Weird electrical issue

[76k5blazerr]

I want to try and get this fixed today while I have some time. Recently after pulling and then re installing the engine in my 72 c10 I have a weird problem with my headlights. The wiring on this truck is not hacked up and I've never had any electrical problems before.

When I turn on my headlights now, the gas gauge needle moves to the left and pegs out past empty. Then if you turn the lights off, it will return to full. Also the left turn signal indicator stays on all the time, but not blinking. The other thing is all my lights work fine, even the signals. The problem is the gas gauge thing. I have not been driving the truck after dark because I am afraid of something getting hot. Is this overly cautious? Obviously there's a short somewhere, I hope it's not something that would heat up and melt. How would you guys go about finding the problem?

 

[rampage]

Shorts fry wires, but that's what fuses are for. It's probably a ground wire you forgot to reattach - they create strange electrical issues like yours. Maybe the one going to the core support?

 

[diesel4me]

I'd start poking around at the fuse box,engine side of the firewall..
The wires for the headlamps and gas gauge enter the cab there,and they may have gotten pinched and grounded or even melted together..

I assume some wiring must have been disturbed during the engine R&R...probably under the hood area more so than inside the cab..

Be sure all the ground leads other than the battery negative cable itself are all bolted up and clean..(is there a ground strap from the valve cover to firewall ?..there should be)...on a '72 vintage truck there is a wire with a weird rubber fuse holder that runs along the radiator support ,that feeds the amp meter if it has one,if those fuses blow it can affect the dash cluster..

The headlamps only have a circuit breaker make into the switch,and far as I know they have 12V all the time to the switch so they will light up without the key "on"..(and drain the battery!)...

The gas gauge wiring is not hot all the time,only when the key is "on"--sounds to me like your getting 12V back fed to the gas gauge somehow or its getting a ground by the headlamp wiring..the fact a filament in a bulb is "glowing" indicates a back feed through that lamp wiring..(sometimes a dual filament bulb has one filament break and touch the other,creating a weird back feed too,when that happens the bulb will glow like that..

Most likely if it had a dead short,the wire(s) involved would release their magic smoke almost instantly,or pop a fuse...sounds more like a back feed or ground issue to me..

 

[76k5blazerr]

I assume some wiring must have been disturbed during the engine R&R...probably under the hood area more so than inside the cab..

This is what I'm thinking. I've already cleaned up the headlight to core support ground connection to no avail. There's no ground strap from the firewall to motor. But I didn't have this problem before.

 

[GoGoGirl]

The gas gauge wiring is not hot all the time,only when the key is "on"--sounds to me like your getting 12V back fed to the gas gauge somehow or its getting a ground by the headlamp wiring..the fact a filament in a bulb is "glowing" indicates a back feed through that lamp wiring..(sometimes a dual filament bulb has one filament break and touch the other,creating a weird back feed too,when that happens the bulb will glow like that..

Amateur radio geek girl here...that's what I was thinking along the lines of as well.

Here's a thought...are the dash gauges all connected to a master mount board, of sorts, which in itself may be grounded? I've never been in the dash of my Blazer, but on some modern cars they are like that.

 

[76k5blazerr]

I think it's the headlight switch. Or something to do with it. With the lights on I can wiggle the headlight knob and the gas gauge will rise back up and everything acts normal. Then if you wiggle it the other way it goes wack again.

 

[diesel4me]

It is probably related to the wiring ,the switch does have the "dimmer" rehostat wired to the dash cluster lighting though,maybe the printed circuit on the dash cluster got some damage from a bad ground or back feed issue..
The rest of the headlamp wiring should be isolated from the dash cluster by the switch..it doesn't seem likely that an engine swap would do something to the switch itself,it would be quite a coincidence if the switch decided to screw up just at that time..

 

[Kain]

time to look at the wiring in the dash

 

[6872xtc]

I think it's the headlight switch. Or something to do with it. With the lights on I can wiggle the headlight knob and the gas gauge will rise back up and everything acts normal. Then if you wiggle it the other way it goes wack again.

You probably found the problem.
The headlight switch must be grounded. Maybe it isn't grounded well, or has another type of ground problem? I would also check the ground for the cluster.