‘72 911 - Troubleshooting Back-up Lights

[McOtto]

My back-up lights are not working. Before I start checking the circuit, can someone tell me the location of the multi-connector circled in the attached wiring diagram? Thank you!

[gled49]

That’s the 14 pin connector on relay panel, left rear of engine compartment, under plastic cover.

[McOtto]

Thank you gled49! Found both, one immediately in front of the relay panel and the second in the back.

[Scott A]

Most common issue is just wire falling out of switch on tranny.
If we are thinking of same problem.

[McOtto]

Most common issue is just wire falling out of switch on tranny.
If we are thinking of same problem.

That’s a pleasant thought Scott A. Only wish it were indeed the case thus a very simple fix.

I was able to get good tests on the switch and all of the wiring circuits that energize the reverse lights, except for the green-yellow feeder wire from the rear 14-pin connector terminal 5 to the switch and the gray wire from the switch back to the same 14-pin connector terminal 6. No continuity on either, which unfortunately implies both have a wiring fault somewhere.

Both wires are protectively cased together (and part of the engine wiring harness). Given the wires run together for their entire length through this thick protective rubber case, a fault in this circuit surprises me. Fuse 8 is not popping so the fault is not grounded.

Has anyone encountered something similar in this particular section of wiring? Am I missing something?

Thank you!

[edmayo]

Highly unlikely for the fault to be the actual wire, it most nearly always a problem at a connector on the end of a wire. Use of an ohm meter from each end of a circuit should isolate a bad connector. I would first check for 12 v. power at the input wire from the fuse box to the proper pin at the 14 pin connector. Or better yet using the voltmeter check for power on both sides of the appropriate fuse.

[McOtto]

Highly unlikely for the fault to be the actual wire, it most nearly always a problem at a connector on the end of a wire. Use of an ohm meter from each end of a circuit should isolate a bad connector. I would first check for 12 v. power at the input wire from the fuse box to the proper pin at the 14 pin connector. Or better yet using the voltmeter check for power on both sides of the appropriate fuse.

Connector(s), not the wiring, being the problem makes perfect sense. I too could not wrap my head around it being a wire fault given the wiring is so well protected. I disconnected and tested all of the circuit legs in isolation by connecting to the male or female connectors, not the actual wire. For the next round, which will be limited to the problem wiring to/from the switch, I will try to grab the wire, not the connector.

As you also appropriately mention, I did test from the fuse connection (top number 8) to the rear 14-pin connection (pin 5) and have 12V all the way down the green-yellow circuit. 12V power on both sides of fuse 8 feeding it as well. Additionally, the gray wiring from rear 14-pin connection (pin 6) is good to the light bulbs.

I very much appreciate the help!!!

[oldlefty]

If by chance you find that your wiring and connections are OK.... I recently had no backup lights on my 68 912. Replaced the switch and pin- still no lights. Removed the sealing washer from the switch and had lights again. I did put some teflon tape carefully on the threads.

[McOtto]

If by chance you find that your wiring and connections are OK.... I recently had no backup lights on my 68 912. Replaced the switch and pin- still no lights. Removed the sealing washer from the switch and had lights again. I did put some teflon tape carefully on the threads.

Interesting, oldlefty. I would not have otherwise considered that. I guess transmission fluid being a poor conductor of electricity, and enough of it leaking in the switch pin socket, could conceivably insulate the connection??? Thank you!