Bizarre fuel gauge

[Broeheem]

1991 4.0 - Originally, the issue was a fuel gauge that just stayed "E". Replaced the entire fuel pump/sender, verified the leads were functioning. Still not reading. Replaced a burnt "anti slosh module" with a non-burnt one. Now I get this:

Turn ignition on, gauge pegs at "F", then settles somewhere else after a mile or two. Not necessarily where it should. Topped off tank today --- ignition on --- "F". For a minute, then read 2/3. WTF? What else is there to fail?

 

[RonD]

5volt Pulsing power supply behind the instrument cluster or the gauge itself

But check the Yellow/white strip wire going back to sender first.

Pull out instrument cluster, the Yellow wire will be on the connector.
Use an OHM meter set to 200 OHMS or 1K OHMS
1989 and up Ford's use in tank senders that have 10 OHMS empty and 158 OHMS full
So Ground one OHM Meter probe, and hook other probe to the Yellow wire.
OHM reading should be steady and reflect gas level in the tank
Now rock the truck, gas in the tank should slosh around and OHMS should go up and down as float goes up and down.

The fuel gauge IS an OHM meter, is shows the OHMS from the sender, so if OHMS are not stable then gauge won't be stable.
Could be Yellow wire is chaffed somewhere, or corroded, causing OHMS to vary

If Yellow wire/sender OHMS are stable then could be gauge or circuit board on cluster is the problem, check plug in for yellow wire.

5volt power supply could be the issue but generally they work or don't, so no level shows at all, it wouldn't vary up and down