Gas gauge problem

[David B.]

I have a 1961 corvette with a frustrating gas gauge problem. When the tank is filled the gauge registers full. However if I step on the brake pedal or turn on the lights the gauge shows 1/4 full. I have run a dedicated ground wire to the sending unit, a dedicated wire to the sending unit from the gauge, and a new wire from the gauge to the source to try to isolate the problem. Nothing makes a difference. The gauge still drops to 1/4 reading when the brakes are applied or the lights are turned on. Can someone suggest what I might try to resolve this problem. Thanks in advance David Bodily

[Bill W.]

Re: Gas gauge problem

My 62 drops about 1/4 tank when the brakes are applied (not to a 1/4 tank ) It did it in 1969 when I bought it and still does even after a new under dash wire loom .I have no idea why .

[Philip P.]

Re: Gas gauge problem

I would check the voltage at the guage, should be battery voltage 12-13 volts on the pink wire ( brake on and off). From the information you gave make sure the sending unit is OK. Checking the sending unit requres it to be removed from the tank not a bad job but a job. I cannot remember off hand what the resistance measurement is but will try to find that out or someone may know. If that checks OK then I would suspect the guage. I know my 1960 has a mind of its own on the first 1/2 tank and I have checked it out similar to what you have done. There could also be some feed back if that is the case I would suspect grounds in the lighting circuit which is normally the first place to check on most electrical problems with these cars. Cannot say enough about proper and clean grounds.
Phil

[Philip P.]

Re: Gas gauge problem

I found this is some notes I had may help
Phil

This is a common problem with 1958 to 1962 Corvettes. Behind the instrument

Panel there is a single ground wire, which becomes overloaded when more than

one device is turned on (brakes, turn signals, radio, Etc.). It is a simple matter of

the ground wire becoming overloaded and giving a false gas tank level reading.

Install an extra ground wire behind the instrument panel up out of sight. Use a

piece of 14 or 12 gauge wire to make sure that you eliminate the problem. No one

will ever see it and your gas gauge will show your true gas tank level.

[John F.]

Re: Gas gauge problem

Solder a ground wire to the fuel sending unit and run the other end of the wire to a good ground.

[Ed T.]

Re: Gas gauge problem

Getting a C1 fuel gauge to read correctly could be a lifelong occupation. My 62 only reads about 1/2 full on a full tank. Empty on an empty tank. I added the suggested new ground and sending unit wire. No change. Got a new sending unit. No change. Pulled the sending unit out and set it up on a bench with extension wires to the in-tank wires. Float high reads full. Float low reads empty. Even tested for a leaking float, and that checked out OK. When I run out of important things to do I'll try at it again...Ed

[John H.]

Re: Gas gauge problem

The root cause of this common (and chronic) C1 problem is that the fuel tank sending unit ground wire is a branch of the rear body wiring harness ground, which serves the rear running lights, brake lights, turn signals, and license light. That rear ground wire at the left rear corner of the trunk then extends forward in the main harness and grounds everything under the dash. Any time you step on the brakes, use the turn signals, or turn the parking/running lights on, the ground side of those circuits (which is all on one wire) affects the inadequate ground for the fuel tank sending unit, which causes the gas gauge needle to move. 90% of the time that issue can be cured by running a separate ground wire from the sending unit down through the hole in the underbody on the passenger side to the frame directly below the hole. Attaching that added ground wire to any of the various convertible top frame brackets and reinforcements won't help - they aren't grounded.