The idle controller does get a signal from the AC compressor to compensate. When the clutch engages there should be a momentary jump and then back in control with maybe an increase of 100 rpm.  It often happens that the idle controller can go bad where it responds to the AC signal but does not regain control. This is often shared with the power steering causing the same effect if the wheel is turned while the van is still or turned to a full end lock. 

Idling at 2,000 rpm with the AC will build pressure and with no benefit of air moving over the condenser from vehicle motion the condenser pressure can raise to increase the fan speed. Especially for a system converted to R-134a and that may also be over charged. As the condenser and radiator have some age on them the space between the fins-tubes does not cool as well as when new due to dirt and corrosion. The fan is needed more to compensate.  A pressure gauge set on the AC will confirm the fan switch operation. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Dan N
Sent: Friday, October 17, 2014 12:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: High idle w. A/C on... please help

hi all,

1990' Carat - recently purchased - rebuilt engine - smooth idle 950 RPM - A/C recently serviced, cold air, new fan switch.

When I turn the A/C on the idle jumps to 2000 RPM - the fan goes to medium speed all the time.

What part or parts that triggers the idle?

What part or parts that triggers the fan?

please help -  I search in the archive and only found an April 2000 article about this..

many thanks in advance

dan