Another fuel gauge mystery… 😖

[Stranny64]

My fuel gauge still doesn’t work! I found a great tech article on this site and bench tested everything with a ohm meter per that write up including the resistor. Everything tests correctly. I can test the sending unit in the truck and it is reading about a 1/4 tank which is about right according to my odometer and gas mileage tablet that I carry with. I’ve replaced the gauge with a new one, the sending unit wire, and I’ve tried two sending units. I’ve checked the truck ground straps and also ran a wire from the back of the gauge to a ground. The gauge shows full all of the time. When I disconnect the sending unit wire and touch it to ground the gauge goes to empty. Ive tried reading every article on here and I finally give up. What am I missing? Thanks!

[pjmoreland]

Does your fuel gauge point to 3:00 when the tan sending unit wire is completely disconnected, with ignition turned on? If it does, then maybe you don't have an adequate connection between the mounting plate of your sending unit and ground. The ground path for the sending unit has a lot of pieces to it:

-Contact between the sending unit mounting plate and the sending unit lock ring
-Contact between the sending unit lock ring and the tank
-Tank bolted to cab
-Cab connected to engine via grounding straps from the firewall to the valve covers
-Battery negative cable connected to front of passenger side head

[Stranny64]

I’ll take a look at those grounds. Thanks a bunch!

[Stranny64]

Checked the tank bolts and the ground straps, no luck😖 Any other ideas and/or help is greatly appreciated! Thanks!!! 🍻

[pjmoreland]

Does the gauge point straight to the right past full when you disconnect the tan wire from the tank sender (ignition on)?

[Stranny64]

Yes it does.

[pjmoreland]

Your gauge seems to be working correctly since it moves to empty when the sender wire is grounded and it moves to 3:00 when the sender wire is disconnected.

When you grounded the sender wire, where did you ground it? You could test the ground of your sending unit by grounding the sender wire to the metal fuel line coming out of your sending unit to see if the gauge still moves to empty.

[Stranny64]

Yes it does.

[pjmoreland]

What is the resistance between the fuel sender wire terminal and the metal fuel line with the wire disconnected?

[Stranny64]

It goes to -139.9 ohms.

[pjmoreland]

That explains why the gauge indicates full all the time. The sender resistance shouldn't be reading that high. It should be between 0 and 90 Ohms, depending on how full the tank is. Sounds like this sender is bad.

You might want to bench test your other sender. Measure the resistance between the sender wire connection stud and the metal fuel line. Move the float through its full range of travel and see if the resistance changes from 0 to 90 Ohms gradually as you move the float.

0 = empty
45 = half tank
90 = full

[pjmoreland]

Originally Posted by Stranny64

It goes to -139.9 ohms.

I just re-read my post #9 and realized the wording was confusing. When you took this 139.9 Ohm measurement, did you disconnect the tan wire from the sending unit and then measure the resistance between the sending unit electrical terminal post and the metal fuel line on the sending unit? I'm looking for the internal resistance of the sending unit that is currently installed in your truck.

[Stranny64]

Just the sending unit shows 45 ohms which is pretty close to what I would guess that I have a half tank of gas.

[pjmoreland]

Originally Posted by Stranny64

Just the sending unit shows 45 ohms which is pretty close to what I would guess that I have a half tank of gas.

I'm stumped.

[pjmoreland]

Does your fuel gauge move at all when you disconnect and reconnect the tan wire to the sending unit?

[Stranny64]

Nope. Only if I touch it to ground then it goes to E. Otherwise, just stays at 2:00-3:00. Thanks for helping me diagnose this by the way. 👊

[pjmoreland]

Do you currently have a reproduction gauge installed? If yes, then did you also reinstall the original ceramic resistor on the mounting posts? The reason I ask is because reproduction gauges are generally designed to work without the original ceramic resistor.

[Stranny64]

I put a new fuel gauge in it also. Maybe that’s the problem…

[pjmoreland]

Try removing the external ceramic resistor from the back of the gauge, and see how it does. You can see the white & blue resistor in the photos in this thread:

https://67-72chevytrucks.com/vboard/...d.php?t=640615

[Stranny64]

I’ll try it tomorrow. I’ll let you know. Thanks!!!

[Stranny64]

pjmoreland,
I owe you a beer or a burger! Or both!!! 👊 That was it. I guess I didn’t know that when installing a new fuel gauge not to use the resistor. I have been going crazy trying to figure this out. Thanks again!
P.S. Let me know about the beer! 😉

[pjmoreland]

That's great to hear! It was a lucky guess. I was wrong to assume the gauge was good based on it moving from 3:00 to empty when grounding the sender wire. Now you'll have extra appreciation for a car feature that most people take for granted

[Accelo]

I'll bet lunch that that little tidbit of information wasn't in the instructions.
Wait, wait! It didn't come with any instruction! At least they could insert a public service announcement. "Product doesn't come with enough information to install correctly." Use at your own risk, and BTW this product can cause cancer if you eat the gauge.

[Stranny64]

😂😂😂 Yeah, no instructions telling me that the resistor is built in to the new fuel gauge. Maybe I should of known somehow… Glad it finally works! 👊