Jim Chambers

Problem solved. Thanks to my near neighbor dr. bob.
Went to Bob's fantastically equipped garage this morning. He went to work with the multi-meter and determined there was a direct short in the wire to the bulb. We removed the right front fender bulkhead to see nothing obviously amiss. More work with the tester demonstrated that the two leads which enter the connector to the light socket were switched, ground wire to bulb contact and hot wire to bulb socket. Swapped them at the connector and now we have light (blinking)!
Best guess is that the mistake was made when the bumper was removed when car was painted. And that was at least 25 years ago. Car came to me with blown fuse 32. Now we know why. Car has been driven for a very long time with no right front signal.
Stay away from the Buss fuses pictured above. The short did not blow the fuse, only heated it to the extreme.

dr bob

So the connector in the wheelhouse area under the headlight has two identical male terminals, ready to accept the two identical-except-for-wire-color female connectors from the car harness. It's readily possible to get the connections swapped, and that's what happened.

Troubleshooting was a lesson in isolating the problem in a systematic way. The front and rear turn signal circuits are common from the switch, then split to go through individual fuses front and rear. We pulled both fuses, then used the Ohms function in the DMM to find where there was continuity to ground from each fuse holder. We pulled the bulb for the front signal, tested from the fuseholder, and found very low resistance to ground. Next we pulled the "N" harness connector at the CE panel, and tested the wiring between the CE panel and the bulb again. Still very low resistance to ground for the front so then removed the right front wheel and the forward liner panel to look for the connectors. Everything looked OK, but the clamp for the harness was dangling, a sign that the wiring had been previously worked on. Pulled the two connections to the light socket, then tested again for continuity to ground with the DMM. Now none in the harness, one in one of the connections to the back of the socket. And one of the socket connections showed continuity to ground while the other didn't. So connected the grounded socket wire to the brown ground wire for the harness. Tested again for ground in the harness and found none. Verified ground OK at the socket. Put the bulb in, connected the other wire, then tested again with the meter from the CE panel end, so we could see the resistance of the bulb in the Ohm meter. Reconnected the battery, reconnected the CE panel and verified proper function before closing everything back up.

soontobered84

When you explain it that way, Dr. Bob, you make it sound sooo easy!!!

dr bob

It was.

Jim had removed the CE panel cover and the carpets, the turn signal lens, and had done some preliminary testing with a blow-proof fuse. I talked him into letting me do the testing with a meter instead of sniffing for burned insulation, not a tough sell at all.

The wiring diagrams for the '78 don't show wire colors, but fortunately the '79 diagrams for this section do and are the same. We spent a few minutes figuring out that the connector letters on the CE panel are not in any particular order, so I was in there with the flashlight and mirror trying to read the letter off the panel under the connector row.

After that, the Ohm meter is used to detect the continuity to ground in the section. We isolated each section and tested both of them. From measurements in the wheelwell with the wires disconnected, it was pretty quickly obvious what was causing the problem.

What is amazing to us is that the problem has persisted through two previous owner's, so 20+ years based on some other evidence. The fuses blew, so they just drove without a right front turn signal.

Mrmerlin

Nice work Dr Bob, I am glad there are smart 928 owners around that give freely of their time.
Thanks for taking the time to write up your troubleshooting procedure.