Stumped with fuel sending unit

[Stranny64]

Hello, I’ve read every article in the last year about fuel gauges and sending units and I can’t figure mine out. I’ve replaced the gauge, circuit board, wire to the sending unit, and I’m on my third sending unit. The gauge reads empty. If I unhook the wire it goes to past full. If I bench test the sending unit out of the truck it reads 90 ohms and I also test it behind the seat out of the tank with the wire on and a ground wire and the gauge works perfect when I move the float. I made sure the grommet is between the wire nut and the unit assembly as well. Is my sending unit itself somehow getting grounded? I’m soooo frustrated! Thanks!

[67 Burb]

First of all, is there a good ground connection from the sending unit and the truck chassis / cab? On my current project, I didn't like the factory idea of using the gas tank and the tank straps as the ground route for the sending unit. I added a ground wire from one of the screws that hold the sending unit to the tank and ran it to a good ground.

Does your tank have a good ground?

[Stranny64]

I’ll double check the grounds. Thanks! 👊

[dmjlambert]

Are you sure your float is floating?

[VetteVet]

Run a resistance check on the sending unit installed, between the terminal and the sending unit flange. You should get whatever reading that corresponds with the fuel in the tank. Somewhere between 0 and 90 ohms.
If the sending unit wire was grounded it would read 0 ohms all the time but, when you disconnect the sending unit wire you get past full. There s a problem with the sending unit wire chafing under the rocker panel and grounding out causing the gauge to read zero or being intermittent, but you ruled that out with a new sending unit wire and by gettting a past full reading when you disconnect the sending unit wire.
You can also get the past full reading if you disconnect the wire at the fuel terminal in the fuse panel, I usually do that first to rule out the gauge. Your checks indicate that the sender is the problem. Have you considered that it might be that the sending unit is binding on the inside of the tank and the float arm cannot rise in the fuel. A hole in the float could cause this but in your case the float is new and 3 of them is unlikely to have any holes in them. Also they probably wouldn't all read zero.
I would try loosening the lock ring with the sender in the tank and rotate the flange a little CW or CCW with the key on and the flange grounded to the tank.

[Stranny64]

I’ll give these ideas 👆 a try after work today. Thanks!

[Stranny64]

Well 67 Burb and VetteVet, drum roll please…. I pulled out the sending unit for the millioneth time and bench tested it, works perfect. I bent the float arm lever forward a tick and also just for fun I switched the brand new float with the other brand new float and it works. Not sure if the float had a hole in it or if it was grounding out inside the tank but I’m so glad it finally works!
Thanks for the advice! 👊

[67 Burb]

Glad to hear that it's working.

By the way, if the float did have a hole, it would be full of fuel when you pulled it out.