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diesellibrarian Samba Member
Joined: November 06, 2013 Posts: 55 Location: Lethbridge, Alberta
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Posted: Wed Jan 24, 2018 9:27 pm Post subject: Back-up lights, 1970 auto Fastback |
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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 ) _________________ Robin
'70 Fastback
'80 Westfalia |
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diesellibrarian Samba Member
Joined: November 06, 2013 Posts: 55 Location: Lethbridge, Alberta
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Posted: Wed Jan 24, 2018 10:11 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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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. _________________ Robin
'70 Fastback
'80 Westfalia |
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Peters Van Samba Member
Joined: March 27, 2014 Posts: 357 Location: Goulburn NSW Australia
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Posted: Wed Jan 24, 2018 10:20 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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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. _________________ 1965 Type 3 Panel Van
1963 EH Holden Station Wagon |
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diesellibrarian Samba Member
Joined: November 06, 2013 Posts: 55 Location: Lethbridge, Alberta
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Posted: Wed Jan 24, 2018 10:50 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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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! _________________ Robin
'70 Fastback
'80 Westfalia |
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rwf8ball Samba Member
Joined: April 30, 2004 Posts: 234
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Posted: Thu Jan 25, 2018 8:01 am Post subject: Re: Back-up lights, 1970 auto Fastback |
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You should have something that looks like this inside your shifter if I'm not mistaken.
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diesellibrarian Samba Member
Joined: November 06, 2013 Posts: 55 Location: Lethbridge, Alberta
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Posted: Thu Jan 25, 2018 1:09 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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Yup, and it appears to be in good condition. Cleaned and greased all contacts. Going to try the test previously mentioned tonight. Thanks! _________________ Robin
'70 Fastback
'80 Westfalia |
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KTPhil Samba Member
Joined: April 06, 2006 Posts: 34018 Location: Conejo Valley, CA
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Posted: Thu Jan 25, 2018 1:57 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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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. |
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diesellibrarian Samba Member
Joined: November 06, 2013 Posts: 55 Location: Lethbridge, Alberta
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Posted: Thu Jan 25, 2018 4:18 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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KTPhil wrote: |
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. |
I did see a fuse holder coming off the coil, but it seems to lead to the kick-down switch. According to Bentley, the power for the back-up lights comes off the positive side of the coil, and up to the main fuse box, then to the switch.
Someone else has indicated that the behaviour I'm seeing might also be caused by resistance on the supply side, ahead of the switch, so I'll also check resistance between coil and fuse/fuse and switch.
Really appreciate the help. It's such a simple circuit, and it's making me crazy that I haven't been able to get this figured out. _________________ Robin
'70 Fastback
'80 Westfalia |
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Donnie strickland Samba Member
Joined: December 21, 2009 Posts: 2403 Location: Moody, AL
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Posted: Thu Jan 25, 2018 5:16 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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KTPhil wrote: |
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. |
No. The fuse for the backup lights on an automatic car is in the fusebox. On my 71, it's fuse 12. _________________ 71 Elm Green FI A/T Squareback |
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KTPhil Samba Member
Joined: April 06, 2006 Posts: 34018 Location: Conejo Valley, CA
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Posted: Thu Jan 25, 2018 7:31 pm Post subject: Re: Back-up lights, 1970 auto Fastback |
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Oh, I think mine was hacked then... two fuse holders back there! Car has been gone 20 years, so I can't check now. |
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