Nothing is more aggravating than a fuel gauge that doesn't give an accurate reading, especially when the gauge shows there's still fuel in the tank when there really is not. Faulty readings can be caused by a bad sending unit, a problem with the gauge, or shorts, opens or weak connections in the wiring that links the two together. But which one is it?

In 1904, the first float arm gas gauge appeared. A float mounted on a hinged arm moved a mechanical pointer on gauge on the outside of the fuel tank to indicate the fuel level. The idea was taken one step further when vehicles started to get better electrical systems by connecting the hinged float arm to a rheostat. This allows an electrically operated fuel gauge to be mounted in the vehicle's dashboard. And that's the basic operating principle that's still in use today.

The successor to the resistance type fuel gauge is the magnetic fuel gauge. Instead of having a heated bimetal strip to deflect the indicator needle, the base of the needle has a small magnet that "floats" in a magnetic field created by three coils. The coils are fed voltage through a terminal that's usually marked "B+" on the back of the gauge. Inside, the voltage follows a split path. Part of it passes through all three coils to ground and part of it goes through only the first coil then on through the sending unit to ground. Depending on the resistance in the sending unit circuit, the strength of the magnetic field created by the first coil compared to the other two shifts the magnetic field one way or the other to deflect the needle on the fuel gauge.

The primary advantages of magnetic gauges (fuel as well as temperature and oil pressure) compared to the resistance variety are faster response and more accurate readings. The resistance variety can take up to two minutes to respond to a change.

The sending unit that's used with a magnetic fuel gauge is essentially the same as before but with higher resistance values. And most follow the GM format of high resistance when the tank is full. On a 1980s vintage Chrysler application, for example, the sending unit reads 145 ohms when the tank is full and 22.5 ohms when the tank is empty.

ELECTRONIC INSTRUMENTATION

Electronic fuel gauges use vacuum fluorescent or LCD graphic displays to indicate the fuel level. Many also read E, 1/2 and F. Electronic displays usually have their own self-contained voltage regulator in the fuel gauge control module. The basic operating principle for electronic fuel gauges is essentially same as that of the other two types in that the tank mounted sending unit produces a variable resistance ground path. The module monitors the current through the sending unit and decides which display circuits to energize to show the fuel level.

Most electronic fuel gauges have limited self-diagnostics. When the key is turned on, most units light up all their display pixels as a way of checking the display itself (like a bulb check). Others go a step further. On Ford, for instance, the fuel gauge will flash if the module detects an open or short in the sending unit circuit.

LOW FUEL WARNING SYSTEMS

Low fuel warning systems are worth mentioning because they're wired into the fuel gauge. On Fords, for example, a low fuel warning switch assembly illuminates a warning light when the fuel gauge reads below 1/4 full. On late model electronic fuel gauges, the light typically comes on when there is about 1-1/2 to two gallons of fuel left in the tank.

FUEL GAUGE DIAGNOSIS

If the fuel gauge does not change (always reads the same, always reads empty or full), or behaves erratically, the list of possible causes include a defective voltage supply to the gauge (instrument voltage regulator), a bad gauge, a defective sending unit, a wiring problem between the gauge and sending unit, or a poor ground connection.

If the temperature and oil pressure gauges are also affected, the problem is not in the fuel gauge or sending unit. It is in the instrument voltage regulator or instrument panel wiring. To get at the voltage regulator, you have to remove the instrument panel.

Check the voltage output of the regulator with a volt meter. If it isn't within specs (usually around 5 volts), it may have a weak ground connection or an open in the resistor wire that supplies it voltage. Refer to a shop manual for the resistance wire specs. and where to find and check the regulator.

If only the fuel gauge is acting up, you can rule out the voltage regulator as a possible cause. The problem is either in the gauge itself, the sending unit or the wiring in between.

It makes no difference whether you start with the fuel gauge or the sending unit to begin your diagnosis. The best advice is to start with which ever one is the most easily accessible. If you have to drop the fuel tank to get at the sending unit, you can save yourself some effort by starting with the fuel gauge. But if the sending unit connector can be reached without having to drop the tank, then start there.

SENDING UNIT CHECKS

There are several ways to find out whether or not the sending unit is doing its job. One is to unplug the sending unit connector and hook your ohmmeter up to the sending unit terminals. Note the resistance reading. If it isn't within the range of minimum and maximum specs, you've found the problem. Replace the sending unit.

To tell how much fuel is in the tank, you can remove the gas cap and slide a piece of wire or dowel rod down the filler neck like a dipstick. Needless to say, you'd better not have a cigarette dangling from your lips when you do this or your widow will be reading your obituary in the newspaper. You don't need an exact indication, just an approximation. Or you can drain the tank or fill it up, checking the resistance readings before and afterwards to see if the sending unit makes the appropriate response.

