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Fuel Gauge Diagnostics for Advance Design Chevrolet Trucks
Fuel gauges seem to be one of the common problems that people have with the electrical systems on the Advance Design series of Chevrolet trucks. The diagnostic procedures below are the same for all Advance Design trucks and might be the same for GMC New Design trucks and other vehicles.
There are four or five parts of the fuel gauge system that need to be considered when diagnosing a problem:
1. The fuel gauge
2. The fuel gauge sender unit that is installed in the gas tank.
3. The line that creates a circuit between the fuel gauge and the sender unit.
4. The grounding of the sender unit.
Other things that might have to be considered:
1. If the fuel gauge is in a truck that has been converted to 12v, a Runtz-style voltage converter IS NECESSARY, or the gauge will eventually be burned out by the 12v current. Note: Some venders sell restored gauges for Advance Design trucks and these gauges can be purchased for 6v or 12 systems.
The most common reasons for Fuel Gauge failures is bad grounding or fried gauges (from using a 12v hot lead).
The diagnoses below will be given for two, testing set-ups: (1) gauges and senders in-place in the truckÕs dashboard; and, (2) gauges and senders tested on a bench (out of the truck). It is also possible to Òpermutation testsÓ, with either the gauge or the sender in or out of the truck.
First, consider the table of possibilities that is found in many Shop Manuals:
1. The gauge gives no reading (ÒEÓ, Empty) at all times (especially with the power on).
2. The gauge gives a ÒFullÓ reading with the power on, when either tank is empty of not full of gas.
3. The gauge does not register accurately (within normal limits)
l The gauge gives a reading with the power off (it should read ÒEÓ Empty).
l The gauge gives too low a reading when the gas tank is full.
l The needle on the gauge moves erratically.
Trouble-Shooting Diagnostic Procedures:
TESTING THE FUEL GAUGE
- Be careful not to reversei the electrical connection (hot/positive lead and ground/negative lead)..
- Do not use 12v current with a 6v gauge.
The fuel gauge has two terminals (both terminals are Òhot) and a ground (the backplane of the gauge touches ground through the gauge housing (which is grounded through connection to the dash, which is gounded to the chassius, which is grounded to the negative lead of the battery)..
1. Disconnect the sending unit wire from the F or Full side of the gauge (on the back of the gauge). The gauge should read empty; if it does not read empty, the gauge is bad.
2. Connect a hot or live wire to the terminal on the E or Empty side of the gauge (on the back of the gauge). The hot lead may come from the ignition switch (an always ÒhotÓ pole), fuse box (a hot pole), or a hot pole on the light switch (or a lead that is attached to the positive pole of a 6v battery.
3.a. . Bench tesing: Ground the gauge – run a lead from the negative battery terminal to the insulated base plate between the two rear poles on the gauge.
3.b. In-truck testing: the gauge should have a ground through the gauge housing. You can test the quality of this ground but using a contrinuity testor between the housing and the negative pole of the batter (it should light, showing continuity – a good ground connection).
4. At this point, there should be a circuit from the positive lead from the battery to the Empty side of the gauge, through the backplane ground on the gauge, to the negative lead from the battery.
If the gauge reads Full, there is no problem with the gauge or the circuit.
If the gauge does not read Full, either the gauge is bad or the hot lead is bad or the ground is not good. At this point the lead wires and backplane grounds should be checked again – if the leads and grounds are good, and the gauge does not read Full. the gauge is bad.
When you obtain a new gauge, it is best to bench test the new gauge (and the old gauge, if you have not yet bench-tested it). Usually, you are able to return the old gauge for a ÒcoreÕ savings (do this after you get the new gauge – it is not uncommon to get a new gauge that does not ÒpassÓ the bench test. After fiddling with the test leads/grounds you get the new gauge to work – since your testing set-up iwas not correct, you might want to retest your old gauge (if it now works, take/send the replacement gauge back).
TESTING THE FUEL TANK SENDER UNIT:
The sending unit is attached through the top ofd the gas tanks. The sending unit is easy to get to on trucks that have the gas tank in the cab. On trucks with the gas tank under the bed, getting to the sender unit usually require dropping the gas tank (all early Advance Design trucks and all Pane/Suburban/Canopy-Express trucks)
1. If the fuel gauge tests out OK, the sending unit is tested by connecting the lead wire from the F or Full side of the fuel gauge to the sending unit (there is only one pole on the sending unit). If everything is working OK< the gauge should now give an indication of how much fuel is in the fuel tank.