Gas Gauge False Reading ??? - Team Camaro Tech
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post #1 of 35 (permalink) Old Sep 29th, 10, 06:26 PM Thread Starter
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Gas Gauge False Reading ???

69 Camaro with a the gauge in the dash. New sending unit.

The gauge reads a little over 1/4 tank with a full tank of fuel. Bear with me I just spent all day testing and re testing.

Tested the gauge. Brown wire disconnected in the trunk with the key on and the gauge reads full. Key on with the brown wire grounded and the gauge reads empty. I am getting 11.96 volts to the back of the gauge.

Tested the sending unit. Bench tested before install and it worked as intended. Tank installed with 5 gallons of gas sending unit read 27 ohms (gauge read just over empty). Filled tank and it reads 96.0 ohms in the trunk, 96.6 at the gauge.

Key on and the Brown wire disconnected in the trunk I am getting 6.30 volts. With the brown wire connected to the sending unit I am getting 4.12 volts.

I have checked and cleaned/added star washers to the ground at the tank and the ground behind the ashtray. I have run a jumper from the metal gas line on the sending unit to the back bumper all with no change.

Any ideas, thoughts, anything left to change/ check.

Thanks for looking.

KR

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post #2 of 35 (permalink) Old Sep 29th, 10, 06:30 PM Thread Starter
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Re: Gas Gauge False Reading ???

I also ran 12 volts to the back of the gauge, ran a separate ground and a jumper from the brown wire to the gauge. Again no change. Gauge still reads just over a 1/4 tank even though the tank is full.

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post #3 of 35 (permalink) Old Sep 29th, 10, 06:37 PM
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Re: Gas Gauge False Reading ???

I hate to suggest this, but it worked for me.

I had the same issue this summer... all the same symptoms, new sender, etc. I ended up pulling my fuel gauge cover window and pulling the needle off, then stuck it back on at F with a full tank (I gave myself a smidge of reserve, maybe too much). Now the needle goes to full when I fill up and decreases as I rack up the miles.

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post #4 of 35 (permalink) Old Sep 30th, 10, 09:42 AM Thread Starter
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Re: Gas Gauge False Reading ???

Thank you for the reply. I'm not sure if that would work with the problem I am having but may resort to that if nothing else works.


Any other Ideas? I am willing to try almost anything....

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post #5 of 35 (permalink) Old Sep 30th, 10, 01:19 PM
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Re: Gas Gauge False Reading ???

Somethings wrong here. You stated the gauge worked properly with and without grounding the tan wire (empty to full as expected). You then state that with a reading at the gauge of 96.6 ohms, the gauge only reads 1/4 tank. Retest by disconnecting the tan wire, the ohm will go to infinity and gauge should go to full. But why does the gauge pass the tan wire test and not the 96.6 ohm unless something is off with the gauge?

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post #6 of 35 (permalink) Old Sep 30th, 10, 01:59 PM
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Re: Gas Gauge False Reading ???

Quote:
Originally Posted by b4wscrambler View Post

Key on and the Brown wire disconnected in the trunk I am getting 6.30 volts. With the brown wire connected to the sending unit I am getting 4.12 volts.

KR

Where are you measuring these voltages?

You should have 12 volts on the positive terminal of the gauge, and 0 volts on the ground terminal. Voltage on the tan/brown wire does not matter, resistance does.

Don't forget there is a resistor on the gauge so measure the brown wire at the gauge with it disconnected.

If you have 96 ohms on the tan wire to gauge ground, and 12 volts on the positive gauge terminal and the gauge does not read full when you reconnect the tan wire, the gauge or gauge resistor may be bad.
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post #7 of 35 (permalink) Old Sep 30th, 10, 02:50 PM Thread Starter
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Re: Gas Gauge False Reading ???

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Originally Posted by alanrw View Post
Somethings wrong here. You stated the gauge worked properly with and without grounding the tan wire (empty to full as expected). You then state that with a reading at the gauge of 96.6 ohms, the gauge only reads 1/4 tank. Retest by disconnecting the tan wire, the ohm will go to infinity and gauge should go to full. But why does the gauge pass the tan wire test and not the 96.6 ohm unless something is off with the gauge?

alan

I retested by disconnecting the tan wire in the trunk and the gauge reads as it should. I checked the ohms again under the dash with the gauge disconnected and got 96.6 ohms from the tan wire.

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post #8 of 35 (permalink) Old Sep 30th, 10, 03:17 PM Thread Starter
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Re: Gas Gauge False Reading ???

Quote:
Originally Posted by 68 Ragtop View Post
Where are you measuring these voltages?

You should have 12 volts on the positive terminal of the gauge, and 0 volts on the ground terminal. Voltage on the tan/brown wire does not matter, resistance does.

Don't forget there is a resistor on the gauge so measure the brown wire at the gauge with it disconnected.

If you have 96 ohms on the tan wire to gauge ground, and 12 volts on the positive gauge terminal and the gauge does not read full when you reconnect the tan wire, the gauge or gauge resistor may be bad.
I got the voltage at the tan wire in the trunk.

I have 12 volts at the gauge. No voltage from the ground. Both were checked at the harness with the gauge disconnected.

I definitely have 96 ohms at the gauge harness and have 12 volts being suplied to the gauge.

Thank you again for the replies.

KR

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post #9 of 35 (permalink) Old Sep 30th, 10, 04:53 PM
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Re: Gas Gauge False Reading ???

Measure the resistor on the gauge separated from the gauge. Value is ?

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post #10 of 35 (permalink) Old Sep 30th, 10, 05:23 PM
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Re: Gas Gauge False Reading ???

I have a '72 with the same issue..reads a bit over 1/4 full...sending unit good etc.

Assumed it was the gauge..maybe a failed resistor leads to the same approximate reading across the gauge meter coil only?

Didnt' know there was a resistor in the circuit...
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post #11 of 35 (permalink) Old Sep 30th, 10, 05:24 PM Thread Starter
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Re: Gas Gauge False Reading ???

What would be the best way to test the resistor on the gauge?

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post #12 of 35 (permalink) Old Sep 30th, 10, 05:25 PM
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Re: Gas Gauge False Reading ???

Remove it from the gauge and test it by itself.

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post #13 of 35 (permalink) Old Sep 30th, 10, 06:05 PM Thread Starter
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Re: Gas Gauge False Reading ???

So is this bad?



I tested the ohms across the resistor and got nothing. no reading. I assume that is bad. Can I get another one and where?

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post #14 of 35 (permalink) Old Sep 30th, 10, 09:08 PM
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Re: Gas Gauge False Reading ???

Would assume the little broken wire in the photo is the cause of the open circuit..

Can't recall seeing them for sale..maybe Heartbeat City or Bob at Camaros.com, or maybe a tach electrical repair shop or Everett or others may have an idea. If you find one let us know..I likely need one as well..

Not sure what the design resistance is or Watt rating -looks like Radio shack goes to 1W which I would think is light in 10, 100, 1K ohm...

Based on the heat disipating insulation looks like the Watt rating required may be fairly high?
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post #15 of 35 (permalink) Old Oct 1st, 10, 01:15 PM
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Re: Gas Gauge False Reading ???

Well, if all else fails, re-solder the wire to the connector on the right of the photo.

alan
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