Looking for some ideas on how to resolve the following issue with the Fuel Gauge on my 1970 442 with Rallye Pac

No Power, fuel gauge shows last reading. With ignition switched turned on, the fuel gauge needle drops to E. Pull the headlight switch to either detent, and it swings to F.

Background:

Instrument harness new (M&H)
Headlight switch new (AC Delco)
All Gauges just redone by Redline
Tank sender new, reliable ground
NOS under hood light installed (ties to headlight switch) - works
Installed jumper wire from the instrument panel grounding strap that is bolted to the headlight switch to the firewall to ensure headlight switch is adequately grounded
Water & Oil Pressure gauges working fine as do all the other instrument cluster lights
No other weird electrical issues that would suggest a chassis ground problem

Anyone run across this issue before? I’m running out of ideas.

 

Did this problem just start, did you just install the harness, is the gauge light illuminated with the lights off?

 

Pegs full = open...ground wire or sending unit or wire between the sending unit and gauge.
Pegs empty = short

1. Check the voltage at the trunk connector (mid-body harness to trunk harness). The wire you are looking for is tan wire.
2. Turn the ignition on, and check the voltage. If it reads 12 volts, you have an open circuit between the trunk harness connector and ground. It could be a bad sending unit, broken or damaged wire leading to the tank, or a bad ground. You can further validate by inserting a wire into the connector and connect the other end to ground, the fuel gauge should read empty. You can move along tan wire and test this way at each connection point. Complements of Harvy White 442.com, July 2013...Im lazy give credit its due.
A general consensus is a 10+ year old car needs its grounds refreshed.

 

I bet you will find a bad ground on the gauge itself. Try connecting a jumper wire from the gauge housing to a good ground.

 

Replying to all three responses, thanks guys:
- Still can’t explain the headlight switch connection, other than potential high current draw of the lights indicating a poor ground, but weird that everything else in the dash is working fine
- Can’t say if this is a new or existing problem. Car was blown apart for years for a resto that ran out of gas. All the parts were added as part of a frame off recently finished. With the gauges needing work, I’m sure it hasn’t worked for ages.
- Good tips, I’ll add these to my to do list. Had to do some fresh paint scraping for the tank sending unit ground, but will chase the lead wire back from the tank.
- Tried that. I have a jumper from the instrument gauge ground strap and cluster mounting screw going straight to the firewall

 

I was just thinking about point #2. Shouldn’t it always have 12V with the switch on? The sending unit acts as a potentiometer, and as the fuel level drops in the tank the increasing resistance draws the gauge needle closer to the E position. Full tank, little to no resistance and the gauge reads F. At least that’s what I think happens, but I could have it wrong.

 

Fuel sender is 0 ohms empty, 90 ohms full. You need to see if the wiring at the gauge and sender is correct. Check the gauge for voltage in the right place at the right time.

 

Make sure that the resistors are on the back of the gauges correctly. I made this mistake and my temp pegged when I turned the lights on. This shows the correct positions of the resistors.

 

Thanks for all the ideas guys. Turns out the D.A. (me) that hooked up the tank sending unit wire to the license plate light feed in the trunk was the culprit. My trusty voltmeter helped me confirm this before taking them apart. As an added bonus the license plate light now works too.... (duh). Still shaking my head as I mutter a few things to myself....

 

I figured it was something out of the ordinary. Glad you found it.

 

I no how you feel, my fuel gauge didn't read right when i got it from the previous owner, changed the sender out and still empty lol, so I shifted to back out of the driveway seen it peg full, that's when it hit me, he had it wired to the reverse light, I re spliced the wire and it was fixed lol.