Back-up lights, 1970 auto Fastback

[diesellibrarian]

Hello, I have a question about reverse lights, but really it’s an electrical noob question. Here’s the scene: back-up lights don’t work on my ‘70 Fastback (auto trans). I have continuity across the switch (when closed) and in every section of wire between the switch and the lamps. With key on, I have 11.5v on the hot side of the switch when in park. When I move the shift lever to R, closing the circuit, voltage drops to zero on both sides of the switch. Does this indicate a ground fault somewhere? Or bad/no ground? Thanks in advance.

(And yes I’ve done lots of searches )
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Robin

'70 Fastback
'80 Westfalia

[diesellibrarian]

Voltmeter is also showing 0v at the lamp socket. I’ve cleaned the grounds and added dielectric grease to the contacts. It just doesn’t make sense to me: as soon as I put the shifter in R, the whole system goes to zero volts.
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Robin

'70 Fastback
'80 Westfalia

[Peters Van]

if you have 12v at hot side of switch at normal then the 1st thing to do is disconnect the wire going away to the lights.
now put your meter across the vacant terminal and put the car into reverse.
you should now have 12v on the terminal that was connected to the light wire.
if you do then its not the switch so you have a wire fault or a light socket fault.
if no 12v then you have a faulty switch.
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1965 Type 3 Panel Van
1963 EH Holden Station Wagon

[diesellibrarian]

Thanks for the reply. I didn't do that test, but I did test for continuity across the closed switch (i.e. shifter in Reverse, both leads disconnected) and there was continuity. But maybe the connection is bad and creating too much resistance. Anyway, I will try your suggestion. Thanks!
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Robin

'70 Fastback
'80 Westfalia

[rwf8ball]

You should have something that looks like this inside your shifter if I'm not mistaken.

[diesellibrarian]

Yup, and it appears to be in good condition. Cleaned and greased all contacts. Going to try the test previously mentioned tonight. Thanks!
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Robin

'70 Fastback
'80 Westfalia

[KTPhil]

Yes, unplug the reverse lights from that shifter switch. It appears you have a short downstream from there, causing your no-voltage measurement when put into reverse.

The fuse for the reverse lights is in the engine bay (of course it is!), so check that fuseholder and the wires to and from it for shorts.

[diesellibrarian]

[Donnie strickland]

[KTPhil]

Oh, I think mine was hacked then... two fuse holders back there! Car has been gone 20 years, so I can't check now.