On my 1980 GMC K15 manual 4-speed, I have the reverse switch out the left side of the trans, near the top/front. There are a pair of wires coming out, one green, one blue. Here they are from ST_352_80_1980_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf:
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They are circuit 24 (light green, Back Up Lamp Feed), and circuit 75 (blue, Feed, Ign. Sw. "On and Crank" Controlled - Fused). They proceed to the firewall and enter the cab. In my case I can trace them to a spot on the steering column, and then they seem to disappear into the harness that comes from near the steering wheel down to the inside cab firewall.
On the next page ST_352_80_1980_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf shows circuit 24 entering the cab (with 75), but then it splits off and exists the cab at the rear light system harness. It is one of 4 wires at that location (24 light green, 9 brown, 18 yellow, 19 dark green):
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That same page shows circuit 75 proceeding on it's own inside the cab and terminating at the fuse block at the "FUSE - TURN, BACK UP LAMPS":
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Can you please help me with a few questions?
1) When trans is in reverse, are reverse lights on only when ignition switch is in ON position or engine is running?
2) Does circuit 75 terminate in the back of the fuse block? Earlier in this thread
@foamypirate showed a picture of the green/blue wires with a connector at the end. I don't know what vehicle that's from so I don't know if I should have the same connector on my truck. In his case the wires come out of the column... in my case the wires go into it, under wrap, and then I can't track them anymore.
3) If it doesn't terminate in the back of the fuse block, then where?
4) If it does terminate in the back of the fuse block, how can I use a multimeter to test if the trans switch and the wiring is good up to that point? Can I do that test without removing the fuse block?
I ask because I'm trying to diagnose non-responsive backup lights. I know there could be other problems as well, but right now I'm trying to diagnose only that part of the system described above.
Thanks,
Mark