Fuel gauge and sender operation and diagnosis

One of the most fielded questions on V8 seems to be, "my fuel gauge stopped working! What do I do??". So I thought I would give a synopsis of how a fuel gauge and sender works. You would be surprised how simple the circuit is. So lets jump right in..

Here is the fuel gauge and sender out of a 70-72 Skylark/ GS-


The sender has a black wire coming off it that is the ground. The eyelet comes out from in between the trunk floor and the tank and is screwed to the sheet metal of the body. You can see the "sock" or strainer for the fuel pick up. Also the float arm and float. On the top of the sender, not shown is a terminal that the tan wire connects to that goes to the gauge.

The sender works by supplying the gauge with a resistance reading. A wiper is connected to the float arm and slides along this rheostat strip. If the wiper is close to the grounded side of the strip, there is less resistive material in the path of the current, so the resistance is small. If the wiper is at the other end of the strip, there is more resistive material in the current's path, so the resistance is large. This one is clearly stamped "90 ohms"





So, with your digital volt ohm meter set to ohms, and the sender float arm in the full position, the sender should read 90 ohms-



And in the empty position, it should read zero or close to it-

 

Now lets hook up the gauge and sender. Since Im doing this in my office, Im going to use a 12 volt power supply to power the fuel gauge. On the back of the gauge, the pink wire supplies power to the gauge. The tan wire goes to the sender. The case of the gauge is grounded to complete the circuit.



So the hook up is as follows- pink to 12v, negative to the metal case and the tan tire to the top of the fuel sender. Now we have a working fuel gauge and sender






So now with the float arm in the full position, the gauge reads full-



And with the float arm in the empty position we naturally read empty on the gauge-



And just for laughs, we'll put the float arm in the middle. And the gauge read 1/2 full. So everything checks out -

 

A common issue/ complaint is that the gauge reads past full. The first thing you want to do is halve the problem. In other words, we want to find out if the problem exists at the gauge or at the sender. The tan wire from the sender passes through the trunk. If you pop the trunk and look near the trunk latch, you'll find the tan wire with a black "Packard 56" connector. Disregard the crusty trunk. Pull that connector apart.




On my desktop set up it looks like this-




Take the tan wire that's going back to the gauge and ground it to the trunk latch. You may need a jumper wire to make the connection. Your now supplying zero ohm to the gauge. By doing this, your basically fooling the gauge into thinking the tank is empty. Turn the ignition to the on position and watch the gauge. So in this picture we have the gauge reading past full. In the foreground is the tan wire we disconnected in the trunk



Now were going to ground the tan wire in the trunk to ground. In this case, we're using the case of the gauge. You'll be grounding it to the trunk latch or any ground you like. The gauge reads empty-



So in this case, we have jumped out the sender and taken it "out of the loop". We have now determined that the gauge and all the wiring from the trunk forward is operating correctly. The next thing you will want to do is check the ground wire from the sender. Its common for the eyelet that is screwed to the sheet metal under the car to deteriorate. You'll want to make sure that black wire has a good ground. You may have to replace the eyelet with a crimp connector, replace the screw and or sand down the area of rust to gain a good ground. Then retest. If the gauge is still non reactive, chances are you have a bad sender

 

Now, if the gauge doesn't move or react when grounding the tan wire in the trunk, the issue is forward of that connection. In that case, we will need to be more invasive and get behind the dash to access the back of the fuel gauge. Next step would be to ground the tan wire at the back of the gauge and turn the ignition to the ON position.

-If the gauge now goes to empty by grounding the tan wire at the back of the guage, you have a break in the tan wire somewhere between the trunk and the gauge. Easiest way to repair that would be to run a new tan wire piggybacked to the harness to the trunk

-If the gauge is STILL non reactive, and you have verified the pink wire has 12v and the case of the gauge has a good ground, your fuel gauge has failed.

So there is it is in a nutshell.

 

Its common for the eyelet that is screwed to the sheet metal under the car to deteriorate.

Where exactly is this located? I have a feeling this may need some cleaning to make my gauge operate correctly again.

 

Get under the car and take a look where the sender lines exit on top of the tank. you should see a black wire exiting from on top of the tank as well. That's the ground wire. If you follow it, it should ground to the left side gas tank brace

 

Is the gauge even capable of going past 3/4's? Disconnect the sender wire and see of it goes past full. It should if the gauge is working correctly. Then ground the wire and see of the gauge goes to zero. If so, the gauge is working. When the tank is full, the sender should test to 90 ohms or close to it. And when its empty, it should be near zero ohms. So it would be easy to compare the ohms reading against the reading on the gauge. All the gauge is doing to reading ohms.

So if 19 gallons (a full tank) is 90 ohms, that's 4.73 ohms per gallon. So with 7 gallons in the tank, the sender should read around 33 ohms or thereabouts. Its not an exact science but you can get close

 

I did try this last year and it worked briefly.

Here is the location, as though you're standing under the driver's side axle tube, facing the back of the car.



And here is the assembly manual diagram.

 

A Body assembly manual but I can't imagine the location is much different.