1977 fuel gauge problem

[John #43802]

I recently purchased a 1977, and the fuel gauge does not operate properly. When I fill the tank, the highest the gauge reads is 1/4 tank. As I drive, it seems to decrease at a normal rate to empty when I have used up about a quarter of the tank. It then stays at empty.

The guy I bought the car from told me about it; he lived with it operating like this. He thought the guy he bought it from (who had done some major restoration work) had replaced the sending unit. His opinion was that there was something not adjusted/calibarated right on the sending unit. From the things I've read, it doesn't look like there's anything that can be adjusted/calibrated.

Is there a way I can determine whether the problem is the sending unit or the gauge itself?

[Tom R.]

Re: 1977 fuel gauge problem

I authored an article a few years ago on rebuilding a late model C3 sending unit that explains and shows how to proceed. If you don't have the past few years, you may want to invest in the CD. I'm not sure how they have them grouped but you may want to look into it.

Essentially though you must remove the unit from the car and do a bench test with a voltmeter. Also when you replace a sending unit, you're actually replacing the filler neck, the emission control check valve and the tank sealer...so you're paying for parts you don't really need.

[Michael C.]

Re: 1977 fuel gauge problem

John,
Another item to consider when encountering inaccurate readings on a 75-77 and later Shark is the safety bladder. The rubber bladder lines the tank in the event of a rupture. When it starts to deteriorate, becomes misshapen, or otherwise collapses it can affect the gauge reading as well as the ability of the tank to hold fuel.

When you evaluate the sending unit you should also evaluate the bladder. The opening in the top of the bladder is where the sending unit bolts to the tank. The bladder edge actually forms a gasket under the sending unit. If it is defective there is no repair that can be performed on it. It has to be removed and a gasket fabricated from appropriate material. If the tank is rusty inside under the bladder then it must be cleaned and sealed.

Is this safe? Well, as safe as any of the preceding model years without a bladder. No companies I know of make a replacement bladder. Quanta sells 75-77 tanks in repro but they are without a bladder. Don't overlook this important issue if you pull out the tank.

[Don 42616]

Re: 1977 fuel gauge problem *NM*

[John L.]

Re: 1979 fuel gauge problem; Additional ?

Tom,
Haven't seen your article yet but will look tonight...quick question; last winter my guage started to jump around from full to empty after a fillup. I have filled up now about three times (don't drive it much) and the same thing happens, it doesn't settle down during the drive, it moves around a lot. Is a ground loose or is the problem in the sending unit? Thanks.

[Tom R.]

Re: 1979 fuel gauge problem; Additional ?

That sounds more like loose connection than sending unit based on my limited experience. My gauge would be fine for say half a tank than drop suddenly to empty. It didn't jump around.

It's some work to remove the sending unit and removing the short rubber hoses that connect to the fuel lines is a "B____" and you can fill in the blank! The ground connect on the top can get corroded and lose contact. Remove it and clean it with some sand paper and than check all your other connection between tank and gauge.

PS: DISCONNECT THE BATTERY BEFORE ATTEMPTING THIS...you don't want an accident