Date: Wed, 5 Jun 2002 10:06:42 -0700
Reply-To: Keith Hughes <keithahughes@QWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@QWEST.NET>
Subject: A/C Advice Needed -- Kind of Long
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> I'd have to dig out my shop manual to verify the specifics, but on my
> '88 Westy with factory air,
Yes, I think you need to review your wiring schematics.
> it is wired in such a way that two things happen whenever the A/C is
> turned on.
> 1.) The radiator fan switches on to high speed
> 2.) The idle gets bumped up a higher speed.
If yours works this way, then someone has modified it! Basically,
here's how the system is *supposed* to work (take a look at the
Bently):
1. Switch on the A/C and IF the temperature is higher than the
thermostatic setting:
a. The A/C relay sends power to the cooling fan resistor pack,
through both resistors, and the fan runs on low speed.
b. IF the freon low pressure switch is made (i.e. sufficient charge
in the system to prevent compressor damage) the compressor clutch
engages.
c. The clutch engaging sends a signal to the idle control relay to
raise the idle.
d. The evaporator fan comes on at whatever setting the switch is
in.
This is pretty much the status quo for normal operation unless, or
until:
1. The compressor discharge pressure exceeds 15 bar (about 220
psi): This will happen soon after the system starts cooling. At
this point, the *2nd* stage cooling fan relay is activated, sending
power to the cooling fan resistor pack *bypassing* one of the
resistors, and the fan runs at a higher, intermediate, speed (still
much less than high speed).
2. The radiator temperature switch reaches the level 2 (high temp)
setpoint: When this occurs, the cooling fan resistor pack is
completely bypassed, and the fan is powered directly by battery
voltage. ALSO - the radiator temp switch level 2 contacts send
power to the A/C Compressor Clutch Relay (please note this is a
*normally closed* relay) which then opens, removing power from the
clutch (i.e. compressor disengages).
Note: the high speed fan is never intended to operate *at the same
time* as the A/C compressor.
> So, I'm not understanding what the A/C shop did since this is how the
> factory wiring is..
Well, no it isn't actually.
> Anyway, since you had such a vibration with the fan, I'm inclined to
> think that there is something wrong with the fan,
Well, on High speed, mine vibrates like crazy. And has for ten
years. Since the Tiico install, however, it's never used since the
motor stays cool (not so my late wbx - however).
> Also, I'm curious as to how the compressor failure is in any way related
> to the failure of the engine fan.
Skyrocketing head pressure?
> I thought compressor failures are usually related to a lack of
> lubrication caused by leaks in the system,
> or incorrectly mixing incompatible freons which use different oils
> (e.g. 12A and 134).
> I'm no HVAC expert - but I just can't see where not having the radiator
> fan work would cause the compressor to fail.
As I said, high head pressure is one. However, given that Ray said
it had stopped cooling, it was probably out of freon due to leakage
(maybe something existing that the shop didn't find - and maybe
not) and the low pressure had switch crapped out. These are
'normally open' switches, and when they get old and gunked, they
can as easily fail closed as open. This is something *I* would have
checked when evacuating the system - takes 2 seconds, and may save
a new compressor. Better yet, always replace it when doing major
repairs - cheap insurance.
Of course, if the low pressure switch is bad, any overpressure
situation (like 'cause the fan wasn't running) could force a major
leak (blown o-ring, swedged hose fitting, hose, etc.) and you might
never know it until the compressor was toast.
Another possible cause is compressor valve damage from high head
pressure caused by the fan not running. Early on, the valve leakage
just caused short-circuiting so that system pressures couldn't be
maintained (stops cooling), followed by seizing up. In this
situation, however, it *should* have overheated one, or several,
times when the A/C was on. If not, there must have been adequate
air flow through the radiator (i.e. not idling around) to keep the
engine cool, and that'd be more than enough to keep the head
pressures from getting critical, unless the system was overcharged.
And Ray, when the system is *NOT* cooling, don't run it. If you
really need the evap fan running, disconnect the wire to the
compressor clutch. Relying on the low pressure switch to keep you
from grenading the compressor is a bad gamble.
Keith Hughes
'86 Westy TiiCo "Marvin"
