Fuel Sending Unit Maybe?

try replacing the sending unit...if its not that go from there....

The red/yellow wire (power supply to gauge & sender) should have 12 volts when the ignition is in the start or Run position.

The next steps require dropping the fuel tank and removal of the fuel level sender. Here are some useful tips...

I have done the tank removal three times, and the main issues are getting the car up on jack stands and getting the gas out of the tank. DO NOT try to do this job without jack stands. Becoming a pancake is not part of the repair process.

Pumping out the old gas:
If the old pump still works, you can use it to pump the tank out.
1.) Separate the pressure line (the one with the Schrader valve on it) using the fuel line tools.
Look in the A/C repair section for the fuel line tools. They look like little plastic top hats. You will need the 1/2" & 5/8" ones. The hat shaped section goes on facing the large part of the coupling. Then you press hard on the brim until it forces the sleeve into the coupling and releases the spring. You may need someone to pull on the line while you press on the coupling.


Use a piece of garden hose to run from the pressure line to your bucket or gas can. Make sure it is as leak proof as you can make it. Fire and explosion are not part of the repair process...

2.) Jumper the fuel pump test point to ground.

Turn the ignition switch to the Run position. the fuel pump will pump the tank almost dry unless the battery runs down first.

Some 5 gallon paint pails lined with garbage bags are good to hold the gas. The garbage bags provide a clean liner for the pails and keep the loose trash out of the gas so you can reuse it. If you decide to use a siphon, a piece of 1/2" garden hose stuck down the filler neck will siphon all but a gallon or so of the gas.

Remove the filler neck bolts and put them in a zip bag. Disconnect the supply & return lines by removing the plastic clips from the metal tubing. If you damage the clips, you can get new ones form the auto part store for just a few dollars. I have used tie-wraps, but that is not the best choice. Then you remove the two 9/16" nuts that hold the T bolts to the straps. Put the nuts in the zip bag with the filler bolts. Pull the plastic shield down and away from the tank. Once the tank drops a little bit you can disconnect the wiring for the pump & fuel quantity sender.

The fuel gauge sender assembly comes out by removing a large metal ring that unscrews from the tank. There is a separate mounting/access plate for the fuel pump and fuel gage. You are supposed to use a brass punch to tap on the ring so that you don't make sparks. Look closely at the rubber O ring gasket when you remove the fuel gauge sender.
When you install the metal ring that holds the sender in place, watch out for the gasket O ring. Some RTV may be helpful if the ring is not in excellent condition.

The tank to filler pipe seal is a large rubber grommet. Inspect it for hardening, tears and damage. At $20 from the Ford dealer, it might be a good idea to replace it.

I used a floor jack to help lift the tank back in place. You may find that it is the only time you really can make good use of a helper.

All resistance measurements should be made with the power off.

Note from bstrd86 - 86 and older fuel tank sender units are 73 ohms empty, 8-12 ohms full.

The yellow/white wire will show a voltage that varies with the movement of the float on the sender unit. To test the sender, set your Ohmmeter or DVM on low Ohms. Then disconnect the sender and connect the Ohmmeter or DVM to the yellow/white and black wires from the sender unit. Move the float arm while watching the Ohmmeter or DVM. You should see the reading change from 22 to 145 ohms +/- 10%.

If the Ohmmeter or DVM resistance readings are way off, replace the tank sender unit.

Use extreme caution if you do the next step. Fumes from the gas tank can easily ignite and cause a fire or explosion.
With the sender unit out of the tank and connected to the body wiring harness, turn the ignition switch to the Run position. Move the float arm and the fuel gauge indicator should move. If you are very careful, you can use a pair of safety pins inserted in the connector for the yellow/white and black wires to measure the voltage as you move the float arm. The voltage will change, but I have no specs for what it should be.
Do not short the safety pins together or to ground. If you do, you may damage the anti-slosh module or crate a spark. A spark with the fuel tank open could cause a fire or an explosion.

If the voltage does not change and the tanks sender passed the resistance tests, the anti-slosh module or gauge is bad.

i had the same problem but mine was going all over the place randomly, i changed the sender unit and now it just says empty all the time lol

If the instrument cluster has been pulled out the anti slosh module may not be plugged in all the way and causing the problem.

