gas gauge stopped working

[dodj]

Here's a basic question about the tank grounding.  Where is the intended ground path.

Inquiring minds want to know.....   

Thanks,

In relation to the fuel gauge,
The intended ground path is from the metal of the sending unit to the body (ground), and from there it gets to the -ve of the battery. The sending unit is a variable resistor and the changing resistance to ground is what influences the gauge needle.
The grounding of the sending unit has nothing to do with grounding of the fuel lines or the tank. The reason the sending unit has it's own ground wire is because it is effectively isolated from ground by gasketing and rubber hose.

[spamtank]

In relation to the fuel gauge,
The intended ground path is from the metal of the sending unit to the body (ground), and from there it gets to the -ve of the battery. The sending unit is a variable resistor and the changing resistance to ground is what influences the gauge needle.
The grounding of the sending unit has nothing to do with grounding of the fuel lines or the tank. The reason the sending unit has it's own ground wire is because it is effectively isolated from ground by gasketing and rubber hose.

I should have specified that my vehicle is a 70 Challenger SE.  I'm pretty sure that there is only one wire running to the sending unit which connects to the insulated, threaded stud.  It's been 6 months since I installed my tank, so I'm going to have to verify that there is no ground wire running.  And the electrical diagram only shows G4-18DBL.  Maybe I'm missing a wire.

[Glennster]

I'm going to run a GOOD ground from the sending unit to the frame.
I do not know a good way to test the sending unit while it is in the tank. The ones I've tested at work are out of the tank and I can move the floater to check it.
 Do you think I could hook up test wires, then shake the car to get the gas to slosh around and move the float a little. Could that show me anything?

[Jamiez]

The sending unit is just variable resistor the set up is simply:

+----------------------Gas gauge--------------------------------------+
|                                                                                              |
|                                                                                              |
+Wire to sending unit  -------- Sending Unit ---------- Ground +

If you have a variable resistor (potentiometer) handy you could wire it like this:

+----------------------Gas gauge--------------------------------------+
|                                                                                              |
|                                                                                              |
+Wire to sending unit  ------potentiometer  ---------- Ground +

By adjusting the variable resistor (potentiometer) the needle on the gauge should move.  If that is the case then you know that the wiring into the tank, and the gauge itself all are functioning correctly.  If it doesn't, then you have a problem unrelated to the sending unit, such as a bad wire going to the sending unit an ungrounded fuel gauge, etc. 

If everything else checks out, that leaves the sending unit as the problem.

Once you know that there are really only 2 possibilities:
1) Sending unit is not grounded
2) Sending unit is non-functional

Using a multimeter, you should be able to set it to read resistance (aka Ohms).  Put one end on a good body ground (frame rail for example) and the other end on the sending unit where it should connect to the ground strap.  If this reads very low Ohms, then the ground is good and the unit itself is bad.  If this reads high ohms/overload/etc then the sending unit is not grounded.

If the unit is grounded and still doesn't work...but the gauge works with a variable resistor...you'll have to drop the tank and pull the sender. 

[Glennster]

I can't believe I didn't think of this .....
I have a like new 5/16" sending unit hanging on the wall. I should have tested with it as soon as I lost the gauge......
I hooked it up and the gas gauge went right uo!
Soooooooo..... It's time for a new sending unit. I think this one is less than a year old....BUMMER!
THANKS again for all the help!!!!!

[dodj]

I should have specified that my vehicle is a 70 Challenger SE.  I'm pretty sure that there is only one wire running to the sending unit which connects to the insulated, threaded stud.  It's been 6 months since I installed my tank, so I'm going to have to verify that there is no ground wire running.  And the electrical diagram only shows G4-18DBL.  Maybe I'm missing a wire.

While I'm not familiar with the SE's low fuel light relay and how it is wired into the cct, if you only have one wire connected to the threaded stud on the sending unit, you must be missing the ground wire connection. That one wire on the sending unit still needs an electrical path back to the -ve of the battery. If there is no ground connection, you are open circuited. I remember reading a thread where you said your low fuel light blinked from time to time. Does the gauge work at all?

[Glennster]

Dodj,
yes the gauge does work. When I wired up my extra sending unit, the gauge worked great. I then hooked a GOOD ground to my existing sending unit to double check that connection.  The gauge did not work, proving that I have a bad SU.
 
Does anyone know of a SU that has two 3/8" lines going in to it?? I would like to go to a set up that letts me recirculate gas , I have the 12V803-BP regulator to go with my Holley Blue pump....

Is it possible to change a SU without dropping the gas tank???

[tman]

You do not need to drop the tank, just make sure the gas level is below the SU opening of the tank. 

[JayBee]

It could just be the float too. My "new" sending unit went bad (read empty all the time) and I found that the float developed a hair line crack that allowed gas into it. Apparently this has happened to other members too. Someone on here found that a certain Ford float works but I can't find the thread.

[Jamiez]

I believe you are talking about this thread by hooD

www.cuda-challenger.com/cc/index.php?topic=85797

Modern fuels don't play nice with a lot of plastics.  A brass float is usually a better option if available.

[dodj]

Does anyone know of a SU that has two 3/8" lines going in to it?? I would like to go to a set up that letts me recirculate gas , I have the 12V803-BP regulator to go with my Holley Blue pump....

I believe the return lines are 1/4" or maybe 5/16. Should be big enough for recirc?

[brads70]

I believe the return lines are 1/4" or maybe 5/16. Should be big enough for recirc?

I just installed a new gas tank ( bought at Carlisle) and a new 1/2" all stainless sender from Dave at Roseville. It has a 1/4" return line on it.  I don't think it's big enough for my set up so I welded a 8AN return bung on the filler tube while I had the tank out.  I'll be plugging the 1/4" return line.

[dodj]

I don't think it's big enough for my set up

What determines how big a recirc line is big enough? 

[spamtank]

While I'm not familiar with the SE's low fuel light relay and how it is wired into the cct, if you only have one wire connected to the threaded stud on the sending unit, you must be missing the ground wire connection. That one wire on the sending unit still needs an electrical path back to the -ve of the battery. If there is no ground connection, you are open circuited. I remember reading a thread where you said your low fuel light blinked from time to time. Does the gauge work at all?

Thanks for your help guys.  I don't want to hi-jack this thread with my problem.  That said, my gauge works, but it was off by about 1/8 - 1/4 tank.  The light was on constantly.  From the service manual, I discovered that the black wire from the regulator should have been replaced with the blue wire from the low fuel relay.  Once I fixed that, the low fuel light blinked on and off with about a 10-20% duty cycle about every 30 seconds while sitting still.  And while driving over bumps it exhibited what I would describe as an intermittent.  I discovered the LFE light and fuel gauge work in the accessory position of the ignition switch.  So I'm gonna have my wife watch the light while I try to move some "groundy" things around back by the tank.  This might turn something up.  Since the low fuel relay contains thermally actuated bi-metalic switches, I think the time it takes to register a change if fairly slow, but maybe the grounding of the relay itself has some affect.

There are a lot of electrical connections that are returned through the chassis, and don't have separate ground wires.  The dome lite, oil pressure sending unit, horn, etc.  I think the fuel gauge is one of them.  But I'll double check this weekend.

I'll copy this over to my thread so I don't drag this off on a tangent from the OP.

[brads70]

What determines how big a recirc line is big enough? 

I'm using a Mallory electric pump and they say as big as the supply line or bigger?