Backup Light Circuit

Hey, I was trying to install a wireless backup light camera/monitor kit on my '67 108 the other week, and found that the voltage on the backup lights has quite a drop. That I upgraded the backup lights to about 70 or 80 watts load might have something to do with it. But there's a 10 amp fuse on that circuit so it should be able to handle even the upgraded bulbs.

With the more powerful backup lights installed, the voltage drops to about 8 or 9 volts.

So... I'm wondering if there is a current leak to ground somewhere in that backup light power circuit. As far as I can tell from the service manual, on this van the backup light switch is on the transmission. And the wiring harness for the backup lights (as well as the stop/signal/license lights) looks pretty small gauge.

I checked the resistivity of the circuit with the fuse removed, light bulbs disconected, and with the backup switch engaged (column shifter in reverse), there's a 140 ohm resistance to ground. There should be infinite resistance! So I'm thinking there must be a current leak to ground that is dropping the voltage.

I suppose I could run a separate power wire to the backup switch, and a new power wire from there to the backup lights. It would be a lot of cutting and splicing though, and sort of pointless if the source of the leak is in the switch itself. Pulling the switch itself looks a bit involved from the service manual, and of course it's gonna be a dirty greasy job. But do-able with enough time and patience. And I'm wondering if this backup switch is even available any more (Saginaw manual 3 speed tranny).

Anybody else experience this sort of problem? The backup camera won't work properly with the existing voltage drop. An alternative is to wire the camera and monitor separately with a toggle switch at the dash... but that sort of obviates the whole "camera automatically goes on when the vehicle is put into reverse" concept. And it does bother me that there may be that much of a current leak going on, so I figure I need to fix the backup circuit anyway.

I think the temp gauge/light might also get its power from the back up light circuit, so you can unhook it and see if the resistance goes up to a dead circuit,,,,(there must be like 4 connectors that the juice has to go through before it gets to the back up bulbs),,,,you can unhook the connector at the back up light switch and bypass it to see if you get more power at the back,,,,,,or,,,,you can grab some power off the starter solenoid, add a  headlight relay close to the transmission,   use the output wire from the back up switch to activate  the relay and send all the power you need down along the frame to run the camera.....  

Good idea with the relay.

What I might do is run a heavier gauge (fused, of course) wire all the way to the back, to run switched interior lights etc, and then put the backup relay there to run the camera. I could also use the rear mounted relay to run auxiliary backup lights like Spacetrucker did.

Although I don't understand why the temp gauge/oil idiot lights would be wired into the backup light circuit. That would mean they are only active when the car is in reverse...? Unless perhaps the gauge(s) gets its power from the circuit between the fuse panel and the backup light switch. I suppose I'll have to consult the wiring diagram.. wherever that might be in the service manual, if at all.

If you like using that meter, unplug the temp gauge wires and test for resistance again,,,,,,,,that would tell you if that gauge gets its power from the feed wire that goes to the back up light switch,,,,,that would account for you finding resistance in the back up light circuit when you think that nothing is connected,,,,,,,the temp gauge/light would be adding resistance to a supposedly dead circuit,,,,,,,but a relay sure is a better idea,,,,,,,,that back up light wiring looks like its stretched to its limit to me.....

Veefre if you go here you can print a real nice color wiring diagram. Lots easier to read than the black and white in the service manual.

https://vintage-vans.forumotion.com/t25631-second-gen-wiring-diagram

Thanks, Wideload. I've seen that jpg collection before but never bothered to cut and paste it into a big foldout diagram in color. Now that I have, I can see it's only slightly less incomprehensible than the B&W version ;-).

Be that as it may, it appears to me that the temp light switch and the temp switch are not on the same feeder circuit as the backup lights, at least not in the V8 with the std trans. In the L6 model or with auto tranny with a neutral safety switch there appears to be additional circuitry involving that switch. But as far as I can tell, this vehicle doesn't have a neutral safety switch. So it's still a mystery where that 140 ohms is coming from, as the diagram show nothing in the backup light circuit between the fuse box and the lights, except for the backup switch on the transmission.

This van however has had a long history, with a Clifford alarm system spliced into the wiring in somewhat mysterious (to me) ways. It's possible that the alarm system installer spliced into the backup light feed circuit to power part of the alarm system, as if the alarm system is part of a neutral safety switch. The alarm system works in a limited manner, with no ignition cutoff. All it's got is an alarm that sounds if the vehicle is hit hard enough (or a door slammed) or if motion is detected inside the cargo box. However something is drawing power even when the ignition is off, probably the radio clock/memory and the alarm system. This accounts for the spark I sometimes see when I change a fuse or connect up a wire to the fuse box. I keep a solar charger on the dash to help prevent battery drainage when the van is parked for days outside without driving. Otherwise the battery may go dead.