Strange fuel gauge problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**ScottH** · 06-06-2011, 02:27 PM

I have a 84 GMC 2WD stepside that came from the factory with dual tanks. When I got the truck the PO had removed the RH side tank but could not remember why. So after checking it out I have come to find that the fuel gauge is off by about a 1/4 of a tank too much. I tested it with 3 different sending units, all at different levels, all with the same results. Also changed out the fuel gauge with another used one I had, same result. Not sure if the LH tank is the same as I dont have it on the road yet.

Does anyone know what might cause the fuel gauge to read more than it should? Anyone have a similar situation?

Any info or suggestions are appreciated.

Thanks
Scott

ray_mcavoy · 06-06-2011, 09:03 PM

Hi Scott,

The fuel gauges in these trucks use 0 - 90 Ohm sending units with zero Ohms being empty and 90 being full.

So if you have some added resistance anywhere in the sending unit circuit, it'll make the gauge read higher than it should.

The extra resistance is most likely due to a weak connection somewhere. Start by making sure the ground wire from the sending unit to the frame is in good condition and that the area where it's screwed to the frame is free of rust, paint, undercoating, etc. Next, check the socket where the wiring plugs into the tank changeover valve. I think there's also a socket/plug where the sending unit wire plugs into the dual tank harness. Anyway, check the terminals inside those plugs/sockets for corrosion. Finally, check for a good connection where the fuel gauge sending unit wire passes through the bulkhead connector and where it connects to the printed circuit on the back of the instrument cluster.

If you don't find any obvious or suspect connections, the problem might be the switch contacts inside the changeover valve. To test that, you could temporarily unplug the changeover valve and jumper the wire coming from the gas gauge (pink) to the wire leading to the sending unit (pink w/black stripe I believe).

**ScottH** · 06-06-2011, 10:53 PM

Ray, great info and much appreciated.

Thanks very much! I have some huntin to do.

Scott

06-06-2011, 02:27 PM	#1
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,193	Strange fuel gauge problem I have a 84 GMC 2WD stepside that came from the factory with dual tanks. When I got the truck the PO had removed the RH side tank but could not remember why. So after checking it out I have come to find that the fuel gauge is off by about a 1/4 of a tank too much. I tested it with 3 different sending units, all at different levels, all with the same results. Also changed out the fuel gauge with another used one I had, same result. Not sure if the LH tank is the same as I dont have it on the road yet. Does anyone know what might cause the fuel gauge to read more than it should? Anyone have a similar situation? Any info or suggestions are appreciated. Thanks Scott __________________ 1963 C10 swb fleetside 1966 C10 swb fleetside 1966 GMC G1000 Handi-bus (future project) 1955 Chevy 2 dr. wagon

06-06-2011, 09:03 PM	#2
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,373	Re: Strange fuel gauge problem Hi Scott, The fuel gauges in these trucks use 0 - 90 Ohm sending units with zero Ohms being empty and 90 being full. So if you have some added resistance anywhere in the sending unit circuit, it'll make the gauge read higher than it should. The extra resistance is most likely due to a weak connection somewhere. Start by making sure the ground wire from the sending unit to the frame is in good condition and that the area where it's screwed to the frame is free of rust, paint, undercoating, etc. Next, check the socket where the wiring plugs into the tank changeover valve. I think there's also a socket/plug where the sending unit wire plugs into the dual tank harness. Anyway, check the terminals inside those plugs/sockets for corrosion. Finally, check for a good connection where the fuel gauge sending unit wire passes through the bulkhead connector and where it connects to the printed circuit on the back of the instrument cluster. If you don't find any obvious or suspect connections, the problem might be the switch contacts inside the changeover valve. To test that, you could temporarily unplug the changeover valve and jumper the wire coming from the gas gauge (pink) to the wire leading to the sending unit (pink w/black stripe I believe).

06-06-2011, 10:53 PM	#3
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,193	Re: Strange fuel gauge problem Ray, great info and much appreciated. Thanks very much! I have some huntin to do. Scott __________________ 1963 C10 swb fleetside 1966 C10 swb fleetside 1966 GMC G1000 Handi-bus (future project) 1955 Chevy 2 dr. wagon



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