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Discussion Starter #1
Hi all,

I’m trying to verify the accuracy of my fuel gauge. Here’s my dilemma: I’m reading 4.4 ohms from the sending unit terminal to the ground (one of the 5 mounting screws), but reading 7.5 ohms from the sending terminal on the back of the gauge to a ground screw on the dash. Doesn’t seem right to me. The gauge seems to be accurate based off the 7.5 ohms as it is sitting around 1/4 fuel level. I am concerned that it is reading erroneously high as the sender side reads 4.4 ohms (or about 1/7 fuel level).

Both units are 0-30 ohms. The gauge does not have a ground terminal. Just the positive and the sending terminals. The sending unit is in a metal tank with metal straps, but is propped up with wood blocks. Though it should be grounded with the straps at least, correct? I ran a short ground wire from sending unit terminal to one of the mounting screws. Would running a a ground from the mounting screw to the body help? Should I remove the short ground from the sending unit terminal to the mounting screw (unnecessary and possibly affecting the connection of the sending wire to the gauge)? Or should I possibly add a ground from the sending terminal on the gauge to the ground screw on the dash?

Any other ideas or checks? I thought adding more gas and checking the resistance at both points as I go.
 

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1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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I don't think you're supposed to have a ground from sending unit terminal. Ground wire could run from a mounting screw to the body. I have my ground wire running from the metal outlet tube to the body. (using a small metal hose clamp around tube)
 

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Discussion Starter #3
Okay. I will try that and see if anything changes and report back. The more I think about it the more unsure I am about that part of the setup. Though if the terminal is grounded through the screws anyway, then I’m thinking that short ground I added shouldn’t make any difference. But I definitely could be wrong.
 

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1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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The terminal on the sending unit that the gauge wire attaches to is insulated from the body of the sending unit by non-conductive washers.
 

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Discussion Starter #5
Yes, you are correct. However, my memory was incorrect. I actually had it grounded from the mounting screw to the screw on the metal strap. So all is good there. I double checked the resistance from the sending unit terminal to the mounting screw and from the sending unit terminal to the metal fuel outlet and got the same resistance. And the reading today matches the gauge pretty well. However it seems if I tap on the sending unit terminal and then check the resistance again, if will be different. I’ve done this several times and had readings any where from 27 ohms (first test which was vastly different than the 4.4 from yesterday, which led me to try taping the terminal) to 6.5 ohms with several readings in between those two. Would that mean the sending unit is bad, or maybe a bad ground? Other ideas?
 

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Install a ground wire from one of the sending unit screws to the body...You can drill a small hole in the body and use a star washer for bite.

 

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X2. And don't think you are ever going to achieve perfect accuracy either, lol. I don't think these gauges were all that accurate even when they were new.
 

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55 belair 3 inch chop, history unknown...
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Install a ground wire from one of the sending unit screws to the body...You can drill a small hole in the body and use a star washer for bite.

I did mine exactly like pops when I recently replaced my tank and sending unit and it works great...
 
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