1978 K20 Gas Gauge

[Rebus]

I have a stock 1978 K20 with dual tanks and the gas gauge has quit working. It's normal resting point is F but when the ign key is turned on it goes beyond F to 3:00, pegged. It doesn't matter which tank is selected. Looking at previous forum posts I see conflicting comments about pegged full scale indicating an open vs shorted to ground circuit. Consequently I'd like to get some clarification if I could and considering the tank selection doesn't matter, does this narrow the possibilities? Thanks.

 

[TurdTaco64]

Dealing with my own fuel guage issues, although different from yours. When I disconnect the ground at the sender the guage pegs to/past full, so open circuit=full.

 

[Rebus]

Dealing with my own fuel guage issues, although different from yours. When I disconnect the ground at the sender the guage pegs to/past full, so open circuit=full.

Interesting. Meanwhile I checked with a professional mechanic yesterday and he said going beyond full indicates a short to ground. Seems there's a lot of contradictory information floating around. I suppose there's no way to check the sending unit grounds without dropping the tanks.

 

[justhorns]

I’d start with the instrument in the dash, because it’s easier to get at. You will need a basic knowledge of electrical circuits. Gas gauges are very simple. A coil working against a spring. There is a lot of conflicting info on the internet because there a lot of dumb;()s. You either figure out how it works or pay someone who knows.

 

[75gmck25]

One point of confusion is that GM gauges don’t work the same as Ford gauges. Ford gauges max out when the lead at the sending unit is grounded, and read zero if the wire is disconnected.

GM gauges also read full when grounded, but may also stay at a full reading when disconnected. They often do not go to zero when the sending unit wire is disconnected.

 

[Albrigap]

I’d start with the instrument in the dash, because it’s easier to get at. You will need a basic knowledge of electrical circuits. Gas gauges are very simple. A coil working against a spring. There is a lot of conflicting info on the internet because there a lot of dumb;()s. You either figure out how it works or pay someone who knows.

99.9% of the time the problem is the sender in the tank or wiring.
Virtually never is the gauge bad.

 

[Rebus]

Just a follow-up, turns out both sending units went bad. Instead of the 0-90 ohms, the senders still vary resistance with float level but in the MegOhm range.
Confirmed the gas gauge goes offscale high with open circuit, to empty if grounded.
Truck is apart awaiting backordered fill and vent hoses. Seems nobody has them.