7747 A/C enable

[steve ravet]

Brent, you might take a look at the pressure switch and make sure it's
OK.  From memory, looking at the schematic last night, the clutch
circuit is like this:

Switched power
  |
A/C controls
  |_______________This wire tees, and becomes the ECM A/C request
signal.
  |
low side switch
  |
high side switch (only some models have this)
  |
relay
  |
clutch
  |
ground

The ECM input may tee after the low side switch, I don't recall off
hand.  But, all the switches must be on (control, pressure switches,
relay) before the clutch engages.  Plus, running the compressor past the
point where the low side switch cuts it off results in lower pressure in
the evaporator which shouldn't blow it up, but may turn it into an ice
cube maker.

I'd go through the system and make sure everything is OK before
installing and charging that new evaporator.  Plus, I'd look into using
an R-12 compatible refrigerant like R-409 instead of R-134a.  see
www.autofrost.com for dealers.  I've been using it for a couple years
with no problems, and it works with mineral oil instead of that nasty
PAG or ESTER stuff.

--steve

Brent Obermiller wrote:
> 
> Unforunately, I now believe that Ludis is right...that is the polarity of
> the computer output is opposite (not the state of the output).  I found the
> A/C wasn't cycling AFTER grenading the evaporator (ESTER oil is very messy).
>   Originally, I JUST switched output contacts on the relay (as many of you
> suggested), then the compressor engaged on request, and doing this
> eliminated the pressure switch from the circuit, somehow; For record the
> blame is mine as I did this BEFORE writing for everyone's advice.  I bought
> a new evaporator and rewired the car to the 92 Roadmaster Schematic. Let
> this serve as a public service message to save money ($125 evaporator and
> Freon charge cost).
> -Brent
> 
> To reply directly
> mailto:ober_b at sssnet.com
> 
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-- 
Steve Ravet
steve.ravet at arm.com
Advanced Risc Machines, Inc.
www.arm.com