Nothing is as annoying as a fuel gauge that doesn’t offer an accurate reading, especially when the gauge indicates there’s still fuel in the tank when there really is not. Faulty readings can be produced by a bad sending unit, an issue with the gauge, or shorts, opens or weak connections in the wiring that links the two together. Lets see which one it is.
In 1904, the first float arm gas gauge showed up. A float mounted on a hinged arm moved a mechanical pointer on gauge on the outside of the fuel tank to show the fuel level. The idea was taken one step further when cars started to get better electrical systems by connecting the hinged float arm to a rheostat. This made an electrically operated fuel gauge to be installed in the vehicle’s dashboard. And that’s the basic operating principle that is still being used today.
There are basically three types of fuel gauges: analog resistance (used almost universally up through the early 1980s), analog magnetic (introduced in the ’80s) and digital/graphic electronic gauges (also introduced in the ’80s and used to this day).
ANALOG FUEL GAUGES
Analog fuel gauges make use of a heated bimetal strip to move the indicator needle on the fuel gauge. The amount of current passing through the gauge heats up the bimetal strip. The strip expands and knows how far the needle moves. Voltage is supplied to the gauge by a small voltage regulator in the instrument panel which reduces circuit voltage to about five volts. The voltage regulator also gives the temperature and oil pressure gauges.
The amount of current that passes through the fuel gauge is controlled by the ground circuit provided by the sending unit in the fuel tank. As the fuel level inside the tank goes up and down, the hinged arm that’s connected to the float rotates a rheostat. This changes the amount of resistance in the ground circuit which lets more or less current to flow through the gauge.
In Ford and Chrysler applications, the sending unit increases resistance as the fuel level drops and decreases resistance as the fuel level goes up. When the fuel tank gets empty, for example, resistance is high (around 73 ohms). High resistance lowers the current that flows through the fuel gauge, producing little or no movement in the needle. When the fuel tank is full, the sending unit has low resistance (around 8 ohms) so more current flows through the fuel gauge. This heats up the bimetal strip creating maximum needle deflection. Now the needle moves all the way to the full mark.
A shorted sending unit or a short in the wiring between the sending unit and gauge would lower circuit resistance causing the fuel gauge to read full. And with nothing to slow the amps, the circuit would probably overload and blow a fuse. An open in the sending unit or wiring, on the other hand, would keep the needle from moving at all and the gauge would read empty.
With General Motors analog resistance fuel gauges, the basic operating principle is the same but electrically opposite. Resistance in the sending unit goes down as the fuel level drops, and increases and the level goes up. When the tank is empty, the sending unit reads about zero ohms, and when the tank is full it reads about 90 ohms. Gauge operation is also the same with maximum needle deflection corresponding to minimum resistance in the sending unit. In this case, maximum deflection is needed to move the needle all the way over to the empty mark.
A short in the sending unit or wiring on a GM system would make the fuel gauge to read empty, therefore, while an open in the sending unit circuit would make the gauge read full.
WHAT ARE MAGNETIC FUEL GAUGES
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 produced by three coils. The coils are fed voltage through a terminal that’s often 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 gulp up to two minutes to respond to a change.
The sending unit that’s used with a magnetic fuel gauge is basically 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.
Digital gauges may not be as accurate as analog gauges.
The fuel level shown is a calculation based on the sending unit input.
ELECTRONIC FUEL GAUGES
Electronic fuel gauges makes use of vacuum fluorescent or LCD graphic displays to indicate the fuel level. Many also read E, 1/2 and F. Electronic displays often 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 power 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 gets an open or short in the sending unit circuit.
LOW FUEL WARNING LIGHT
Low fuel warning lights 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 acts 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 issue between the gauge and sending unit, or a poor ground connection.
If the temperature and oil pressure gauges are also affected, the issue 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 take out the instrument panel.
Confirm 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 gives 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 problem. The problem is either in the gauge itself, the sending unit or the wiring in between.
It makes no difference whether you begin with the fuel gauge or the sending unit to start your diagnosis. The best advice is to begin 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 begin with the fuel gauge. But if the sending unit connector can be accessed without having to drop the tank, then start there.
The fuel level sending unit inside the fuel tank
can sometimes stick giving a false indication to the fuel gauge.
It may read high or it may read low.
If it reads high, you might run out of gas!
FUEL GAUGE SENDING UNIT CHECKS
There are many 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 discovered the problem. Change the sending unit.
To know 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, inspecting the resistance readings before and afterwards to see if the sending unit makes the appropriate response.
Another alternative is to take out 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 give 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 stops the float from moving or reading the level accurately.
What if the sending unit seems great? Then the problem is in the wiring or the gauge. Corroded or loose wiring terminals, opens or shorts in the wiring can be found by checking wiring continuity. That leaves the gauge.
FUEL GAUGE CHECKS
Like the sending unit, the gauge can be inspected in 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 replacement in the gauge reading. If the gauge hasn’t responded within a couple of minutes, you’ve confirmed the fact that you have an issue. But you’re still at square one since you don’t know where it is.
To inspect 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. Link 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 does not respond appropriately when the resistance in the sending unit circuit is changed, check the hot terminal at the gauge to see if it’s getting 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 equally check the gauge’s internal resistance with an ohmmeter. You should generally discover somewhere between 10 to 15 ohms resistance. No resistance would show a short in the gauge while very high resistance would show an open.
ELECTRONIC INSTRUMENT CLUSTER SELF-CHECKS
On many late model vehicles, the instrument cluster can be placed on a self-diagnostic mode to check its internal circuitry