Incorrect fuel gauge readings on cluster

[TedC]

Incorrect fuel gauge readings on cluster - FIXED!!!

My cluster fuel gauge readings started to bounce around between what it should be and full. According to the FSM, at 0 ohms the gauge should read empty, 45 ohms it should be 1/2, and at 90 ohms it should read full. The cluster fuel gauge posted the right results using correct valued resistors, so my attention shifted to the fuel sending unit. After removing it from the tank (same process as replacing the fuel pump, and bleed the rails), I could now attach a VOM and measure the readings at different float levels. As I wiggled the float sideways at say the 3/4 position, the 60 ohms reading would spike up to 90 ohms or higher. Found my issue. Now how to fix it. First I sprayed contact cleaner inside the metal canister that contained the variable resistor. It helped some, but I was hoping for even better results. The canister has three tabs which I carefully bent back to open up the unit. Tarnish like buildup. Inside the variable resistor is a coil made of real thin wire, so I sanded it very lightly with 1000 grit sand paper in the directions of the wires (not across). Reassembled the canister and measured the results again without bending the tabs down. What a difference. Stable readings now on the VOM. Feeling that I solved the issue, bend the tabs down and reinstall the sending unit in the tank. 1.5 hours to complete from start to finish. After a few test drives yesterday and today, all is working properly.

A lesson learned: When I tested the cluster fuel gauge using different resistors, I discovered that the fuel gauge has a delayed response. So when you go from open circuit to a direct short of purple and black wirse, you need to allow time for the gauge to move to the correct reading. At first, I didn't give the cluster enough time to react, and I thought my cluster was messed up. It makes sense why the cluster is designed this way, but it threw me a curve for a few minutes. I could not find any comments about this delayed reaction in the FSM.

 

[bogus]

That and the senders are JUNK. They are well known for being woefully inaccurate.

 

[John Robinson]

Thanks for this information. I am going to copy it and file it with my other Corvette trivia and help stuff. I have a folder labeled Corvette in my file that is getting to be about 3 inches thick. I am all the time going back to it to find answers like this to a problem.

 

[boomdriver]

Yep...the gauge system has a "dampner" in the circuit to prevent the display from bouncing around as fuel sloshes in the tank...if it were a direct simple circuit you'd never be able to see what your level was until you sat still for a minute or two....

One word of caution..
I'd stay away from the sand paper. Those windings are extremely delicate and can easily be scratched thru so that the gap between gallons is bridged and there has to be greater consumption before the gauge can register a difference....

What I did that worked well was to use a diluted acid wash to basically etch the scum off of the contacts. A watered down phosphoric acid works well. CLR thats been diluted will do the trick and so will Eagle-1 etching wheel wash. This will dissolve the varnish and leave clean contacts again without abrasion and having to risk breaking the tiny tabs that hold the assy together. The breaker point that sweeps the coil is pretty delicate as well. Apply the solution with a small painters brush or Q-tip...allow it to work for a few seconds, (depends on strength) and as scum vanishes rinse it off and immediately apply some WD or some other oil to prevent immediate oxidation. Re-install and you're on your way. Word of warning: if you break those tabs or damage that cheap, goofey simply sender....you're in for a new pump/sender assy thats gonna cost ya at least $175(no pump).
IF you're even lucky enough to find one for sale. A not very 'gently' used one will cost $100.(without pump).

Just a very few years ago that sender assy sold for $600 at Chevy dealers and Delco distributers but the licensing has changed and its now more reasonably priced in the after-market world.

 

[TedC]

Boom,
Where were you when I got into this project? Hey, I was very careful when using the 10 grit sandpaper.

The good news is the fuel gauge is reading like new again. At near empty the reserve light turns on properly, and when I filled the tank 5 gallons at a time, the readings were pretty accurate. Been driving around for almost a week and everything is back to normal. A pretty simple fix to an annoying issue.

Is it the ethanol that causes the tarnish build-up? I store my car in the winter with a full tank. This past winter I did not use any stabilizer. Once in a while I will use techron.

 

[boomdriver]

Boom,
Where were you when I got into this project? Hey, I was very careful when using the 10 grit sandpaper.

The good news is the fuel gauge is reading like new again. At near empty the reserve light turns on properly, and when I filled the tank 5 gallons at a time, the readings were pretty accurate. Been driving around for almost a week and everything is back to normal. A pretty simple fix to an annoying issue.

Is it the ethanol that causes the tarnish build-up? I store my car in the winter with a full tank. This past winter I did not use any stabilizer. Once in a while I will use techron.

lol...the voice of experience (bad ones):eyerole is all that I am. I used to sand mine to clean them as well...till I sanded thru the coating and bridged the coils and it then only moved when 2-3 gallons had been used. Annoying to park thinking you still have 5 gal and get in later to see it bottomed out at empty.

As far as the varnish and crud..? I've been told that its the alcohol and other additives in contemporary gasoline blends that cause this. I recall something about sulfer...but I forget exactly what that was about. Techron seems to be a good cure-all for this, as well as seafoam and a few other top shelf gasoline/fuel system cleaners if used as preventive maintenance.

 

[Janice]

Cleaning

Just a thought. When cleaning any metal contacts including brushes ect.... A hard rubber eraser is by far the best and safest. like the ones that came with old slot car tracks to clean the track and shoes.
Great fix by the way

 

[vigman]

The other Oh By the way

there is a white coupler between the incoming fuel pump wires and the pump... this is an RF noise elminator ( for those of you who still listen to AM radio ) ....
I have had that R/C network open up on 2 vette's leaving you with NO POWER getting to the pump. I recommend removing it next time your in there....

Mike