Those of us with AC'd Westies know this system isn't the best in the world.
Due to the large volume of air between the AC discharge and the front of the
vehicle the coolness of the AC often does not reach the front.  When we
lived in South Carolina I often considered installing a small roof top AC
unit in the luggage rack just to cool the front cab of our Westy.  This
would have required sizing up the alternator which until recently I didn't
think was an option.

As an alternative I thought about building a parallel evaporator coil which
could be mounted on the headliner between the driver and passenger seat.
Basically this would be an air handler unit consisting of a solenoid valve,
expansion valve, evaporator coil, fan, controls, and louvered vents all
contained within a single housing.  A liquid line and suction line would
have to be run to the unit as well as power and control wiring.  I would
probably also install a solenoid valve before the existing expansion valve
to permit independent control of the rear zone.

My questions for the list are:

1)  What is the size of the existing system in BTUs/HR or tons?

2)  If I install a parallel expansion loop which has a smaller refrigerant
capacity than the existing expansion loop do I need to include a pressure
relief to avoid over pressurizing the high pressure side?  I'm thinking if I
only operate the front zone then the volume of refrigerant required will be
less than the compressor is sized for.  This would trip the high pressure
switch and disengage the compressor pulley clutch.  When the pressure
dropped the high pressure switch would close and the compressor clutch would
engage again.  If the front zone is very small then the compressor pulley
clutch would cycle frequently possibly leading to premature failure.

Thank you,

Tom Moritz
'90 Westy