Could be a couple issues....the fuel gauge system relies on resistance in the wiring from the fuel tank sending units.
If the sending units are bad, then the resistance is off and the gauge reads high or low. ....bad wiring, corrosion or loose connections between the fuel tanks, transfer switch and dash gauge could also affect resistance. Or the gas gauge itself has a bad resistor or other problem with the printed circuit for the gauges...bad contact.
Since it's not likely that both tank sending units are bad at the same time and same amount, it's more likely a problem in the wiring from the sending units to the transfer switch and gas gauge ...or the gauge itself is bad...or a problem with the printed circuit panel for the gauges. Make sure the ground wire connections at tank wiring are clean and tight. What we would often do in the shop is unplug the sending unit wire harness at the tanks and connect it to a known working sending unit...then move the float up and down and see if the gas gauge worked correctly or not in the transfer switch settings.
You would have to do a resistance test on the wiring from the gauge/switch to the tanks and test the gauge.
Related info on the topic....
"If gauge reads EMPTY AT ALL TIMES:
A. Probable Causes
Wire from sender to dash gauge is shorted to ground.
Sending unit is shorted internally
Float in sender is defective
Testing
Disconnect wire from sending unit at tank. If gauge now reads Full, the trouble is in the sending unit.
Disconnect sender terminal wire at dash gauge. If gauge now reads Full, the wire to the sending unit is shorted to ground. Replace wire.
Repairs
Look for broken or loose wire in the fuel gauge circuit and within the sending unit."
https://www.fillingstation.com/articles/...
The above testing method can also be used to see if the wiring from the gauge to the tanks is bad which can cause the gauge to read low.....or do a resistance test of the wire between the gauge and tanks in the wire harness....as compared to a similar length of same size wire. If the truck wiring has higher resistance, then check the harness for problem areas....corrosion, loose connectors, etc..
"
TEST PROCEDURE - GAUGES
Using an ohmmeter set to the appropriate scale, "zero" the ohmmeter if the meter requires it. Then, while referring to the subsequent gauge-specific charts, with the ohmmeter connected to the tester leads, adjust the tester to the stated resistance for testing the 'Low' index line of the particular gauge and year vehicle.
Once the tester is adjusted, substitute it for the gauge sending unit by unplugging the wiring harness from the sender, connecting one lead of the tester to the sender wire, and the second lead of the tester to a verified, good ground. Turn the ignition ON. The gauge needle should point to the lowest reading (leftmost index line) on the gauge face.
Similarly test the 'Mid-range' and 'High' index lines of the gauge by systematically resetting the tester resistance according to the appropriate chart.
A correctly reading gauge indicates there are no problems with the gauge or its wiring, limiting any faults to the respective sending unit.
An incorrectly reading gauge calls for further testing and verification of 12-volt ignition power, grounds, connections, and associated wiring.
TEST PROCEDURE - SENDING UNITS
The subsequent charts provide precise sender resistance values for three specific gauge readings (Low, Medium, and High), hence, can be used to estimate sender resistance when actual physical parameters are known (e.g., engine temperature that is verified with a thermometer, actual fuel tank level, engine oil pressure that is verified with a manual gauge, etc). Therefore, a sending unit’s resistance can be measured and “subjectively” compared to its interpolated value between ‘Low,’ ‘Medium’ and ‘High,’ based on the charts and the observable/verifiable physical conditions that actually exist on the vehicle.
FUEL LEVEL GAUGE PARAMETERS
For Truck Years: 1967 - 1990
The Fuel Gauge Needle Should Point to:
Left Line (Empty) when sender resistance = ~1 Ω
Middle Line (Half) when sender resistance = 44 Ω
Right Line (Full) when sender resistance = 88 Ω"
http://forum.73-87chevytrucks.com/smforu...
http://www.gmsquarebody.com/forum/showth...
http://67-72chevytrucks.com/vboard/showt...
The gauge is dead and gone. If you really need to know more, then look it up. It's not worth replacing on any car much less a '79. Just fill it up every week or every two weeks depending on how much you.
If it shows 1/4 on both tanks, you have a bad gauge.
Gauge could be bad, or the float
change the gauge
I have a '79 chevy. I filled up both tanks, but the gas gauge only showed it to be 1/4th full. What's wrong with it?
