Subject: Help! a/c compressor stays on
  From: "SteveM" <>
  Date: Tue, 20 Jun 2000 18:27:15
  X-Message-Number: 62

I just changed the a/c compressor pulley because the bearing went out on   the old one. Now the compressor stays on all the time. The pulley was   identical to the old one and I'm pretty sure I re-assembled everything right. The curious thing is that the pulley was a Ford part which makes me believe the PO didn't use OEM part.
Any suggestions before I bring it in?
Steve '83 928S


1. The compressor could be "on all the time" for many reasons, let's start at the beginning.
a) since the pulley was removed and replaced with another it's possible that the air-gap between the pulley surface and the front hub surface is not great enough; it was not shimmed properly.  Inspect the air-gap, you want approximately .015" to .032" of space.  To make it easy check this measurement using two feeler gauges. A .014" inch should go in and a .033" should not.  Check the gap at three evenly spaced increments around the "clock".  If the gap is not correct you should remove the nut on the front hub and place a shim washer on the compressor shaft to shim out the front hub.

b) the relay on the ac board could be hanging up causing the coil on the clutch to be active all the time, usually they simply burn out.  You can check this using a simple circuit light or volt meter, checking the current at the wire on the front of the engine belt cover that goes to the compressor coil.  If the relay is kaput you can either replace it (check the Rennlist for past posts on the procedure) or buy a refurbished or used board from either 928 Specialists, 928 Int. or a dealer.  If the relay and board check out OK then the board is still receiving a signal to stay on which is related to either the evap being warm or system is malfunctioning.  You can check the evap temp with a thermometer, it should not go below 31 degrees (typically) else it will freeze up.

2. I don't know who "PO" is.  Porsche to the best of my knowledge only uses OEM parts from NipponDenso.  The Nippon (aka Denso) on your car looks like some compressors found on older Fords, but it does not have the same mounting ears.  The pulley could be look like those found in other vehicles but from what we know the pulley clutch assembly is unique to the 928, though we have come across some strange attempts to duplicate the unit by aftermarket rebuilders.

Check your air-gap first.  If that does not correct the problem then check for constant current at the coil.  If these suggestions do not resolve the problem give me a call and I'll attempt to help you (Friday I have a 12:00 noon T-off... sorry, no cell phones while we are on the course).



At 01:44 PM 8/15/00 -0400, wrote:
>My guess is perhaps that it maybe getting tooooo cold and the temp sensor in the evaporator is shutting it down so it doesn't freeze over.

This is the function of the freeze (perhaps more accurately, the anti-freeze) switch. However, when the freeze switch cuts the compressor off, the evaporator will quickly warm up above freezing, and the compressor will cut back on. You should never be able to detect the switch operation, as the evaporator should never get above 35 deg or so. The goal is to hold the evaporator temp at 33 deg F.