I've seen a lot of threads about the fuel gauge stuck on full - that's how mine has been for many years. Sure enough, like the other posts, the resistance of the sender was infinite (open circuit - confirmed when taken apart...the coil in there visibly open circuit). So, I replaced the sender unit. I did make a test prior to installing the sender unit - just connected it to the car, and then varied it. All seemed fine, the gauge responded correctly.

Fast forward....everything is installed and connected and there's about 3 gallons of gas in the tank. The gauge reads right on "E". I disconnected the sender from the car and the gauge goes to "F". Okay seems about right so far. I measured the resistance of the tank sender and get about 10 Ohms. Sadly, i did not check the resistance with the tank empty. I grabbed some resistors and connected them up to the car to emulate different fuel levels - and now it's not responding correctly at all. Still gets "F" when unplugged, and "E" when shorted. But most resistance value don't change the gauge - it sits at just past the RED mark.

Added some more gas, a little at a time, and measured the sender resistance each time. As the tank filled up, the resistance goes up. There's probably about 6 gallons in there now, I forgot the resistance reading but expect it's around 6/16 * 90 = 33 Ohms, which I expect should read 1/4 tank - but it doesn't. Just reads the same RED mark.

Oh - one other thing. With resistors wired in series in place of the sender unit, I think about 40 Ohms - the gauge read correctly for a little while then went back to "F". Those little resistors were definitely getting hot though...

Any ideas what I am missing here? Does anyone have a schematic of this part the wiring?

thanks

 

I will follow up my own message....

It's now working. There's something fishy going on. With a 47 Ohm resistor connected, the tank was still reading E. I was measuring the voltage across the resistor and it was very low. Suddenly, it started going up! Eventually it reached a few volts, I think 3.5 but I'm not 100% sure. Anyway, at that point the gauge correctly read 1/2. It appears the system uses a current source to drive the circuit, then must be measuring the voltage at the gauge, maybe through a bridge circuit. It's clever enough, just surprised me how much current it uses.

Ultimately, I think there are 2 problems on my car. First, the circuit didn't work, then did work. Based on my analysis, I think a crummy connection somewhere in the system didn't allow the current to flow. Possibly something actually wrong with the electronics that drive it. This will worry me moving forwards as I didn't "fix" anything so it might let me down. Second, the new connector on the sending unit has somewhat thinner pins, and one was crooked - meaning it didn't point straight. Keep in mind that these pins are sort of shaped like an "S" and the connection relies on it - so don't ever straighter this. I'm just saying mine was pointing the wrong way. I don't like these types of connector, but it's worked for GM for a long time so won't change it.

Hope this is helpful to someone else.