John, take a look at this diagram. It's really quite a simple circuit.
http://www.pbase.com/dennygraham/image/159356931 If the sender is controlling the circuit proper when the unit
is out of the tank, then your problem is obviously at the tank
not at the gauge. And you obviously have the correct
sender/gauge combination, i.e., both matched to the 90 ohm
sender. It has to be either an obstruction to the float
movement, the float not set to the correct position or a bad
connection. You've said that it's intermittent, that is, that
it works for a while then not. Could very well be that the
ground return is not solid if it's intermittent. The ground
has to be a solid continuous path from the sender back to the
dash gauge just as the hot lead is. Make sure your tank isn't
insulated from the rest of the cab or frame and vise versa at
the gauge. Your ground wire originally would have been
connected to the frame under the driver seat.
Just so we don't muddy the water John, "resistance" is the
correct electrical term we would want to use in a DC circuit
to control the voltage.
"Impedance" is the term for a complex resistance in an AC
circuit. To put it simply it's a combination of the physical
resistance and shift in phase.
The confusion in terminology is often because both values are
expressed in ohms. Ohms resistance (R) and ohms impedance (Z).
Denny Graham
Sandwich, IL
