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Fuel Gauge Issue

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Jimjific View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jimjific Quote  Post ReplyReply Direct Link To This Post Topic: Fuel Gauge Issue
    Posted: Sep/15/2022 at 8:07am
Hello all,

Here are is what I know.
- 1972 Gremlin 232 Auto
- I bought the car with the gas gauge not working.
- When grounding the sending unit the gauge goes to full
- Pulled the sending unit and the float was full due to pin hole. (this is what I assumed the issue would be.
- Since I had the sending unit out, I figured I'd clean and check it out.
- I hooked up the gauge to my fluke meter and it reads 75 ohms empty and 12-13 ohms full. Seems to respond well to various positions.

So, just as a test, I decided to hook the sending unit back up in the car but not in the tank so I could move the float position around. I attached the ground wire and the power source. In the empty position, the gauge reads correct. In the full position the gauge only goes to about 1/4 full.

I'm not sure why this isn't working.

Thanks,
Jim


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304-dude View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2022 at 8:21am
Measure the voltage at the terminal, without fuel sender connected. It should read around 6v DC. It could be behind the gauge cluster... the IVR or the gauge terminals and circuit contacts need cleaning with pencil eraser.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2022 at 8:32am
When you say " attached the ... power source", do you mean the sense wire? (that is the power source to the float assembly)

Where did you attach your ground wire? What gauge of wire did you use?

Since you tested the float assembly, and it looks like it is working correctly, your symptoms indicate excessive resistance in the circuit. Given that you had grounded the sense wire and the gauge went full scale (past full scale, right?), I would guess there is a problem with how you hooked up the float assembly.

Possibly, and this is really grasping at straws, the sense connector needs a good cleaning.
69 Javelin SST BBO 390 T10
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2022 at 8:38am
Originally posted by 304-dude 304-dude wrote:

Measure the voltage at the terminal, without fuel sender connected. It should read around 6v DC. 
If you're trying to measure the output of a working IVR, you will never see a constant voltage. It switches between 0 and 12V at a slow rate - too slow for a voltmeter to follow. When you do the math, the average voltage will be 5-6V.
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Jimjific View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jimjific Quote  Post ReplyReply Direct Link To This Post Posted: Sep/15/2022 at 8:48am
"When you say " attached the ... power source", do you mean the sense wire? (that is the power source to the float assembly)"
Yes the sense wire that has fluctuating voltage. I checked that first to see tht I had a signal.

"Where did you attach your ground wire? What gauge of wire did you use?"

I attached it to where it screws into the body. It was getting late and dark so I didn't have time to clean the ground area and contact. When I did the test for the gauge, I ran a wire from the negative battery terminal to ground the sense wire.

Thanks for the replies,
Jim
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Jimjific View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jimjific Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2022 at 12:10pm
I was able to work on the Gremlin again.

- So I rechecked the fuel gage by grounding the source to the battery terminal.
- The gauge only goes to about 5/8th full.
- My meter has a min/max function. The voltage to the sending unit fluctuates up to max 12V
- So I hooked up the sendng unit again (outside of the tank). I ran a line from the negative battery terminal for gound. The gauge only reads a little over a 1/4 full.

So it looks like the gauge might be the problem? I have a new float and gasket coming.

Thanks,
Jim
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mstrcrftr View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mstrcrftr Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2022 at 12:52pm
If the gauge goes to full when you ground it, its not the guage. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jimjific Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2022 at 1:30pm
It only goes to 5/8th when grounded.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mstrcrftr Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2022 at 1:56pm
Originally posted by Jimjific Jimjific wrote:

It only goes to 5/8th when grounded.


In your original post you had said it went to full. 
My bad
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2022 at 2:27pm
If the gauge only goes to 5/8th full when the sense wire is grounded, you definitely have excessive resistance in the circuit.

The IVR always fluctuates between 0 and 12V. It's the amount of time the output is at 12V vs 0V (called the duty cycle) that changes, depending on how much fuel is in the tank and how hot the coolant is.

Since you are taking it back to the battery, I'm thinking that's 10 to 20 feet of wire? What gauge did you use? At that distance AWG24 or larger should have little affect. 26 or 28 might be too small.

I'm more familiar with the early AMX/Javelin gauges, but I'm assuming the Gremlin gauges are similar except for having the IVR built into either Fuel or Temp gauge (I can never remember which one). The mounting posts are also the electrical connection to the circuit board for the cluster. It's not uncommon for these connections to oxidize over time. On one cluster I had, I needed to unscrew the nuts, remove the gauges, and clean the pads on the circuit board, to get the gauges to work. Yours hasn't totally failed, so maybe just loosening off the nuts and re-tightening them will fix the problem.


Does the Temp gauge work properly? Since it is identical in function to the Fuel gauge, and you have the fuel level assembly out of the car, you could attach the Temp sense wire to the fuel level assembly and get the exact same readings as you would with the Fuel gauge.
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