Gazz,
If you have an ohmmeter, you can take the sending unit out of the
tank, and measure the resistance from the center terminal to the
housing (disconnect the wire from the sending unit first!)
It should read 0 ohms when the float is all the way down, and 30 ohms
when the float is all the way up, and the resistance should change
proportionally as you move the float up and down. The actual readings
full and empty may be off a bit, but they should be within a few ohms
of spec.
Check the float to see if it has a leak and is full of gas, if so that will
make it sink and read low all the time.
Check to see that the wire to the gauge is not shorted anywhere; it
will read low all the time if this is the case.
Check to see that the sender is properly grounded to the truck's body,
it will read full or past full if there is much resistance.
Check to make sure the sender and power wires on the gauge are not
switched.
Check to make sure the gauge housing in the dash is properly grounded.
Harold
