Hi Tom -
At 12:01 PM 1/27/2013, Mister Tom wrote:
>Both problems come and go.

>My question is whether this is the typical symptom for the electrolytic cap
>on the instrument cluster going bad.

If it varies with humidity then it's completely 
typical;  otherwise maybe/maybe not.  During one 
of the spells while it's misbehaving, check for 
+10 V +/- 0.5 V between ground pin on the 
thermostat (the skinny one) and whichever of the 
other two gives the higher reading.  If bad, you 
have a bad panel regulator or a contact problem 
on the panel.  Then remove the level controller 
module from the relay panel.  If the misbehavior 
continues (and +10 V is good) then it's 
definitely inside the gauge.  If not it's either 
the controller itself or more likely an 
intermittent open in the wiring to the sender.

>2) The fuel gauge needle will drop slowly, from an accurate reading, all the
>way down to the reserve mark.
>
>I watched it fall from a little over half to the reserve mark in about �
>hour while driving.

If the temp gauge is dropping at the same time, 
then it's the panel regulator or its 
connections.  If the temp gauge is steady then you can eliminate the regulator.

It sounds like a flaky connection that's 
gradually increasing resistance when live.  Or 
conceivably a heater short inside the 
gauge.  There is only a heater inside the fuel 
gauge, no electronics.  If you drop the rear of 
the tank an inch or so you can get your hand far 
enough in to unplug and replug the sender in case 
that connector is flaky.  Also cycle the B and E 
connectors on the back of the fuse/relay panel, 
and the instrument panel connector.

>The fuel level is indicated correctly after turning the ignition off and on
>again.

I can't explain that, if you meant that just 
cycling the ignition cures it temporarily.  There 
aren't any electronics to reset except the 
regulator itself - and if it were giving a 
problem you'd see it on the temp gauge as 
well.  This could make it a real stinker to find.

This would be a good time to have a spare panel 
to swap in (or spare fuel gauge ditto) to 
eliminate the gauge as a possibility.

The wiring goes from panel connector pin 7 to B3 
on the back of the fuse/relay panel; then back 
out through E5 and straight to the sender.  With 
the panel unplugged you should see about 35 ohms 
to ground with the tank near full, and about 170 
when you've just hit the reserve.

You could break the panel supply lead (black, pin 
8) and insert an ammeter, then let it sit while 
the gauge goes through its cycle.  If the current 
rises as the gauge falls it's a short in the 
gauge; if it drops it's a resistive connection in the wiring.

Yours,
David