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Does anybody have an effective method for testing a fuel gauge?

When I turn the key on, the gauge goes to full all the time. I've installed a new float and checked wires for shorts, etc. I've also confirmed good ground for the cluster. However, I don't have a proper wiring diagram (I only have a '68 diagram). Anybody have one or can you tell me which colors go where (perhaps post an image)?

In advance, thank you.
 

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I doubt the wiring diagram changed much between 1967 and 1968.
What other diagnostics have you done? Things like:

  • Are you sure the sending unit is well grounded?
  • Are you sure the two wires going to the sending unit aren't reversed? What happens if you swap them?
  • When you say you "installed a new float" are you referring to the whole sending unit assembly?
  • If you pull the gauge wire off the sending unit, does the gauge still go to "full" with the key on?
 

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Discussion Starter #3
I doubt the wiring diagram changed much between 1967 and 1968.
What other diagnostics have you done? Things like:

  • Are you sure the sending unit is well grounded?
  • Are you sure the two wires going to the sending unit aren't reversed? What happens if you swap them?
  • When you say you "installed a new float" are you referring to the whole sending unit assembly?
  • If you pull the gauge wire off the sending unit, does the gauge still go to "full" with the key on?
Yes, well grounded.
Did not try reversing wires though will this evening. Makes sense given that one is for light and other for gauge.
Entire sending unit
I did not pull the wires off sending unit but will try that as well.

Thank you.
 

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Discussion Starter #4
One more thing - wiring diagram may not have changed but the colors of the wires are not the same as on the 68 diagram. I believe the wires on my car are original or at least the connectors are of original type - why I wanted to see a 67 diagram.
 

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Did not try reversing wires though will this evening. Makes sense given that one is for light and other for gauge.
If the wires somehow got reversed, you would get odd behavior at the gauge. You can't damage anything by reversing them.

I did not pull the wires off sending unit but will try that as well.
Yes, pull them off and then try grounding each in turn with the ignition on. Obviously when you ground the wire for the "low fuel" light, that light should come on. Then you know which wire controls the light. And, when you ground the wire for the gauge, the gauge either goes to "full" or "empty" (I'm forgetting which). And the gauge needle goes to the opposite end of the range with the wire disconnected. So that acts as a quick test that the gauge is working OK, which it probably is - most of the time, the sending unit is the issue.
 

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I've got a similar problem with my 1300 TI. The guage shows full, or very nearly full, until the tank is almost empty, at which point the red light comes on, and the needle abruptly moves to the left, towards empty. I tried replacing the sending unit with a new one from Centerline, but after wiring it up, the gauge was totally unresponsive. No action of any kind when I turned the ignition on. My suspicion was either a defective float/sender, or an electrical issue in the car. Strange business.
 

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Does anybody have an effective method for testing a fuel gauge?

When I turn the key on, the gauge goes to full all the time. I've installed a new float and checked wires for shorts, etc. I've also confirmed good ground for the cluster. However, I don't have a proper wiring diagram (I only have a '68 diagram). Anybody have one or can you tell me which colors go where (perhaps post an image)?

In advance, thank you.
You are testing my memory on wire colors etc (one is red the other white), but these are classic symptoms of ground problems on 105 cars. There are two wires from the gauge to the sender. One goes to the empty light and the other goes to the gauge. By connecting either one to the body either the light should come on or the gauge should read empty. Often people try to ground the sender using a screw on the gas tank. That usually doesn't work because of the gasket between the tank and the body. The ground should be directly into the body. If solidly connecting the gauge wire to the body does not read empty then you have a problem with the gauge. The sender unit should read between 0 ohms and 300+ ohms over it's travel. If that is not true then you have a wrong or bad sender.
 

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You are testing my memory on wire colors etc (one is red the other white), but these are classic symptoms of ground problems on 105 cars. There are two wires from the gauge to the sender. One goes to the empty light and the other goes to the gauge. By connecting either one to the body either the light should come on or the gauge should read empty. Often people try to ground the sender using a screw on the gas tank. That usually doesn't work because of the gasket between the tank and the body. The ground should be directly into the body. If solidly connecting the gauge wire to the body does not read empty then you have a problem with the gauge. The sender unit should read between 0 ohms and 300+ ohms over it's travel. If that is not true then you have a wrong or bad sender.
I have a feeling the answer to this question is not uncomplicated, but...how does one go about resolving the grounding issue, if that's what's causing the gauge to misbehave?
 

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I have a feeling the answer to this question is not uncomplicated, but...how does one go about resolving the grounding issue, if that's what's causing the gauge to misbehave?
There must be a ground wire between the sender unit and the body. Attach one end to one of the screws holding the sender to the tank and drill a hole in the body and attach the other end by screw to it. Don't try to use one of the tank to body screws (the only screws in the vicinity), they often don't make good body contact because of the gasket between the body and tank. The tank gasket is the heart of the problem.
 

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There must be a ground wire between the sender unit and the body. Attach one end to one of the screws holding the sender to the tank and drill a hole in the body and attach the other end by screw to it. Don't try to use one of the tank to body screws (the only screws in the vicinity), they often don't make good body contact because of the gasket between the body and tank. The tank gasket is the heart of the problem.
Thanks for the instructions. If that's the case, then the sender's definitely note grounded. I'll give it a whirl. Just got to figure out where to drill a hole...

Cheers!
 
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