Dash cluster problems

[Beekeeper]

Was helping a friend today with his 75 Dart sport and we ran into something odd. I know it's not an E body but you folks always seem to have good advice for any Mopar.

The fuel and temp gauges were not registering but they did a few months back. I started with the fuel gauge today but checking the wiring and ground. Seemed okay. I then removed the wire to the sending unit and grounded it out. My understanding is this should peg the fuel gauge since you are bypassing the resister in the sending unit. What we noticed is the needle moved erratically up and down. At the same time, the brake warning light would dim and flicker on and off. Normally the brake light would turn on with the key on and go off once the engine was started.

I am thinking the circuit board in the gauge cluster is messed up. Partly because two gauges stopped working at the same time and also because I can't think of a reason why the brake light circuit would be affected by bypassing the fuel sending unit.

Am I on the right path before we rip apart his dash gauges? All advice is appreciated.

[Chryco Psycho]

Interesting, the regulator emits a pulsing voltage so ti would seem the regulator is working causing the erratic movement , it would also seem the gauge is working ok & feeding voltage to the tank connector or the gauge would not have moved at all .  The way it should work is 5v pulse is fed to the gauge first , goes to the sender & grounds through the sender completing the circuit , so if you have power all the way to the sender then that should be where the problem is .
 The brake circuit is another mystery altogether , power is fed to the light at all times but ground either at the park brake pedal when pushed or at the proportioning valve in the engine bay if there is an imbalance in the system , so why would the brake system be grounding though a powered circuit ? There is nothing on the circuit board that connects to the brake warning light , it just pops into a blank hole with a blue power wire in &  a second Black ground wire out .
 Are you sure nothing melted under dash ?

[Beekeeper]

I started helping my friend and his son on this project a while back. It's quite a Frankenstein car. Considering the rest of the car, a melted dash cluster is certainly possible. I suggested to him that we pull the whole dash cluster so I can see it for myself. The guy who sold him the car gave him an extra dash cluster. I don't believe in coincidence. I think he knew something.

If we pull the cluster and connect the two wires on his ammeter, the car should run and drive right? It's a daily driver for his son so I don't want to take it out of service but I'd like to open it up and study things better.

[Beekeeper]

I reread your post again CP. If I get under the car and measure the voltage at the wire going to the sending unit, am I correct in understanding it should read 5 volts and rather than be constant, it should pulsate? If that's true, I'll try that first since it's easy to do.

If the voltage checks out, I'll yank the sending unit and test it further. The sender in it now is a replacement. Maybe he got a bad one. I just assumed his problem was elsewhere and he replaced it when he didn't need to.