Inaccurate Fuel Gauge

[jackjeckel757]

Hey all I have searched the forum and can't find an answer for my problem so here goes...
I have a '77 C10 with dual tanks, I removed all the gauges to clean them up and paint the needles which were pretty badly sun burnt. All the gauges returned to working normally again except the fuel gauge. It was accurate before but now it reads about a half tank below what is in both tanks, so the left one I just filled reads half full and the right one which I know is empty reads about a half tank below the empty mark. Is there any way to calibrate the gauge? I have tried removing the gauge, disconnecting the battery and putting the gauge back in on full where I know the tank is and it just goes back down to a half tank lower. Does anyone know how to fix this? thanks.

[Captkaos]

There are several posts on troubleshooting the gauges.  There is one in the Tech section also.

[LTZ C20]

Just wanted to let you know, painting the needles is not a good idea. The needles are weighted and setup at the factory with a curtain balance. After time they get sun burned and what not, mine are faded and a little cracked. Painting the needles throws of the balance. When the balance is off, the mechanism on the back of the needle (which is adjusted by the level of fuel in the a tank) no longer has the same resistance against the wait of the needle. I considered painting my needles, but when I was told this info i didn't. It just maybe a loose connection from unplugging it in the back if the instrument cluster, the fuel gauge connections may just be loose on the back, it could also be do to a sending unit that has a float not working properly, (which is exactly the problem I have, a tank down job). My suggestion is to double check electrical for the gauges, the tanks, and the switching unit. Also, next time you have the truck on and you know the gauge isn't reading right, give it a light tap on the clear plastic cover, my needle will rise or lower to the right level, stay for a while then stop again. At that point you can tap it again or switch to the other side and back again to reset it. Make sure you allow the needle to settle on the level of the tank your not using when you switch over and back to allow the mechanism to make a complete reading. This problem is very frustrating and I feel for ya. Best of luck with it.

[jackjeckel757]

I might agree with painting the needles being a bad idea except for the fact that my volt, oil, temp, and speed are still accurate. Plus I have left the cover off the gauges and no matter where I try to move the fuel needle by hand it always goes back to where it wants. The old paint seemed thicker and heavier then the Testors paint I used, and I cleaned off the old sun burnt paint carefully before I painted. As I said the truck didn't move while I did this and pulling the gauges and re installing was all I did. Still trying to figure this one out...

[bd]

I might agree with painting the needles being a bad idea except for the fact that my volt, oil, temp, and speed are still accurate. Plus I have left the cover off the gauges and no matter where I try to move the fuel needle by hand it always goes back to where it wants. The old paint seemed thicker and heavier then the Testors paint I used, and I cleaned off the old sun burnt paint carefully before I painted. As I said the truck didn't move while I did this and pulling the gauges and re installing was all I did. Still trying to figure this one out...

I agree with you - painting the needles did not cause your problem.  The gauge motors have sufficient torque to overcome any slight increase in mass; besides the needles are balanced, end-for-end.  The effect of adding paint along their length is negligible.  You have an electrical circuit issue.

First, an important question:  Is/was there a small, rectangular, ceramic block mounted on the back of the fuel gauge between two of the gauge pins?

If you feel "crafty," you can figure out your fuel gauge pin connections by inspecting the PC board and tracing the foil circuit paths back to the harness plug.  You should have three connections: 1) ignition (pink, ~12.6 volts), 2) ground (black, 0 volt), and 3) signal from the tank sender (tan, 0-90 ohms of resistance that provides a "variable" ground path; 0 ohms is Empty, 90 ohms is Full).  With the information gained, you can bench test your gauge.

The culprit is most likely a poor connection or open circuit.  The most likely locations for connection issues that might cause your particular set of symptoms (since the problem didn't exist before painting the needles) are the gauge connection to the board through the "clips" and the board connection to the harness plug.  Testing, as above, while referencing a wiring diagram should provide your solution.

[LTZ C20]

I will be dropping my fuel tanks tomorrow and inspecting my electrical, hopefully to fix the same problem I am having. Would anyone know where to find a complete wiring diagram for both Jack and I to use. I'm not sure what Jack has equipped, my truck is a 79' c20, duel tank, 1 switching unit, I tank selector switch, and 1 big fuel gauge on the right side of the dash, next to the speedo in the large gauge spots. Diagrams would be a big help! Good luck Jack!

[bd]

Look in the Technical Section of this site:
http://forum.73-87chevytrucks.com/smforum/index.php/topic,11766.0.html

[bake74]

Look in the Technical Section of this site:
http://forum.73-87chevytrucks.com/smforum/index.php/topic,11766.0.html

     Beyond the tech section on this site, you would have to work in a place that has access to the books from the manufacture.  I know of at least 2 people on here but do not want to drop names so they don't get inundated with Emails.  I will however PM them this thread with a note and see if they will respond.

[VileZambonie]

http://forum.73-87chevytrucks.com/smforum/index.php/topic,9072.0.html

[bake74]

     The man does it again.  Thanks Vile.

[LTZ C20]

Thanks Vile! THats gonna help alot!