Fuel gauge not working.

[stdyhand]

My fuel gauge isn't working on my 91 GT. It was like this when I got it. I have been trying to find out why it's not working. I checked the two wires by the fuel tank that run to the sending unit. When I touch the wires with the engine running, I get no voltage. From what I've read I should have 12 volts. I am getting zero.

Also these two wires should NOT be connected, right? The reason I ask is because when I use my meter to check for continuity between the black and yellow wire I get the steady beep sound to show those two wires are connected somewhere.

Does this mean these two wires are shorted or does it mean the anti-slosh module is bad?

I read a post by Jrichker on this forum from the past that said not to short the black and yellow wire together to peg the gauge because it could fry the voltage regulator. Well if these two wires have continuity between them, does that mean it is already fried?

 

[stdyhand]

Oh, the fuel gaue never moves. Whether the car is on or off, it never moves. It always shows 3/4 of a tank. When I turn the key on, it doesn't cycle up and then back down like the other gauges.

 

[jrichker]

Fuel Quantity gauge troubleshooting 87-93 Mustangs

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

Troubleshooting the gauge and sender circuit:
Since the sender uses a variable resistor, sum the resistor values of 22 Ohms (empty value) & 145 Ohms (full value). That gets you 167, which you divide by 2: that gets you 83.5. So in theory, 83.5 ohms is 1/2 full. A trip to Radio Shack for the closest combination of resistors to make 83.5 ohms gets you one 68 Ohm (Catalog #: 271-1106) + one 15 Ohm (Catalog #: 271-1102) for a total of 83 Ohms at the cost of $2 plus tax. Wire the resistors in series to make a resistor pack and cover it with heat shrink tubing or electrical tape. The 83 Ohms is close enough to the 83.5 Ohm figure that it shouldn't matter. Disconnect the electrical connector shown in your for the tank sender unit. Connect one end of the resistor pack to the yellow/white wire on the body side fuel sender electrical connector and the other end of the resistor pack to ground. Make sure nothing is touching that isn't supposed to and turn the ignition switch to Run. If I am correct, the fuel gauge will read 1/2 full, or very close to it. If it does not, then the odds are that the gauge or anti-slosh unit are bad.

How and why the test works…
Most of the fuel gauge failures give a stuck on full or stuck on empty as a problem symptom. Using a resistor combination that mimics 1/2 tank allows you to decide if the gauge and anti-slosh module are the problem source.

If the gauge reads about 1/2 tank with the resistor combination, that points to the sender as being the culprit.

If the gauge reads full or empty with the resistor pack in place of the sender, then the gauge or anti-slosh module is at fault.

Fuel gauge sender testing and replacement
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.

Anti-Slosh module pictures courtesy of Saleen0679

Copied from DrBob

I worked on an 88 Mustang today that had similar symptoms. Short version, I took the “anti slosh module” off of the back of the instrument cluster and replaced the electrolytic capacitor. Fixed it for $1.39 with a part from Radio Shack.

In an attempt to help other folks, here’s the long version.
Remove the “anti slosh module” located on the back of the instrument cluster. There was a single Torx screw holding mine to the cluster.

Find the electrolytic capacitor. It will be the largest, 2 wire component on the board. The capacitor may have a red or blue plastic wrapper on it. Mine was red.

The wrapper should have printing on it. Look for printing that looks something like this:
100uF+25V

The “100uF” tells you this is a 100 micro Farad capacitor. The “+25V” tells you the capacitor is rated for 25 Volts. Yours may be different. You may use a higher voltage part but don't use a lower rated voltage part. If you use a lower voltage part the capacitor might open later on down the road or it could be as bad as catching fire.

If you can’t find the printing you’ll need to remove the part. You have to anyway so nothing wasted. However pay close attention to the way the capacitor is oriented on the board.

One end of the capacitor will be bare metal with a wire sticking out. The other end should have some sort of insulation over it with a wire sticking out. The bare metal end is the negative end while the insulated end is the positive end. Pay attention to which end is connected to which hole on the board.

Get a replacement part. I got mine at Radio Shack, $1.39. Here’s the info:
100µF 35V 20% Axial-Lead Electrolytic Capacitor
Model: 272-1016 | Catalog #: 272-1016

 

[stdyhand]

I'm confused on something. The two wires that appear to be coming from my sending unit are a solid yellow wire and a solid black wire. I don't see a yellow/white wire.

I also don't see a solid yellow or yellow/white wire behind the instrument cluster. I pulled the cluster in about 10 minutes and I see a yellow/red wire, but not the others. I checked voltage with the key on and it did show 12 volts at the connector, but that's not what I'm getting back at the tank.

I'm wondering if the previous owner switched some wires or spliced some in somewhere.

I also pulled the anti slosh module out. It looks ok. It doesn't have any leaking or swollen capacitors. I used my multimeter on the large red 100uf 25v capacitor and it does charge up. I shorted the pins to discharge the capacitor and then checked the resistance and I watched as it slowly climbed to infinity. From what I read elsewhere about testing capacitors, that is what it should do and it means the capacitor is good and functioning as intended. Just in case though I purchased another capacitor for 1.50 to test it and it did the same thing. So I don't believe the voltage regulator is bad.

