Ground Issue

[HB Chris]

Still chasing a ground issue in the coupe. When panel lights are turned up high (only position that works which is common) the white Tank light comes on. I checked ground in trunk and cleaned it. Turning light switch also can cause gas gauge to fluctuate. And then sometimes the window motors don’t want to work but then they start to work. I replaced several grounds under the dash as it had a few melted grounds when I got it. Frustrated!

 

[Thomas76]

I believe the light switch dimmer uses a potentiometer which takes unused voltage and turns it into heat, or ground.
1. All of your issues sound like ground issues.
2. Your dimmer isn't working

Do you have another light switch to try? It seems plausible that your dimmer may be doing strange things with voltage by putting it where it doesn't belong.

 

[HB Chris]

1. yes
2. yes

No other switch to try

 

[Arde]

The tank indicator light always receives +12V and only gets ground when the tank is low. If it comes on due to light switch rheostat operation it means it is getting another path to ground. You can disconnect the path to the tank to confirm the issue is local to the instrument and light switch. The other check is if it comes on solid or weak. If it is weak, then the best theory is that the light switch is shorting it to the rheostat voltage divider, but that also requires some short or conduction at the PCB/connector of the instruments between two separate circuits. If it is solid, then it is a short like Thomas suggested but still requires a leak at the PCB/connector.
In the voltage divider (not solid tank light) case the tank light would come on when the panel lights are the dimmest. Else the switch short theory seems better.
I still think there is a PCB/connector problem in both cases.

 

[Stevehose]

Not sure if this is related, but i had a random tank light issue from a faulty plastic light bulb insulator that mounts in the gauge hole where the bulb goes.

 

[Arde]

Not sure if this is related, but i had a random tank light issue from a faulty plastic light bulb insulator that mounts in the gauge hole where the bulb goes.

Good point. Given that these indicator bulbs always get +12V and the ground is switched, any insulator fault to ground and they light up. What Chris faces is correlated to rheostat position, suggesting the leak is not directly to ground but to another circuit.

 

[HB Chris]

I’m searching for a used light switch to rule that out as well, you make great points.

 

[HB Chris]

Not a ground issue! I reread the wiring diagram and the tank bulb has two wires and not one. I checked colors and I had the gas gauge illumination bulb in the Tank socket and vice versa, problem solved!

 

[Bmachine]

Isn't it an immense relief when a problem that had stumped you for days turns out to be super easy to fix? Especially when you are the one to figure it out!

Time for a well-deserved IPA

 

[Arde]

Not a ground issue! I reread the wiring diagram and the tank bulb has two wires and not one. I checked colors and I had the gas gauge illumination bulb in the Tank socket and vice versa, problem solved!

The tank bulb is dual filament?

 

[HB Chris]

No. It is next the illumination socket which is single wire.

 

[Arde]

Ah, the socket can be single wire (gets the ground from the metal around it) or dual wire (gets power and ground through wires). The bulb is always single filament. Isn't then the bulb the same bulb?

 

[HB Chris]

Single grey wire is illumination, I had this bulb in the tank socket so it made sense it illuminated. The tank bulb holder has two wires, power and ground from the sending unit.

 

[Arde]

Single grey wire is illumination, I had this bulb in the tank socket so it made sense it illuminated. The tank bulb holder has two wires, power and ground from the sending unit.

Got it, you replaced the wired bulbs. Makes sense.

 

[HB Chris]

Got it, you replaced the wired bulbs. Makes sense.

Yes, I swapped them into correct position, dumb error on my part in the first place.

 

[Arde]

Yes, I swapped them into correct position, dumb error on my part in the first place.

Not dumb at all. I once played a prank on my mentor. He had two terminals on his desk, one for a computer at our own site, the other to a mainframe in Arizona. I swapped their keyboards. When he typed on one keyboard the screen of the other was updated. He was in awe, unable to figure out how these disjoint computers could possibly be talking to each other. I had to tell him.