77 Fuel Gauge pegged past full?

[rwhite1]

I have a 77 chevy k10. Replaced the dual tanks with new sending units. I took the fuel tank wiring harness out of another 77 chevy truck I have. The Fuel Gauge is pegged past full and I cannot figure out why it wont work or go down and read properly?

The sending units are new and properly grounded to the frame. The fuel tank selector valve operates correctly from the selector switch on the dash. (it is the solenoid type selector switch) I replaced the fuel tank wiring from another 77 chevy truck I have in to this one. The wire that runs down the passenger side of the frame rail is also hooked up correctly (I think) the plug is tough to see if it plugged in right. But the selector valve works just fine. I have pulled the instrument cluster down to check the connections on the back of the fuel gauge and everything seems to work fine. Although I can move the gauge towards empty w ignition off w my hand, and when I turn the key on it pegs way past full. I have also checked the sending units (they are new) with the multimeter and have been getting 8.2 ohms out of the sending units with the tank near empty. I have also checked the wiring plug that runs off the firewall for the fuel tanks and it appears to be just fine as well.

I can't figure out why the gauge keeps peging out past full.. Any ideas or suggestions?

Thanks,

Richard
1977 K10 Chevy shortbed w/ 350

 

[chengny]

Check the pink wire (maybe tan in your case). It runs from the fuse panel to the selector switch on the dash. It splits at the switch (tan/white & tan/Lt blue) and those leads feed the level sender on each tank. Those are the hot wires and should always supply 12VDC to the individual senders.

Because you are pegging high regardless of which tank is selected, it is safe to assume that the problem is in the common run of the hot wire (i.e. between the fuse panel and the selector switch).


Look for breaks in the insulation which might indicate a intermitent connection within the hot wire. A broken/loose connection on the hot wire (tan/pink) will peg the gas gauge to FULL. This is the same reaction that a broken ground wire between the sender head and frame produces.


With the gas gauge, higher current flow through the sensing circuit (pink wire, variable resisitance sender unit & ground from sender to frame) will drive the needle down towards E.

If anything lowers this flow of current, the needle will move towards FULL.

If the hot wire is completely broken or disconnected, current flow drops to zero and the needle on the gauge moves way past F and pegs out high.

Make sure there is 12 VDC coming out of the selector switch at the harness plug (at pins B & C).


BTW - you did check your fuses, correct?

Schematic for 1978 dual tanks (didn't have the 77 but it should be the same):

 

[rwhite1]

Excellent suggestion, but that's not it. I checked the pink wire and it is plugged into the fuse panel correctly. (I have another 77 chevy truck to compare it to as well) With the pink wire unplugged the fuel tank selector switch does not work and the fuel gauge is still pegged at full. With the pink wire plugged in the fuel tank selector switch works correctly. (you can hear it switching) I also tried a new fuel tank selector switch thinking it may be a problem in the switch but the fuel gauge is still pegged at full. I can't figure out what the problem is...?

 

[rwhite1]

Yes and all the fuses are good.

 

[rwhite1]

Ok, I just tried to see if any power was coming from the plug on the firewall to that Im referring to as the fuel wiring harness. I tried all of the prongs and non of them are showing any voltage. How come they wouldn't be getting any power? With the plug plugged in the fuel selector tank switch works, with it unplugged it doesn't (fyi) Gas gauge still pegged at full.