Need help with Fuel Gauge

[AslanRules]

I'm having trouble with the fuel gauge on my 1966 C-10. I thought it may have been a mismatched resistance problem between my sender and gauge. I read this thread but I'm having a slightly different problem.

When I got the truck, the fuel gauge didn't work. I pulled out the sending unit and the resistance per my meter was extremely variable and did not smoothly register increasing resistance as the float was moved. I then took out the dash gauge and found that a piece was broken off and it needed to be replaced. I decided to replace both components, since I didn't want to go through this again. I ordered a new sending unit from Brothers.....they assured me that it was 30 ohm. When I received it, I installed it, but I failed to test the resistance with my meter prior to installation to see if it worked properly and that it indeed was 30 ohms. Next, I found 2 used dash fuel gauges on eBay and bought them. I bench tested both of them and they pegged to E and F when they were supposed to, according to the thread referenced above, as well as in the Shop Manual.

When I put gauge #1 in the dash, it reads 1/2 Full when I have a Full Tank (with ignition on). I then placed one lead of my meter on the brown wire in the gauge plug (sending wire) and the other lead to the chassis ground. The resistance reads 21 ohms. To make sure the problem wasn't in the wiring, I tested the resistance on the sending unit itself (while installed), and it only reads 21 ohms as well. When I install gauge #2 in the dash, it behaves the same way.

So, my problem is this: I have a new sending unit and "new" fuel gauge. The gauge registers only 1/2 full when I have a full tank of gas. I have taken the wiring out of the equation by testing both ends of the circuit. I wish I had checked the new sending unit prior to installation, but I didn't. (This is kinda like the 30ohm-90ohm mismatch problem in reverse.)

I suspect that the problem is with the sending unit.....perhaps it is defective, or I installed it incorrectly and the float doesn't read correctly. Any other ideas before I remove my sending unit (again) and check it for problems?

Thanks,

Randy