I have a 1988 740 turbo, recently the tail lights on the right side stopped working. All other lights on that side work (brake, ts, rev) and the left side works fine. I checked for ground, checked good.I checked for voltage, almost ten volts. Checked the left side same measurements with bulbs working.

The bulbs were getting voltage but not lighting up, so i decided to check voltage with bulbs in. While monitoring the voltage on the running light (center, 1156 bulb) i plugged the running/brake light bulb socket in and the voltage dropped to zero immediately. For peace of mind I did the same on the left side and the voltage stayed.

Now I know wires corrode from the inside out, but I'm not quite sure this is the problem as all the wires in the brake circuit short of the printed circuit in the fixture are pretty much sheltered from the elements. Any suggestions?

Edit: I suspect a wire may be hanging on by a thread. I can't think of any other reason why a wire would provide voltage but fail under load besides corrosion, and even then the voltage would be on the low to very low side. I finally went to Fry's and got a Fox and hound to see if there may be any breaks I'm not seeing. Plus, a previous owner installed, although very poorly, an alarm on this car which I disabled. Perhaps I can get a schematic for this car that is more notable and legible than a shotty haynes manual? This would help tremendously. Thanks in advance.

 

Replace the board in te tail light. Look to see if you are getting water in the tail light. If so replace the whole thing. Try to find one used....new they run from 100-180.

 

It's about time I replaced that assembly anyways, it has holes that are taped up and the tape is becoming brittle. I used epoxy to re-fasten two of the standoffs that broke off. I was planning on replacing both at the same time if I couldn't repair the existing ones. I found the existing ones to be far beyond reasonable repair, but I did a few tests on the printed circuits and believe it or not they hold up quite well. It seems everyone blames these innocent little printed circuit boards for all their brake woes. It must be for a reason. So I'll repost with results at about the beginning of next year when I finally get the new RTI tail light assy's in the mail :P

 

I did some troubleshooting. I started with the ole' fox and hound to trace the wires and look for obtrusions such as screws penetrating the jacket etc.., traced it all the way to the bulb failure indicator relay. I used the multimeter again on the printed circuit, still reads 11v, 0v when bulb is plugged in. I found a 12v bus bar straight off the battery, conjured up a test lead with a blade on one side and bullet connector on the other, slipped it in the socket where the relay plugs in to send voltage to the right tail light assembly and there was joy. The lights work, the relay is bad. Now I run into the problem of finding the relay. There's one for $75.66 plus shipping at thevolvosite.com, and there's one on ebay for $15, but my account is suspended because ebay gave me the shaft and I gave it right back to them :P So, if anyone knows where I can find a working 1362370 relay I would be much obliged. In the meantime, I took the liberty of removing a very poorly installed alarm system and will take off all the zip ties and aircraft-style string tie the wiring harnesses. Gotta get rid of that rat's nest.