1988 - JGW - Fuel Gauge

[DukeJeep]

My fuel gauge seems to always be on FULL, or past full a little. I ordered the "Sparta" brand sending unit and replaced it with no change.

Where do I go from here? Start tracing wires?

Thanks!

[babywag]

You potentially have an open in wiring or disconnected.
Check voltage & ground @ sending unit wires.

Does gauge drop with key off?

[joe]

Trace the wire from the gauge to the sending unit. Bet you've got a chaffed worn spot and it's shorting to ground on the frame/body somewhere.

[babywag]

A short would be ?E? all time.

[joe]

A short would be ?E? all time.

OK, the term 'short' was/is not correct. How about "grounding"? In my experience when determining if it's the gauge or sending unit, with the key on I would remove the wire at the sending unit and ground the hot wire to the frame. If the gauge pegged it told me the gauge was ok and the sender bad. I suspect with a normal good gauge and a good sender and the wire with a worn spot up stream grounding to the frame would cause the same reading. Semantics aside, "I" would still check with the key "on" the integrity of the hot wire from the gauge to the sender.

[babywag]

on a late model if you ground sending unit wire it pegs ?E? not ?F?.
if you disconnect sending unit wire it pegs ?F?

[Mass]

Don't mean to hijack but what's the voltage supposed to be at the sending unit? I understand the unit is passive and works by varying resistance to ground. So what's the input voltage suppose to show?

[gpcl16]

If you have the wire disconnected at the sending unit you should see battery voltage with the key on. That's because there is no ground. If you have it connected to the sending unit and pierce the wire you will see less than 12 volts (unless your sending unit has an open circuit) because there is a resistor in the instrument panel gauge and another variable resistor in the sending unit. The actual input voltage will vary depending on your fuel level because you have two resistors in series, the second of which (sending unit) is variable.