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I'm having trouble with the reverse lights after a 5sp conversion. It's been a couple years since the last one but I hooked it up the way I usually do and was getting no love. I assumed it was something with the PRNDL multi-connector that I'd overlooked but it appears that the wiring is connected properly. When I cross this plug the reverse lights light, so I've obviously got the correct circuit.



When in neutral I get an open circuit off the reverse light switch



When in reverse I get 22 ohms of resistance from the switch. In any other scale it reports a dead short, so I assumed it should be lighting the lights. Is 22 ohms enough resistance to not pass the voltage? Apparently?? Of course I did this test PRIOR to installing the gearbox in a higher scale.

You may want to try another switch, but you already knew that.

Nothing wrong with a new switch but you can try some experimenting first if you like. Brand new moderate current switches measure .01 to .025 ohms (after subtracting test lead resistance) but ancient but barely acceptable ones might measure .1 to .25 ohms. 22 ohms suggests some cleaning in place with two different methods could be tried:

manually with ignition key in off position, actuate into reverse 25 to 50 times with the shifter, very quickly is fine, see if the subsequently remeasured ohms drops from 22 down into single digit ohms or better;

electrically, rig up a 12 V small motor motor load like a 12V/20W auto powered dustbuster vacuum to run by the reverse switch. Want a motor that draws less than 4A when up to speed, temporarily substituting for the backup lights 3.5 amp normal load. Leave the vacuum switch continuously on and cycle the reverse switch manually with the shifter about 10 times. If you don't have a dustbuster the window washer motor might be a substitute. Many of the other electric motors are just too much of a load for the reverse switch.

Electrical cleaning occurs when current is interrupted going to off position, and is better as current being drawn increases so if vacuum cleaner or other small motor starts to rotate continue switching and see if you can get it to eventually run up to normal speed. What is desired is an inductive kick voltage spike at turn off from the motor inductance burning thru silver oxide plus sulfides that has filmed over the contacts from lack of use. If the contacts can be gotten down into the 1 ohm or less region the normal tungsten lamp surge current of the backup lights in use will probably further clean (and maintain) the contacts down into and hopefully below the .1 to .25 ohms range. If you jury rig a small motor load don't try to hold any bare wires together with your fingers or you might feel a bit of electrical shock trying to clean your contacts too.

Wally, that sir, is a hell of a write up. And kind of my style actually. Unfortunately before I read it I wandered over to Zygmunt's at lunch and grabbed a new switch. This fixed my problem. You were correct and I learned something about electronics. 22 ohms is too much, and always test in the proper scale. Luckily the car was still on jackstands from the 5sp conversion and the switch was a breeze to swap out.

That was a nice write-up!

FYI for the future.
Ohms Law says V=I x R (V voltage = I current x R resistance).
Voltage drop across the switch (V) is current (3.5A per Wally's write-up) x switch resistance (22 ohms).
That would be 77 Volts dropped across the switch. Since you only have 12 to start with, no voltage to the bulbs. It's all lost in the switch. Hell, even 1 amp would drop all of the voltage across the switch with that amount of resistance.

But now you know that already!