light would go out when fuses were taken out.

Discovered the removal of 9 would make the light go out, but only when C and 9 we both out at the same time.

If 9 was taken out, but C was left in, the light would stay on.

EDIT: 9 controls the diagnostic module, among many other functions.
Noticed the transmission selector lever position light is not working!


What does this mean?

The transmission selector lever position light was fixed with a change of the bulb.