I have installed 12 volt led indicator lights from Radio Shack on my dual
evaporator AC system to monitor both thermostats and the pressure switch.
Just connect the light between ground and the wire on the thermostat switch
that drives the compressor clutch.  To monitor the pressure, use the same
type of led across the terminals of the pressure switch.  The pressure
switch is a normally closed switch so there will be no voltage through the
led under normal operation.  When excessive pressure is present, the led
will be on when the thermostat is calling for the compressor to be engaged.

The indicator lights are very useful.  With them you can adjust the
thermostat in a very fine range around the current setting.  No guesswork is
involved.

Ed

On Sat, 25 Jul 2009 02:37:50 -0700, Todd P Last <Rubatoguy@COMCAST.NET> wrote:

>I want to add an LED indicator on my Vanagon that will show when the A/C
>compressor is engaged.
>Has anyone done this?
>I'm thinking that I just need to use one of the wires on the A/C clutch
>to connect to a relay to activate the light - but I don't know what wire
>to use.
>
>Anyone have any suggestions?
>
>
>Todd
>'88 Westy