Check Control Indication for Taillights (E30 cars)
Whenever the headlights were switched on in my wife's 84 325e, it always showed a license lamp warning on the check control panel. Replacing the old bulbs and even going so far as to replace the license lamp fixtures (they were all cracked and ugly anyway and they're cheap) did not extinguish the warning lamp.
Finally, not being able to stand it any more, I carefully read the Bentley manual and discovered the location of the rear lights check control relay (near the power antenna on the left wheel arch in the side of the trunk). A visual check of all the wiring showed no problems. Figuring that the relay may be the problem, I removed it from the car and opened it up. The aluminum "can" is crimped at the bottom and can be easily removed by using lineman's pliers to "uncrimp" the bottom edge. Then, the guts can be slid out of the can and inspected.
The cause of the problem was immediately apparent - corrosion! Evidently, at some point, a small amount of water had collected in the "top" corner of the can (the relay is actually installed "upside-down" i.e.. connectors-up) and corroded the PCB. There are two PCBs mounted with component sides facing each other and a couple of copper posts join them physically and electrically. One of the posts had come completely unsoldered where the corrosion was worst, while a couple of the other connections looked very suspicious.
10 minutes spent cleaning and resoldering the joints (I did them all while I was at it) and the relay was functional again. Yee haww!
I am not sure what this relay is worth, but I am sure it would not be cheap. Electrically and mechanically, there is no reason for it to ever wear out so I would consider this a 100% successful, satisfying repair.
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