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Discussion Starter · #1 ·
Interesting mystery. My gas gauge reads erratic. I can fill it up and it will show Full, drive it awhile and it shows normal use. But shut it off and start up again later and it shows a lot lower. Next time it might be normal again. a new sender unit and still reads erratic.

Last night I filled up and gas gauge read full, turned the head lights on and noticed it went to less than ¾??? I guess that has been the erratic readings mentioned above, sometimes normal and sometimes not.

Testing the grounds and voltage at the gas sending unit I get 0 ohms resistance from body or frame to the sending unit. Reading from sending unit terminal to body, key on (not running) I get 2 volts (presently 3/4 full), but lights on I get 3 volts. Where is this extra voltage coming from and how do I get rid of it on the gas gauge circuit?

Awhile back I put my head lights on relays right off the alternator lead to the horn relay and close to the head lights, they are activated by the dimmer switch leads. Also I run a three wire alternator with remote sensing so I have very good voltage through out the car, 14.2 volts while running.

I have plenty of grounds, star washers between gas tank and trunk floor, no padding between tank and hanger straps, two engine to body ground straps, two engine to frame ground straps, one frame to body ground strap and one ground wire from negative battery cable to the body, all with star washers.
 

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Trifive Automotive Electrical Wiring Expert
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Check the voltage at the "ign" terminal of the gas gauge with the lights on and with them off. If it varies a lot, look for a bad connection powering the gas gauge.
 

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Here is my story. I fill my tank and it shows full until about 1/2 tank is gone the the gas hand goes back and forth from 1/2 to full. At half a tank I put 3 gallon in and it shows full until the 3 gal is used up, Then it starts going half to full. Have new sending unit installed. I did the grounding at the tank as requested.....HELP :)
:gba:
bowtie-trifive
 

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Here is my story. I fill my tank and it shows full until about 1/2 tank is gone the the gas hand goes back and forth from 1/2 to full. At half a tank I put 3 gallon in and it shows full until the 3 gal is used up, Then it starts going half to full. Have new sending unit installed. I did the grounding at the tank as requested.....HELP :)
:gba:
bowtie-trifive
Could be the guage :)
 

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Discussion Starter · #5 ·
Fixed my erratic gas gauge.

Took the instrument cluster out and tested the continuity with an ohm meter from back of gas gauge to instrument cluster, speedo and water temp gauge and had no reading (very high resistance). Bad or insufficient grounds. Found the gas gauge’s two 8-32x3/8” mounting screws a little loose.

Tightened the screws and got very good reading (0 resistance). Tightened all the instrument cluster screws, even added some jumper wires from the mounting screws on the gas gauge to speedo and to water temp gauge. Now all the instruments are grounded together and to the instrument cluster by 10 screws and it to the dash by 6 screws.

Appears the voltage from the dash light in the gas gauge was searching for a ground and could not find a good or sufficient one so bled off on to the gauge circuit affecting the reading.

Now I’m getting good and consistent gas gauge readings and is not affected when I turn on the lights.

I love hot rodding.
 
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