1989 ford fuel gauge reading incorrectly

Stumped!! I have a 89 ford 150 that has a fuel gauge reading incorrectly. Was advised to change the sending unit for which i did and the gauge started to read excessfully full instead of reading the way it should. Completely disconnected the wiring for the sending unit and the gauge still continues to read over the full mark. All wiring looks good but able to figure out what is going wrong. Any suggestions to what is going on would be great. Checked for power at the connector to the tank and found power going to the low pressure pump but no power to the sender unit. Traced wire back to fuel gauge and found that there is no power coming from the fuel gauge but the fuel gauge will read 3/4 full with no power and reads well over the full mark with the power on. Got me scratching my head.

since you didn't say anything about either tank I am going to assume that you have a single tank truck. The fuel gauge has a yellow and white wire that goes back to the sending unit in the tank. It connects to the A12 pin in the instrument cluster. with the wire open or disconnected it should make the gauge go to empty (or below empty). If you gauge reads anything above that with the sender disconnected I would look for a short to ground. Because it doesn't read full to the top I would look for a short that has high resistance. A short to ground will take away the voltage measured at the fuel tank sender connector at the tank. (because it is already completing a circuit)

If you can access the dash wire harness and remove the wire to the sender either by taking it out of the connector or cutting it where you can see the wire is not accessible to any ground, check if the gauge still reads high. If it reads over empty you probably have a bad gauge.

When you say it reads 3/4 with no power and over the full mark with power, are you talking about with the key on and with the key off? If your gauge doesn't go to empty when you turn the key off I would try another gauge.

yes it is a single tank and i had ground the fuel gauge wire with power and the fuel gauge went to empty. took off ground and gauge went back to reading full, there is 8 volts at the fuel gauge wire. checked ohms at sender unit and is reading 14 ohms empty and 153 ohms full this was on a new sending unit, was told by another mechanic that i should only have 10 ohms at full and 70 ohms at empty. returned sending unit and got another did the ohms check and got the same readings. not sure what to think at this point. :cry:

thanks 4 the help, solved the problem myself.

that's great Bill. would you like to post your fix so others that have similar problems might be able to benefit from your findings?