Electrical SNAFU ????

[Busted Knuckles]

Here's one for any electrical techs on here .....
My 96' Tahoe has been a PITA . One thing or another has kept me working on a "Hanger Queen" .
Now the damn thing has got an eletrical problem I've never seen in 35 years of wrenching .

This only happens at night . No lights etc. on and only happens when I put it in reverse , the voltage gage drops down to 9-9.5 volts as soon as I slide it into drive right up to 13+ volts . I turn on the lights , heater blower , wipers etc. basically everything and it drops to just over 12 volts .
I've checked grounds , cleaned the battery posts etc. It hasn't been real cold no rain ,couple days of mid 20's .
Could the switch down on the selector shaft be going bad ?
I did install a "Painless" dual headlight relay on it a couple weeks ago . But like I said it's only at night and only in reverse . WTF , over ?!

 

[99Yuk]

well here is a pic of the reverse cicuit wiring diagram.


Let's put the bits together.
Only in reverse, ok. - Throws out a bunch of causes.
Only at night, ok. - Throws out a bunch more causes, and narrows it down further.
The rest of it seems normal.
The gear selector doesn't have the reverse switch there. The actual reverse switch is in the tranny? So odds are it's not the gear selector switch.

So what would affect the voltage in reverse at night? Well, at night your running parking lamps, so now we have to look for what happens with the park lamps and reverse, and how are they connected to each other?
Technically, nothing. Those are different circuits, isolated from each other. Not even a bulb is common to them. But what is common to them is the tail light housings and grounding.
So, does your taillight housings use circuit boards? Can you unplug and take off one housing at a time to see if the problem goes away?

I dunno, but I'd start there, it's the easiest thing to try first.


EDIT: You didn't mention it, but a trailer plug would definetly be common to both and those 4pin flat plugs are bad for corrosion. Maybe try disconnecting the trailer harness first. Usually the trailer harness can be unpluged from where it's spliced into the factory wiring, unless it was installed by a butcher. Mine plugs into the factory wiring behind the rear drivers wheel, just before the bumper along the frame rail.

 

[Busted Knuckles]

Electrical Snafu

Geek ,
Thanks for the schematic . Great Stuff !
Yep, I have a flat plug and getting ready to ****-Can it and install a 7 Pin now that I have a trailer brake controller .
I scratched my ass raw trying to wrap my bald head around this and figured some of you younger techs may have run into something like this or be lucky enough to afford "All-Data" .
I can t/shoot/repair classic cars electrical all day (usually they need it , as some "hacking" has been done by someone with a plastic knife and a mentality to match) but these new vehicles with 50 miles of wires,relays, etc. tend to piss me off more often than not . I'm a bit torn up and can't get into the contortionist positions that I used to do .

Hey how are you guys fending up in the Great White North , seeing winter yet ? Sick of the warm weather down here in "Missery" . Gimme snow !!!!!!!!
It's still 70*F + and humid .

Thanks again ,
Steve
"The Desert Rat"

 

[99Yuk]

No worries, glad I could help. I know jack sh*t about mechanical stuff as evidenced by my brake pad thickness posts! But I do know electricity.

A couple of mornings of white stuff on the ground so far, but by the afternoon it's gone again. My boys are waiting for a good snow fall. They want to try out their new to them 1970's Yellow Bombadier Olympiques. I bought four of em last spring when the snow melted, got three running good, the other needs a motor swap. The boys just turned 10 this year, so I figured a little single cyl 250 they could handle.