[white-hot]

Originally Posted by enderw88

When the CCM was failing was it giving false indication of failure, or did it actually fail to illuminate the bulbs? In other words did it say the lamp was failed, but the lamp was actually working?

This. I tried everything, new bulbs, cleaned the contacts and checked the harness time after time. Felt kinda dumb after it turned out to be the damn CCM after all. In any case, sounds like they are not the same as the E30 ones, as I recall no IC's in the ones I looked at, however if it is cheap try the swap and cross your fingers. If that ain't it then it will be lots of fun tracking down the short.

[enderw88]

Originally Posted by white-hot

This. I tried everything, new bulbs, cleaned the contacts and checked the harness time after time. Felt kinda dumb after it turned out to be the damn CCM after all. In any case, sounds like they are not the same as the E30 ones, as I recall no IC's in the ones I looked at, however if it is cheap try the swap and cross your fingers. If that ain't it then it will be lots of fun tracking down the short.

Yeah, it'll be fun. It is located directly above the splice panel that I haven't figured how to get out. German Engineering: place everything you need to get out behind something you can't remove...

[enderw88]

I finally got the CCM out and popped it open the CCM to take a peek. I really didn't expect to see anything obvious, but I was wrong, the failure was completely obvious:


The two shunts on the right are burned through. Once I got it out I measured resistance from the license plate light pins in the connector to ground and got 3 ohms, which seems right for cold light bulbs.

Not sure why these shunts burned, but the lines to the bulbs seem fine. I have a used part on order.

[John Firestone]

Which pair of shunts burned out: the taller RM8 and RM9 or the shorter RM9 and RM10? I might double check all the fuses supplying power to the CCM (e.g. F37) and confirm they are the right size. Someone may have substituted a larger one and burnt out the shunts (the post #8 scenario).

[enderw88]

RM7 and the one next to it burned.


I thought about this all night and did a little forensic research this morning before I left for work. I think this started as the dreaded trunk harness problem. There is evidence of repair of the license plate positive wires just below the deck lid seal. I think they shorted and either that burned out the CCM or a PO tried a larger fuze, before repairing the harness. In any event, I checked all of the fuze values and they are at spec.

[John Firestone]

Thanks, I see it now. I thought the grey might have been the lighting: it appeared similar to me on both shunts but isn't really.

[enderw88]

Originally Posted by John Firestone

Thanks, I see it now. I thought the grey might have been the lighting: it appeared similar to me on both shunts but isn't really.

It is hard to photograph, naked eye it really stands out.

[white-hot]

Glad to see you found the problem! Lets hope that is the last electrical issue you have!

[stefanom3]

I think I am having a similar problem. My trunk wire harness was totally messed up... PO repair turned out to be ridiculous. I crimped the wires again and now just my right license plate light works. I measured 1.8v at the left one (where is this low voltage coming from?). Could this be an issue with the check control ?

[enderw88]

Originally Posted by stefanom3

I think I am having a similar problem. My trunk wire harness was totally messed up... PO repair turned out to be ridiculous. I crimped the wires again and now just my right license plate light works. I measured 1.8v at the left one (where is this low voltage coming from?). Could this be an issue with the check control ?

From my observation of the CCM it seems to be binary, it works, or it doesn't on a specific function. More likely you have a high resistance connection upstream of your left bulb. Try cleaning all the contacts you can find in the circuit. Also try to get a resistance measurement between the hot side of the left bulb and where the harness splits up near the left tail light. you may have to remove some installation to do that.