I am trying to learn how to work with these electrical things, and I have three issues to work with. The car is a 1970 F85. At some point aftermarket under-dash gauges were put in, and I recently swapped the idiot lights with an original gauge pod. When both gauges are connected to the temp/pressure senders, they run backwards, and with just the pod plugged in they don't move at all. For the other issue, my fuel gauge pegs to empty when I pull the running lights on or step on the brake. Lastly and of least importance to me, the lights in my gauge are really dim. Swapping bulbs has no effect.

My initial troubleshooting steps have been to -

1. verify pressure sender resistance with wire disconnected. It read 270 ohms, -2 ohms at idle, steady on acceleration
2. clean all the grounds I could find - so far 2 points in the trunk and 2 points by the headlights
3. look at wiring diagrams and scratch my head

This picture is the wiring behind the bumper. It looks damaged to me.



What should be my next step?

 

The fuel gauge going to E when the lights are on is a bad ground, but I don't know where.

 

Actually it’s a short that causes the needle to read EMpty. an open causes the needle to read way past Full.

 

The aftermarket gauge and the factory gauge use different sending units. Even if they weren’t different, you can’t run both gauges off the same sending unit.

 

Kenneth, I'm staying with a weak ground because the OP stated that the gauge goes to E when the lights are turned on or the brake is applied. The lights are chasing the path of least resistance to find a ground which in this case is via the fuel gauge circuit. SB

 

I see what you mean. That makes sense as the light switch is on the ground side. So turning the lights on gives the gauge a ground through the light circuit.

 

I think the light circuit has the weak ground and gets what it needs through the gauge circuit.

 

Make sure the ground wire on the back of the dash gauge is there and intact.

Years ago, I removed the fuel gauge from my car to repair a loose pin on the circuit board. When I reinstalled the gauge, I accidentally omitted the ground wire. The fuel gauge did some funny things until I reinstalled the ground wire.

Not saying that’s the issue, but someplace to start.

 

I took the advice on looking for the gauge and switch grounds. I didn't find the switch inside the dash, but it occurred to me that I hadn't fully tightened down the gauge onto the ground strip - durrrrrr... At least the fuel works now while it runs.

I'll also have to get new senders to see what happens.

 

Before replacing the senders verify whether or not they work after fixing the loose ground connection. I've never seen a bad sender make a gauge run backward,

Good find on the ground connection. The crazier the electrical symptom the more likely it is ground related.

 

My own lesson(s) learned years ago when dealing with gauges running in reverse was related to something I didn't know at the time and it was exactly as Kenneth stated Post #4.

Basically, sending unit(s) must match the resistance (ohms) of the gauges. And, most certainly you can't run both gauges off the same sending unit. You're going to need to match each sending unit to each respective gauge. If you're tempted (elect) to reverse wires on a particular gauge, thinking this may fix the issue, be prepared to buy a new gauge as it will most likely damage/destroy the gauge. It's nearly impossible to determine resistance(s) for each type sending unit + gauge. Some gauges are adjustable & allow you to set (calibrate) the resistance (ohms). IMO, the backward readings are most likely related to incompatible sending unit gauge resistance which you'll need to confirm via manufacturer.