Another alternative is to remove the sending unit from the tank and bench test it using an ohmmeter. Moving the float back and forth between the full and empty positions should produce a corresponding change in resistance. On Ford, for example, the sending unit should read between 8 and 12 ohms when the float is at the full position, and 60 to 86 ohms when it's at the empty position. No change, "skips" in the reading or readings that are out of range would all tell you a new sending unit is needed.

NOTE: "Bad gas" can occasionally cause fuel sending unit failures. The amount of sulfur in the fuel may corrode the contacts on the sending unit, causing dead spots or a loss of signal. Bad gas can also cause fuel pump failures.

Nonelectrical problems that can affect the sending unit include a leak in the float (it will sink or read low), a binding or broken float arm, or damage to the fuel tank that prevents the float from moving or reading the level accurately.

What if the sending unit checks out okay? Then the problem is in the wiring or the gauge. Corroded or loose wiring terminals, opens or shorts in the wiring can be isolated by checking wiring continuity. That leaves the gauge.

GAUGE CHECKS

Like the sending unit, the gauge can be checked several ways.

One way is to remove the sending unit from the tank, reconnect it, turn the key on and move the float arm up and down while watching for a change in the gauge reading. If the gauge hasn't responded within a couple of minutes, you've confirmed the fact that you have a problem. But you're still at square one because you don't know where it is.

To check the gauge, you can simulate input from the sending unit. A test box that simulates various resistance readings can be used in place of the sending unit to test gauge response. Such testers are designed primarily for the older resistance type gauges. They include: GM J-24538A, Chrysler C-3826A and Ford 21-0015.

If you don't have access to a tester for checking your fuel gauge, you can fabricate some resistor jumper wires by buying a 5 ohm and 80 ohm resistor at a local radio parts store. Connect each resistor to a fused jumper wire and use them to simulate high and low sending unit readings. The jumper wires (or test box) can be used either at the gauge or sending unit.

If the gauge doesn't respond appropriately when the resistance in the sending unit circuit is changed, check the hot terminal at the gauge to see if it's receiving voltage. If it is but the needle doesn't move, then it's time for a new gauge.

With resistance and magnetic gauges, you can also check the gauge's internal resistance with an ohmmeter. You should generally find somewhere between 10 to 15 ohms resistance. No resistance would indicate a short in the gauge while very high resistance would indicate an open.

ELECTRONIC INSTRUMENT CLUSTER SELF-CHECKS

On many late model cars, the instrument cluster can be put into a self-diagnostic mode to check its internal circuitry. See your owners manual for details, or the factory service literature (OEM website or ALLDATA).

On a late model Ford Mustang, for example, you can initiate the diagnostic self-check by pushing and holding the odometer reset button, and turning the key to the ACC (accessory) position. When the word "TEST" appears in the electronic odometer display, release the trip reset button. All of the analog gauge needles will then sweep from minimum to maximum. When the word "GAGE" appears, push the trip reset button again, and all the digits in the odometer display will light up. When the word "BULB" appears, push the reset button again to illuminate all the bulbs and warning lights in the cluster. Keep repeating this, and after three or four cycles the odometer will display "DTC" followed by any code numbers for faults it has detected.

Here is a list of the Ford instrument cluster codes:

9202 Fuel sender open circuit
9204 Fuel sender short to ground
9213 Anti-theft number of programmed keys is below minimum
A103 or 9232 Antenna not connected-defective transceiver
9317 Battery Voltage high
9318 Battery voltage low
9342 ECU is defective
9356 Ignition run circuit open
9364 Ignition Start circuit open
9600 PATS Ignition Key Transponder Signal is Not Received - Damaged Key or non-PATSKey
9601 PATSReceived Incorrect Key-Code from Ignition Key Transponder (unprogrammed Encoded Ignition Key)
9602 PATS Received Invalid Format of Key - Code From Ignition Key Transponder (Partial Key Code)
9681 PATSTranceiver Signal is Not Received (Not Connected, Damaged, or Wiring)
A139 PCM ID does not match between Instrument Cluster and PCM
A141 NVM Configuration Failure (No PCM ID exchange between Instrument Cluster and PCM)
A143 NVM memory failure
5284 Oil Pressure Switch Failure
D027 SCP Invalid or Missing Data for Engine RPM
D041 SCP Invalid or missing data for Vehicle Speed
D043 SCP Invalid or missing data for Traction Control
D073 SCP Invalid or missing data for engine coolant
D123 SCP Invalid or missing data for Odometer
D147 SCP Invalid or missing data for vehicle security
D262 Missing SCP message.

If an electronic gauge cluster is not reading correctly or display segments are dead, the only fix is to find a service online that specializes in rebuilding electronic instrument clusters (do a Google search for "automotive instrument clusters", or replace your old cluster with a new or used unit.