Testing the fuel gauge

[VegasFastback]

Does any one know how to short the fuel gauge? I'm trying to determine if its the gauge or the wiring that is bad. I have already verified that the IVR is working.

 

[BigPinoyHunk]

just ground one of the wires, and if the guage goes slammed to the right, then its g2g!!!

 

[72fastback]

do I ground the wires at the Tank?

 

[72HCODE]

the body is ground, so jump a wire to the tank itself you could also jump the connector with a paper clip.

 

[marks73]

That would check itall the way back.


Jump it across at the tank. This will test wiring and gauge all at one time.

 

[72fastback]

I unplugged the connector and used my test light. I put the alligator clip in one side of the connector and the needle in the other side. The test light was flashing. I asked my girlfriend to check if the needle went to full and she told me it only moved to E. So basically it moved from below E to E about 1/8 of an inch.

That is what it does most of the time. Sometimes it might go to 1/4 and it went to 1/2 a few times. Does this sound more like a gauge problem rather than sending unit.:huh:

 

[marks73]

O.K. I'm assuming ignition was on due to flashing test light. 2 questions, 1. How much gas is in the car?. 2. Is light flashing constantly or just once in a while while your moving the lead?

 

[c9zx]

Does your test light have an incandescent bulb or an LED? The LED will not act as a near short as an incandescent bulb would. If you use a jumper wire don't leave it connected very long as it will cause more current to flow through the gauge than it is designed to carry. The minimum resistance the circuit is designed to se is 8-10 ohms. Some Radio Shacks still carry inexpensive 10 ohm resisitors. Let us know what you find. Chuck

 

[72fastback]

the light seemed to flash at regular intervals the car was pretty low on gas but I assumed since the connector was not on the tank the amount of fuel didn't matter.

As for my tester It was not led

 

[marks73]

It's either wire or gauge. If you have the schematic for the gauge, Find the wire underthe dash and do the same test with your test light should run along the floor and up to the gauge cluster. Leave the wires disconnected at the tank. Don't skin the wire just penetrate it with the test light point with the other end to ground and see if it does the same thing. If it does, odds are you have insulation broke down in the wires in the cluster going to the trunk. Let us know.

 

[72fastback]

great thanks for all the info. guess I'll have to get under the dash to figure this out. I'll have to find the time to do it. But I will post here as soon as I do.

Once again thanks and a couple of rep points have been sent your way.

 

[marks73]

There is a 5 volt pulsating DC from the instrument cluster going from the gauge to the tank. The float in the tank varies a resistance that changes the current to the gauge. When the tank is at low level the resistance at the sender is high and gives you a low level reading. When the tank is full there is little resistance (just like a dead short to ground) and the level will read high on the gauge. If you are reading E that tells me you either have a gauge that that has a bad bi-metallic strip or you have 2 wires making contact in your harness. Odds are it's the gauge, but wires are easier to repair and get to vs taking your gauge cluster out. So we troubleshoot hoping for the simpler solution first.

 

[c9zx]

There is a connector in the trunk, left side you can try the ground/10 ohm resistor from there. If it moves the gauge upscale toward full you'll know it is the wire to the sender, connector at sender, sender, or the ground that should be screwed to the trunk latch support. Good Luck, Chuck

 

[marks73]

Excellant idea, since he's getting the voltage that will tell if it's gauge or sender. If it stays at E with the resistor the strip in the gauge is gone. Quality ::thumb::

 

[trainey]

One other thing to consider....because it happened to me. The float came off the arm that moves the resistor arm, thus it showed empty at all times, while there was some resistance measured on the sender, and I had proper voltage from the instrument cluster, and no loss of ground.

DOH~

 

[Jeff73Mach1]

Resurrecting this thread while I chase similar demons.

 

[scottyrocks]

One other thing to consider....because it happened to me. The float came off the arm that moves the resistor arm, thus it showed empty at all times, while there was some resistance measured on the sender, and I had proper voltage from the instrument cluster, and no loss of ground.

DOH~

My '73 shows empty all the time, too.  Considering that my gauge stopped working immediately after I put a new fuel tank in, I would think this is the first thing I should check.  It'd be amazingly coincidental that a problem occurred elsewhere along the line right at that moment.

 

[scottyrocks]

My float is attached. Everything looks kosher.

Is it possible for the enclosed resistor in the sending unit to be kaput? Is there a way to test it when it's on the bench (doesn't appear so, but what do I know)? Could it have died just from removal or installation (I was pretty gentle and didn't bang it around).

At any rate, I'm gonna put it back together and do some of the electrical tests mentioned in this thread. If anyone has any info regarding the enclosed resistor, please let me know.

 

[vintageman]

Did you add a bend in the float rod? Most of the time it needs to be bent a little. I would probably test the float with the key on, have the arm fully extended, and check gauge, then lift the arm on the float to the top and see if the needle moves to full. If the gauge goes up, then add about 5 gallons of fuel in the tank, put a slight bend in the float arm, then see if you are near a 1/4 tank.

I will be going through this same thing in the next few months, let us know how it goes. Good luck

Tom