I haven't figured out how to check the ohms at the cluster yet because I am not certain on the wire colors since they seem to differ from the tank to the cluster. I think I'm just missing something so I am going to go back out tomorrow and take another look.

 

[jrichker]

I'm confused on something. The two wires that appear to be coming from my sending unit are a solid yellow wire and a solid black wire. I don't see a yellow/white wire.

I also don't see a solid yellow or yellow/white wire behind the instrument cluster. I pulled the cluster in about 10 minutes and I see a yellow/red wire, but not the others. I checked voltage with the key on and it did show 12 volts at the connector, but that's not what I'm getting back at the tank.

I'm wondering if the previous owner switched some wires or spliced some in somewhere.

I also pulled the anti slosh module out. It looks ok. It doesn't have any leaking or swollen capacitors. I used my multimeter on the large red 100uf 25v capacitor and it does charge up. I shorted the pins to discharge the capacitor and then checked the resistance and I watched as it slowly climbed to infinity. From what I read elsewhere about testing capacitors, that is what it should do and it means the capacitor is good and functioning as intended. Just in case though I purchased another capacitor for 1.50 to test it and it did the same thing. So I don't believe the voltage regulator is bad.

I haven't figured out how to check the ohms at the cluster yet because I am not certain on the wire colors since they seem to differ from the tank to the cluster. I think I'm just missing something so I am going to go back out tomorrow and take another look.

Unfortunately I only have a wiring diagram for an 89 Mustang. Most of the wire colors are the same up to the 1990 model year. The circuit parts and wiring connections are the same from 87-93. The wire colors may be different on the sender, but you can bet that the black wire is signal ground and the yellow wire is signal.

The next step is to use the resistors to mimic 1/2 full tank. You will need to disconnect the sender harness and place the resistors across the body side connector pins.

 

[stdyhand]

Unfortunately I only have a wiring diagram for an 89 Mustang. Most of the wire colors are the same up to the 1990 model year. The circuit parts and wiring connections are the same from 87-93. The wire colors may be different on the sender, but you can bet that the black wire is signal ground and the yellow wire is signal.

The next step is to use the resistors to mimic 1/2 full tank. You will need to disconnect the sender harness and place the resistors across the body side connector pins.

I can see how the resistor pack would work for some, but for me I wouldn't see a change because my fuel gauge is already showing 1/2 a tank. It always says that. So the resistor pack wouldn't show me any different. If it were pegged at full or empty I can see how it would move. The resistors however are cheap to pick up so I'll probably go ahead and get them and try it, just in case I'm wrong.

Is the fuel gauge suppose to move when you turn the key on? I mean does it cycle like the voltage and temperature gauge? The reason I ask is because my fuel gauge never moves when I turn the key on or off. My temperature gauge moves when I turn the key on, even though I do have it disconnected since I'm running an aftermarket gauge. But the other gauges move when I turn the key and then settle at the range the engine is showing.

 

[jrichker]

The fuel gauge may move a little, but not much.

While you are doing the testing with resistors, try 22 ohms (empty tank sender resistance) and 145 ohms (full tank sender resistance). If you get accurate readings with those resistor values, the gauge and anti-slosh module are good.

 

[stdyhand]

Well I think I might have found the problem. I was looking at the back of the cluster and checking all that copper lining that connects the gauges and I found the copper power connection was broke in half to the fuel and oil pressure gauges.

It's that copper film that carries the 12v source to the gauges. I used a few strands of copper wire to bridge that gap and sealed it all back up. After I plugged it back into the cars harness and turned the key over, the fuel gauge went completely below empty.

It might be right because I think the car is actually close to an empty tank.

I am going to get some fuel tomorrow and see if the gauge rises at all. If it does I know it is fixed, but if not then I'll be pulling the tank. I already picked up the resistors so I'll already have them if I do end up pulling it.

 

[stdyhand]

Update on this subject. I got some fuel this morning and my fuel gauge still shows empty. I check the OHMS from the wires behind the cluster. I received 39 ohms which is about right because I only added 2-3 gallons of fuel just in case I had to pump it all back out for dropping the tank.

So I am getting 39 ohms from the sending unit but the gauge shows empty since I fixed the broken copper stripping. The gauge does move if I peg it, so that only leaves the Anti slosh module left.

I have a new capacitor already so I am going to solder it on and see if that fixes the problem.

 

[stdyhand]

I changed the red capacitor on the voltage regulator and soldered a new one on. When I used a battery and resistors, I can get the fuel gauge to move up or down based on the OHMS applied, but it is not adjusting fully to the correct ohm level. For instance, when I apply 83 ohms which is about half a tank, it doesn't go up to half a tank. It only goes up to about 1/4.

The odd part is when I put it back in the car and started it, the fuel gauge again drops to zero and shows nothing at all. The fuel sending unit wires are reading 39 ohms but the fuel gauge says zero so I still suspect its the voltage regulator.

