Date: Mon, 1 Jul 2002 16:26:41 -0700
Reply-To: Keith Hughes <keithahughes@QWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@QWEST.NET>
Subject: ac pressure question
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Gary,
> Is it safe to assume that your r134 pressure is
> neither too low or too high if your high speed
> radiator fan cycles on and off at reasonable
> intervals?
Absolutely not! The a/c high pressure switch turns the compressor
off, and has no connection to fan speed at all.
> I can confirm the high speed cause by
> turning the ac off when the high speed fan is on and
> observing that the high speeed fan goes off?
No, you can't. The HIGH speed fan is controlled by the second stage
contacts in the radiator fan switch only. If you have an '86 model
year or newer (not sure about 83.5-85 model years but I think
they're the same) you have *3* radiator fan speeds. The lowest
speed is when the radiator temp switch hits the lower setpoint, and
the fan is powered through two series resistors. The Middle speed
is engaged when the a/c compressor kicks in (and the low pressure
switch is made), at which point the a/c relay powers the fan,
bypassing one of the resistors. HIGH speed *only* occurs when the
radiator switch hits the high setpoint. The fan is then powered
directly from the battery, and the a/c compressor is disengaged.
> The assumptions I make is that at very low r134 pressures
> the high speed fan will not be activated by the high
> pressure ac relay
If you're looking at the wiring diagram in Bently, please observe
that the a/c compressor relay is "normally closed". That means when
power is supplied to the relay, the contacts *open*, removing power
from the compressor clutch.
> and at very high pressure the high
> speed fan will not go off. This will only work with
> the 86 on up models?
Sorry, but your assumptions are incorrect. The cycling of the high
speed fan is caused by engine overheating due to the extra load,
*and* the extra heat generated in the engine compartment by the a/c
compressor (although probably minor compared to that generated from
the extra load), and by significantly increasing the air temp
passing through the radiator. Due to these heat issues, here in
Phoenix, WBX + A/C is a useless combination. When it's 115°F, the
compressor runs about 30% of the time (due to high speed fan
kicking in) which results in Zero cooling.
Pressure monitoring is for A) not allowing the compressor to start
when the pressure is too low, to avoid damage to the compressor,
and B) disengaging the compressor when the pressure is too high,
same reasons.
Keith Hughes
'86 Westy Tiico "Marvin"