I have a 1970 nova. the gas gauge is not working properly, when i disconnect the sending unit wire and ground it the gauge will move to full and back. with sending unit on work bench i check the ohms readings and get 0 with float down and 92 with float up. when i connec sending unit to harness the gauge moves but with float all the way up i get a reading just under 1/2 tank. im lost any help would be greatley appreciated. thx tinman t

It has to be the sender unit in the tank, or the gauge in the dash, or the wiring between them. In most cases, it is the sending unit-they are quite fragile often. Usually best to replace them (not very expensive-just difficult to get to.) The sending unit will be located with the fuel pump inside the tank. They are removed as one single unit.
There is a way to test if the dash gauge itself is working properly. You have to find the wire going into the fuel tank that is from the gauge, then with key on, ground that wire to the frame-you can use a jumper wire into the fuel tank electrical connector. When the wire is grounded, the gauge will definitely move to one place (either half full, or full-but it moves and stays there). If you see that, you know the sending unit in the gas tank is bad. Do not ground that wire for more than a couple of seconds, it is lo0w voltage, but it may cause a shorting problem.

If your other dash gauges work, which eliminates the gauge fuse, what you need to do first is determine whether it's the "sending unit" which is in the gas tank. This may not be easy as you may have to lower the gas tank to get to the wires that attach to the sending unit. If you're lucky, the wires may have a connector visible close to the tank. There will be one or two wires. Disconnect them, and then have someone turn the ignition on while they watch the gas gauge. While they watch, ground the wires one at a time to a good, relatively clean metal part of the frame. You may need to use a length of wire connected to a good ground on one end, and touch the other end to each tank wire one at a time. If you dash gas gauge is good, when you ground one of the wires the gas gauge will go all the way to the "full" limit.
If that happens, the gas-tank sending unit is probably bad. If there are two wires going to it though, you need to make sure neither one is broken somewhere. If the gas gauge does NOT go all the way to "full", then you have either a bad wire or a bad gauge.

Well friend, you know the dash gage is good cause that was a proper test procedure. Since your not getting proper readings it has to be the sending unit. Pull it apart and clean the rheostat with a pencil eracer and light pressure, the assemble it and try it again. if still nfg, replace the sending unit. they're cheap from any auto parts store.

ground out the power wire from the gauge and it it moves to full and back to empty as you touch and remove the wire , the gauge is ok
then check the grounding for the sender unit in the tank

The fuel level indicating system is a magnetic type system, which consists of the sending unit located in the fuel tank, an anti-slosh module located on the back of the instrument cluster, and a fuel gauge located in the instrument cluster. The sending unit changes resistance according to the level of fuel in the fuel tank, which varies the current flow through the gauge.The pointer position varies proportionately to the current flow. In this system, the sending unit resistance is low (22 ohms) when the fuel level is low and high( 145 ohms) when the fuel level is high. The pointer of the magnetic gauge remains in position when the ignition is turned to the Off position. On some models, an anti-slosh module is used to dampen out fluctuating fuel signals from the sender. Some vehicles are equipped with a low fuel warning indicator. The anti-slosh module will also actuate the low fuel indicator when the fuel level in the tank reaches 1/16 to 1/8 full.


Do your other gauges/warning indicators work properly with the ignition on? If not, check for power at the fuse panel/fuses.

If they do then partially remove your cluster with connector still installed and check for voltage at cluster connector and gauge. Check continuity of cluster and gauge for ground.

Sometimes the anti-slosh module (if installed) goes bad in the Grand Marquis but not likely.

Sender troubleshooting:

1. Perform test with resistors as follows:

1. Disconnect wiring connector at sender unit, then connect resistor between gauge lead and a suitable ground.
2. Turn ignition switch to On position.
3. With 145 ohm resistor, the gauge pointer should contact the Full mark.
4. With 22 ohm resistor, the gauge pointer should contact the Empty mark.

1. Perform test with fuel sender of known quality as follows:

1. Turn ignition to Off position.
2. Disconnect wiring connector from sender and connect it to test sender.
3. Move float rod away from fuel filter against Full stop position. Wait approximately 30 seconds, then turn ignition switch to On position. The gauge should read on or above the Full mark.
4. Move float rod toward fuel filter against Empty stop position. Turn ignition switch to Off position. Wait approximately 30 seconds, then turn ignition switch to On position. The gauge should read on or below the Empty mark. If gauge works properly replace sender, which involves dropping fuel tank.
   

It may be the sending unit in the tank that needs replacing. Here's how you can check: Turn the key on and disconnect the sender unit at the gas tank. Ground the sender unit's wire with a jumper wire from the truck's harness to ground. Watch the gauge. If the gauge on the dash is good, grounding the sender wire will cause the gauge needle to react and should move to full. This would indicate a new sending unit is required. If the gauge doesn't move when the sender unit is grounded, the gauge is bad and needs replacing. Pull the instrument cluster out to replace the gas gauge. Likely any old ford gas gauge might work-back then, there were not so many production changes.

Ensure that all of the wire connections are clean, secure and corrosion-free.

2 Check your fuel gauge fuse if the gauge's indicator stays on empty when your turn on the ignition. If the fuse is good, join the sending unit connector to ground. Replace the sending unit if the gauge indictor moves to the full line.

3 Ground the terminal located on the back of the fuel gauge with a jumper wire if the indicator doesn't move. If it does move, consider replacing the wire between the fuel gauge and the sending unit. A poor ground is frequently responsible for fuel gauge problems.

4 Release the wire that connects to the fuel tank's sending unit. Confirm that there's no worn insulation that would permit wire contact to the body or frame. A fuel gauge reading of full generally indicates a problem with the tank unit, not the wiring.

Dont we wish it be full all the time but gas in it. Okay for the sending unit reading full its getting a full ground reading,Key on engine off. does it move and goe down. It should if it doesnt the problems with the gauge itself. 2 test disconnect the wire harness, did the gauge go down. the problem will be in the sending unit can be a hung float(Rare).

There are three possibilities. The gauge could be defective. The instrument panel voltage regulator could be bad or not grounded or (most expensive) it might be the sending unit.

Voltage from the battery goes through the ignition switch, through a device in the instrument cluster called a voltage regulator (though it is really misnamed), through the gauge, through the wiring to the gas tank, through the sending unit in the gas tank to ground.

The sending unit is an electrical wiper that slides across a peice of resistance wire. the movement of the float moves the wiper. If the sending unit no longer makes electrical continuity, the gauge will read zero. If the sending unit wiper is shorted out, the gauge will read full.

Try grounding the wire at the tank to see if the gauge maxes out, then disconnect it to see if the gauge zeros out. If all that happens, your problem is the sending unit in the tank. If none of that happens, your problem lies closer to the front of the car. While you are under there, you should measure the resistance from the terminal on the tank to ground, just to verify that the sending unit makes some electrical contact.

If the other gauges work OK (temp, oil pressure) then the "voltage regulator" is probably working fine, and the problem is the gauge itself.

If you have no other gauges (excluding speedometer, tachometer and battery voltage meter -- they work differently), then try this:

Put a voltmeter on the part of the fuel level gauge that is supplied by the "voltage regulator." The voltage there should regularly cycle between near zero to 10 or 12, back to near zero, etc. If you get that cycling, then it is most likely the gauge.