I tested ALL of the resistors on the voltage regulator. I researched the color band for all of the resistors and they all come up with correct readings except the large tan resistor. It's the resistor in the photo jrichker put up...its the tan resistor with the brown-red-brown-black-brown stripes.

According to my search, that resistor should be 121 ohms with a 1% tolerance. But the reading I got on it was 96 so it is well below spec. I tried to find that resistor and replace it, but no one has it. Radio shack didn't have one like that.

With what I've described, where would you go with this problem? Since the ohm reading from the sending unit is 39 ohms and the fuel gauge says it is empty, do think it is worth while to drop the tank or is it a safe bet to assume the regulator is the culprit?

 

[jrichker]

The parallel paths in the circuit will cause the resistor to read a lower value that it actually is. I got similar results to yours until I unsoldered the resistor and measured it's resistance. I got 121.4 ohms. With that in mind, replacing the resistor would probably not be productive.

If you do decide to replace it anyway, here's the source for it...
http://www.newark.com/jsp/search/br...ParametricAttributeId=&prevNValues=422+200625
will have it... The shipping will cost more than the resistor. Since you have a 1% tolerance, a resistor from 120-123 ohms will do just fine. The only problem I foresee is that resistors have a wattage rating that usually isn't marked, so it's a guesstimate of what the wattage really is. The larger a resistor is physically, the greater wattage it is. All the resistors on the anti-slosh module except that one appear to be 1/4 watt. My guess is that it is a 1 or 2 watt resistor. I would go for 2 watts or more...

 

[stdyhand]

My regulator looks like the one you put up in an earlier post. The resistor is tan colored with the stripes on it. I found the exact same color bands on a resistor at digikey's website. It is a 121 ohm 1% tolerance resistor but it is only rated at 1/4 watt and has a voltage rating of 400v. The dimensions looked very similar to the one I'm looking for, but the wattage part leaves me uncertain. I was thinking that the larger resistor was probably 2w as well.

Would it be safe to just get a 3W resistor? The wattage is what can be safely passed through the resistor anyhow, right?

 

[jrichker]

My regulator looks like the one you put up in an earlier post. The resistor is tan colored with the stripes on it. I found the exact same color bands on a resistor at digikey's website. It is a 121 ohm 1% tolerance resistor but it is only rated at 1/4 watt and has a voltage rating of 400v. The dimensions looked very similar to the one I'm looking for, but the wattage part leaves me uncertain. I was thinking that the larger resistor was probably 2w as well.

Would it be safe to just get a 3W resistor? The wattage is what can be safely passed through the resistor anyhow, right?

A 3 watt resistor would be OK. I would advise you to unsolder one lead of the resistor and recheck the resistance. There is evidently a parallel path for the electrical current that alters the resistance reading. It could save you some time and money.

 

[stdyhand]

I went ahead an unsoldered one end of the resistor and checked it again. It is now reading 121 ohms as it should. So apparently all of the resistors and capacitors are good, unless one of those small black resistors that are attached to the board is bad.

I left the voltage regulator out of the cluster and installed just the cluster in the car. When I started the car, the fuel gauge went up to 3/4 of a tank.

When I shut it off and install the voltage regulator (yes with battery disconnected) and restarted the car, again the fuel level gauge went to zero.

Can the fuel gauge work at all without the regulator? I though it was only meant for when the car was turning, climbing hills, etc.

I guess I'll just have to buy another regulator since the gauge obviously moves good when the regulator is out, but as soon as I plug it in it stops working.

 

[jrichker]

I went ahead an unsoldered one end of the resistor and checked it again. It is now reading 121 ohms as it should. So apparently all of the resistors and capacitors are good, unless one of those small black resistors that are attached to the board is bad.

I left the voltage regulator out of the cluster and installed just the cluster in the car. When I started the car, the fuel gauge went up to 3/4 of a tank.

When I shut it off and install the voltage regulator (yes with battery disconnected) and restarted the car, again the fuel level gauge went to zero.

Can the fuel gauge work at all without the regulator? I though it was only meant for when the car was turning, climbing hills, etc.

I guess I'll just have to buy another regulator since the gauge obviously moves good when the regulator is out, but as soon as I plug it in it stops working.

The regulator can supposedly be bypassed, and I suspect that all it takes is a jumper wire to do it. The simple matter of where the jumper wire goes is something that I have not given any thought to...

 

[stdyhand]

I decided not to bother with replacing the anti slosh module. I don't plan on selling this car and the whole gauge setup is very outdated. I decided since my speedo is off, the fuel gauge isn't working, and I am using aftermarket gauges for the oil pressure and water temp for better accuracy I am going to go ahead and do a florida 5.0 type setup.

Other than the cost factor of a new speedo, fuel level gauge and tachometer I think I'll be much happier with aftermarket gauges since they are easier to calibrate and more accurate.

It just doesn't make sense to me to pay $50 for a used voltage regulator to get my fuel level gauge working when I can buy a aftermarkget gauge for the same price.

I wanted to say thank to Jrichker for the tech help. It saved me a lot of time and unnecessary work plus money that I may have spent for something that may or may not have fixed the problem.