I had a problem with my '95 C280 wire harness (like everyone else) and replaced it yesterday. The car runs great but the check engine light came on again after 8 miles. I really doubt it came on for any specific purpose because I just installed the new harness. Unfortunately I don't have a scanner and don't have time (or patience) to take it to the shop. If I disconnect the battery for a couple of hours, will it reset shutting off the check engine light?

It's possible. It's also possible that some sensor connection was flaky with the old harness, so the computer noticed a change, thus the MIL. It's also possible that something is hooked up incorrectly with the new harness.

If you do disconnect the battery, the ECU will have to retrain. That usually takes fifty to several hundred miles (at least on other brands - not sure about this one).

One last thing - ensure that your gas cap is on tight.

Hi Matt L,

I had to disconnect the battery when putting in the new harness. I'm fairly sure that everygthing is hooked up correctly and I already checked the gas cap (thanks for the suggestion). Could it be that the ECU is in the process of retraining and I should just leave the car alone? Alternatively, is it worth disconnecting the battery once again and reconnecting after a couple of hours? Is it normal for the check engine light to come on after a new harness is installed?

Either you made a mistake while replacing the harness, or there is actually something wrong. There is no reason for the MIL to light other than a fault in one of the monitored circuits.

Since the MIL showed up after driving 8 miles would seem to indicate that it is not a hard fault, but rather an intermittent problem, or a problem in a system that is only tested by the PCM only under certain circumstances. If you disconnect the battery, you'll erase the code, and it may not return until the PCM once again tests that system.

I'd suggest finding out what code or codes have set before erasing them. That way, you'll know if the next MIL is the same system, or something different.

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1989 300 SEL that mostly works, but needs TLC

Unfortunately, Mercedes agrees that something is probably wrong. I admit that I am tempted to disconnect and then reconnect the battery and see what happens. My hunch is that the check engine light will come right back on. Worth a try or just bring it to the mech. and have the code read?

Check all of your vacuum lines. Especially the ones on top of the engine, under the black plastic cover.
Good Luck, Dan

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2012 E350 Bluetech