2003 GT Mustang 4.6 SOHC Fuel Sending Unit Empty and Full Ohms values

Here is what I found concerning the fuel tank sending unit empty and full ohms values for a 2003 GT Mustang 4.6 SOHC engine. Also, included are instructions on checking your fuel gauge operation. Really good info. Hope this helps.


The sending unit on your vehicle should read 160 ohms empty and 15 ohms full. Here is supporting document from a ford tester that shows this.Hope this answers your question.

CHECK THE FUEL GAUGE OPERATION
Key in OFF position.
NOTE: The fuse must be removed to reset the fuel gauge timers. Failure to complete this step may result in erroneous test results.
Remove the central junction box (CJB) fuse 21 (5A). Wait 1 minute and reinstall the fuse.
Disconnect: Fuel Pump Module Assembly C463.
Connect 1 lead of the Instrument Gauge System Tester to the fuel pump module assembly C463 pin 8, circuit 29 (YE/WH), harness side and the other lead of the Instrument Gauge System Tester to the fuel pump module assembly C463 pin 5, circuit 327 (BK/OG), harness side.


Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Key in OFF position.
Set the tester to 160 ohms.
Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Key in OFF position.
Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Set the tester to 15 ohms.
Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Observe the fuel gauge. The fuel gauge should read F (full) or above.
Key in OFF position.
Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Set the tester to 160 ohms.
Key in ON position.
NOTE: Wait 1 minute for the fuel gauge to respond.
Observe the fuel gauge. The fuel gauge should read E (empty) or below.
Does the fuel gauge operate correctly?

GT Tommy said:

Here is what I found concerning the fuel tank sending unit empty and full ohms values for a 2003 GT Mustang 4.6 SOHC engine. Also, included are instructions on checking your fuel gauge operation. Really good info. Hope this helps.


The sending unit on your vehicle should read 160 ohms empty and 15 ohms full. Here is supporting document from a ford tester that shows this.Hope this answers your question.

CHECK THE FUEL GAUGE OPERATION
Key in OFF position.
NOTE: The fuse must be removed to reset the fuel gauge timers. Failure to complete this step may result in erroneous test results.
Remove the central junction box (CJB) fuse 21 (5A). Wait 1 minute and reinstall the fuse.
Disconnect: Fuel Pump Module Assembly C463.
Connect 1 lead of the Instrument Gauge System Tester to the fuel pump module assembly C463 pin 8, circuit 29 (YE/WH), harness side and the other lead of the Instrument Gauge System Tester to the fuel pump module assembly C463 pin 5, circuit 327 (BK/OG), harness side.


Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Key in OFF position.
Set the tester to 160 ohms.
Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Key in OFF position.
Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Set the tester to 15 ohms.
Key in ON position.
On vehicles equipped with an automatic transmission, apply the brake and move the transmission shift lever from P (park) to D (drive). Wait 10 seconds, and move the transmission shift lever back to P (park). Wait 30 seconds. On vehicles equipped with a manual transmission, apply the parking brake. Wait 10 seconds, and release the parking brake. Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Observe the fuel gauge. The fuel gauge should read F (full) or above.
Key in OFF position.
Wait 30 seconds. Do not change the tool settings or the ignition switch position until 30 seconds have elapsed.
Set the tester to 160 ohms.
Key in ON position.
NOTE: Wait 1 minute for the fuel gauge to respond.
Observe the fuel gauge. The fuel gauge should read E (empty) or below.
Does the fuel gauge operate correctly?

Click to expand...

The fuel gauge in a 87-93 Mustang is different from the 2003 Mustang. The instructions probably won't work the same way since the anti-slosh module and sender resistance are not the same. The body wiring is definitely different.

Mustang fuel tank sending unit empty and full ohms values

jrichker said:

The fuel gauge in a 87-93 Mustang is different from the 2003 Mustang. The instructions probably won't work the same way since the anti-slosh module and sender resistance are not the same. The body wiring is definitely different.

Click to expand...

jrichker, yes, you are right. Just thought I would post the info to let others know that the fuel systems do vary between the different years. So, just be aware of that. Good luck!