Need help with fuel guage ....

OK...a 1976 PA 28 151..

After a few minutes of running, the port guage will start to fall
towards E.

It is showing 3/4 now (fuel at tabs), and will sag to 1/4 within
about 15 min, while I am operating on the starboard (other) tank.

Shut down for a few hours, guage resumes normal (correct) indication.

We have checked / replaced, tank grounds, sender, wiring to guage.

I think the guage terminals handle 12 V to the left terminal
(looking at it from under the panel) (starboard terminal) , The other
to the tank sender (port terminal).. (grounding this terminal when
the fault is present will cause a "full" indication).

From what I recall, there is a "ground" connection internal in the
guage. Anybody know how this is grounded?

I suspect a bad connection here somewhere..

Driving me up the wall..of course every time I attack the problem,
meter in hand, it works fine.

Anybody any ideas? Suggestions or test procedures?

Dave

Electric fuel gauges are electromagnetic devices, with a
reference resistor inside them against which the sender's resistance is
measured. If that resistor is going bad, with its resistance decreasing
as it warms up, the gauge will sag.
However, except for an outright short, most resistors
increase in resistance when warm, so I would suspect a corroded
connection in the wiring of the gauge or in the line to the sender.
Look closely at the crimp connectors in the sender line at the gauge
and sender. Make sure the sender's body is properly grounded to the
airframe. There's also a connector where the wire passes from the
fuselage to the wing, and these get dirty and corroded and loose.
Perhaps a resistance measurement immediately after
disconnecting the wire from the gauge when the sag has occurred
(everything still warm) might reveal something. Full tank should be
around 30 ohms, empty about 150, IIRC.

Dan

Most likely you have a connector or connectors somewhere that are
corroded and are getting hot after the power has been on for some
time. Your best bet is a connector somewhere near the wing root.

Put a battery charger on the battery to keep it charged while you are
looking for the problem. Turn the master on and leave it on long
enough to get the problem while it is on the ground and you can
measure things with the engine off.
John

On Tue, 28 Feb 2006 23:20:49 -0400, Dave
wrote:

OK...a 1976 PA 28 151..

After a few minutes of running, the port guage will start to fall
towards E.

It is showing 3/4 now (fuel at tabs), and will sag to 1/4 within
about 15 min, while I am operating on the starboard (other) tank.

Shut down for a few hours, guage resumes normal (correct) indication.

We have checked / replaced, tank grounds, sender, wiring to guage.

I think the guage terminals handle 12 V to the left terminal
(looking at it from under the panel) (starboard terminal) , The other
to the tank sender (port terminal).. (grounding this terminal when
the fault is present will cause a "full" indication).

From what I recall, there is a "ground" connection internal in the
guage. Anybody know how this is grounded?

I suspect a bad connection here somewhere..

Driving me up the wall..of course every time I attack the problem,
meter in hand, it works fine.

Anybody any ideas? Suggestions or test procedures?

Dave