Fuel Gauge stuck on full

[project02]

So I went to drive my car this morning and I discovered my half tank of gas had turned into a full tank! Or at least that is what my fuel gauge told me. Now I don't think I have ever owned an old car that didn't have a fuel gauge problem, however I was wondering if anybody else has had their gauge get stuck on full? Bad ground perhaps. I checked the on by the tank and it looks fine. It looks like the sending unit is relatively new also. Any suggestions?

[BillWilliams]

I think a full tank reading is more of a short than a ground. If you disconnect the wires at the sender, does the gauge drop off of full?

[jimk]

Loss of ground will for sure make it go to full. but the ususal circumstance is that both of the resistive type gauges, fuel and temp flip high when the ground is lost because it is a board problem, not a gauge problem.

[allbim]

One terminal of the fuel gauge connects to +12v, the other to the fuel level sensor (resistor) which more or less grounds it depending on the fuel level. When the ignition is turned off on a 2002, the fuel gage will read empty. This means that the gage reads empty when there is no current flowing through it. A loss of the ground connection cannot increase current through the gauge. It can only decrease it. Therefore, a gauge that always reads full means that you have a short to ground in either the wire going to the fuel level sender or the sender itself. Disconnect the brown wire from the fuel level sensor. If the gauge still reads full, you have a short to ground in that wire.

[Jerzy]

You're "G" (gauge) wire may be shorted to ground. Look at the wire's insulation where it connects to the sender. The wire can get pinched between the sender and the floorboard if its not routed correctly at the sender.

[allbim]

Obviously you need to be careful not to generate any sparks near the tank when the fuel level sensor is removed from the tank.

[jimk]

One terminal of the fuel gauge connects to +12v, the other to the fuel level sensor (resistor) which more or less grounds it depending on the fuel level. When the ignition is turned off on a 2002, the fuel gage will read empty. This means that the gage reads empty when there is no current flowing through it. A loss of the ground connection cannot increase current through the gauge. It can only decrease it. Therefore, a gauge that always reads full means that you have a short to ground in either the wire going to the fuel level sender or the sender itself. Disconnect the brown wire from the fuel level sensor. If the gauge still reads full, you have a short to ground in that wire.

There are 3 terminals on the gauge, the ground was forgotten above. Loss of instrument panel ground causes the full voltage to be applied to the tank sensor. Lift the panel ground and the gauge will peg high as though the intank sensor had low resistance, just like the temperature gauge does when it's ground is lost. The intank sensor never goes to zero resistance even with the float at full up postion and as such never acts as a gauge ground. Me being mechanical, I had my electronic engineer explain how this works.

[project02]

Thanks everybody for the advice. I know what my project will be this weekend.

[allbim]

One terminal of the fuel gauge connects to +12v, the other to the fuel level sensor (resistor) which more or less grounds it depending on the fuel level. When the ignition is turned off on a 2002, the fuel gage will read empty. This means that the gage reads empty when there is no current flowing through it. A loss of the ground connection cannot increase current through the gauge. It can only decrease it. Therefore, a gauge that always reads full means that you have a short to ground in either the wire going to the fuel level sender or the sender itself. Disconnect the brown wire from the fuel level sensor. If the gauge still reads full, you have a short to ground in that wire.

There are 3 terminals on the gauge, the ground was forgotten above. Loss of instrument panel ground causes the full voltage to be applied to the tank sensor. Lift the panel ground and the gauge will peg high as though the intank sensor had low resistance, just like the temperature gauge does when it's ground is lost. The intank sensor never goes to zero resistance even with the float at full up postion and as such never acts as a gauge ground. Me being mechanical, I had my electronic engineer explain how this works.

The range of resistance of the fuel tank sensor is about 3.5 ohms full, to 75 ohms empty. If you apply full voltage (12v) to the tank sensor when the tank is full you will get a current of 3.4 amperes flowing through the resistance wire in the fuel level sensor (12volts divided by 3.5 ohms). Since this is a .004" diameter nichrome wire, it would get vaporized. Obviously the way your electronics engineer friend described this, can't be correct.

At this point I have to admit that my own description was not correct either.

Here is the way it really works.

The gauge is a differential meter. As you (and now I) can see, disconnecting the instrument ground would have the same effect as shorting the sensor wire to ground. Clear as mud?

Edit: The differential meter is used to minimize the effect supply voltage variations have on the gauge reading.

[radkopff]

Very briefly the impedance of two resistors for the fuel tank sensor and fuel gauge is

Z = RtRg / (Rt+Rg), the current passing through the circuit is I = V/Z.  In a short condition for either the gauge or sensor the Impedance goes to zero current goes high and the gauge pegs full. If the gauge goes full that is the likely scenario and the fault is a short. Comments?

[jimk]

I don't know anymore if my assessment is good or bad, but the Figure 1 above is not correct.  The full and empty sensor resistance shown is reversed.  When the float is high, the length of resistance wire in the circuit is at minimum (low resistance) and when the float is low the reverse (high resistance).