Short Circuit Diagnosis

Short Circuit Diagnosis

Just back from the shop (put the crossover in the exhaust) and ... no: backup lights, clock, interior lights, door lights, dash shift indicator, or hatch release. Fuse, right? Right. By my count, No. 24, counting left-to-right. Blows instantaneously upon replacement. Anybody got any thoughts on where to start looking? (All the work done in the shop was under the car, although they did drain the battery flat and recharge it. There was some welding.) The manual is damn opaque in this regard, near as I can tell - tons of info for someone looking for a problem other than mine. My thinking is that some portion of the ancient harness was bumped and that decrepit insulation failed as a result, leading to a short to the body in the circuit fused by good ole' No. 24. Can't seem to figure out where it is though. Maybe I'm not the first to have dorked this up .... thanx!

-Tim Westby
86.5 928S indischrot

1) Take one of the blown fuses, and solder 3 feet (approx 1 meter) of lamp cord (two conductor electrical wire) to the small metal test contacts on the face of the fuse, where the number is, with one conductor to each of the wires.

2) Solder the two conductors to a small 12 vdc light bulb, so that when 12 vdc is applied to the blades of the fuse, the bulb lights up.

3) Plug the modified fuse into the circuit containing the short circuit. As long as there is a short circuit, the bulb will light up, but the small bulb limits the current flowing through the circuit enough to prevent any damage. This allows you to start disconnecting things - when the light goes out, you have found the portion of the circuit containing the short. The long wire allows you to put the bulb atop the dash or out the door, so you can see it from where you are working.

Keep the tester - you'll need it again!

One word of advice - If you have worked on your car and shortly thereafter something doesn't work, go back to the area where you were working. You probably knocked something loose, misconnected something, etc. In this case, check the wiring for the oxygen sensor. There's not much else on the bottom in that area.

Wally Plumley
928 Specialists

Aha! Eureka! Bingo! It was the interior light (top, between the sun visors) wherein the plastic mounting tab for the hot lead had cracked, allowing the lead to contact the bare edge of the sheet metal cutout that the light fixture fits into. Thanks to y'all's responses, I saved hours of fooling around! And Wally Plumley's diagnostic definitely goes into my personal archives! My backup lights, interior lights, hatch, and clock are now working again (gotta replace that taped-off lightfixture though) and fuse #24 is happy. Y'all did so good I'm going to try again: How can I lose 10 pounds, get back my thinning hair, and gain 50 HP?

Tim Westby
86.5 928S indischrot (no shorts, silk boxers only)

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