Your description of the A/C fan control is not correct. The A/C has no
control over the high speed fan circuit but rather just the opposite.
The high speed temp switch in the radiator controls the high speed fan
relay and also disables the compressor clutch whenever the high speed
fan is on due to excessive radiator temp.

The refrigerant high pressure switch does control the middle speed
radiator fan relay but it does not turn off the A/C clutch when this is
triggered. The A/C cooling ceases only when the radiator can't keep the
engine coolant temp below the trigger point for the highest speed fan
temp switch. The high speed fan temp switch output has a parallel
connection to the A/C clutch relay which is a normally closed relay.
Whenever the radiator temps require high speed fan operation the A/C
compressor is turned off by this relay to reduce the load on the engine
and reduce the heat from the condenser that is in front of the radiator
airflow.

There is no low side A/C pressure relay but rather just a simple low
side pressure switch that disables the A/C compressor clutch if there is
not enough refrigerant in the system.

Mark

Kim Brennan wrote:

> ......
> In your AC system you have a relay behind the rear most drivers side
> pillar. This relay checks the pressure of the refrigerant line on the
> high side. If it is too high, then the AC system is either overcharged,
> or you aren't getting enough cooling up front. No matter what the cause
> the relay will turn off the compressor and turn on the radiator fan to
> high speed until the pressure in the high side comes down to acceptable
> limits.
>
> At highway speeds you might not even realize this is happening until
> you suddenly realize your AC system isn't giving you cooled air
> anymore.
>
> Note there is a low side AC relay too. This makes sure that there is a
> minimum charge of refrigerant in the AC system before it will allow the
> compressor to kick in.
>