Backlighting/Dimmer Switch Fun -- Cherokee 140

I have been experiencing an odd problem with my digital EGT/CHT gauge.
The backlighting on the guage is controlled by the panel light dimmer
potentiometer (or rheostat, which ever it is). When the dimmer switch
is off, the gauge works fine. Once I turn the dimmer past the click,
the instrument backlighting comes on but the digital readout goes
blank. As I turn it further, the digital readout may or may not show
up, and if it does, the reading is inaccurate. Once the dimmer rotates
to full-dim, the display behaves normally again. I took a quick look
behind the panel, and didn't see anything obvious. Of course, there are
about a million wires crammed back there and I was laying upside down
on my head at the time. Any ideas?

My best guess is a bad ground.

wrote in message
ups.com...
I have been experiencing an odd problem with my digital EGT/CHT gauge.
The backlighting on the guage is controlled by the panel light dimmer
potentiometer (or rheostat, which ever it is). When the dimmer switch
is off, the gauge works fine. Once I turn the dimmer past the click,
the instrument backlighting comes on but the digital readout goes
blank. As I turn it further, the digital readout may or may not show
up, and if it does, the reading is inaccurate. Once the dimmer rotates
to full-dim, the display behaves normally again. I took a quick look
behind the panel, and didn't see anything obvious. Of course, there are
about a million wires crammed back there and I was laying upside down
on my head at the time. Any ideas?

These symptoms, and a little common sense as to the sorts of errors an
installer might make, suggest that the ground connection for the gauge is
not connected to the airframe ground but rather to the power source for the
internal lighting. When the internal lighting is turned off or is set to
max dim, the source voltage is very near ground potential, so the gauge
works properly. At high or intermediate brightness settings the source
voltage is closer to the power bus (battery +) voltage, so the voltage
across the gauge is insufficient to allow it to work properly.

This should be easy to check with a simple voltmeter, or even an ohmmeter

-Elliott Drucker