air conditioning clutch not engaging?

[chris b]

My aircon is not working... only had the car a few months so don't know if it's ever worked while I've had it.  Have tried to do some fault analysis and noticed that when AUTO is selected, the revs change slightly, but there is no "clunk" noise like I've heard before when the air con starts on 9000s.  The centre section of the air con compressor - the one with the 3 circular "bolts" on it, does not spin either with AUTO engaged.  I've checked the voltage on the blue/yellow wire to the compressor and it shows 14V when AUTO selected (and pretty much zero when in ECON).  My best guess is that the clutch is not engaging.  It's a SANDEN compressor.  I've tried knocking it with a hammer when running but no luck there either!

Grateful for any advice on how to fix it... bloomin hot in the car today!

[sgould]

If the clutch is engaging, you may have no fluid in it.  Get it checked and regassed.  If it hasn't been used, the seals will be dry and leaky.

[phoenix]

It may be that the clutch is worn and the gap too great to pull it in to engage. Full procedure here:

http://www.saab9000.com/procedures/ventilation/airconclutch.php

[chris b]

I've had a look and it appears to affect seiko and not sanden compressors.  Mine's a sanden.  I hit it with a hammer when running and it wouldn't jump into engagement which made me think it is more than just the gap...... will take another look though in the morning - ta

[Max Headroom]

I hit it with a hammer when running and it wouldn't jump into engagement which made me think it is more than just the gap...... will take another look though in the morning

In aviation we just go and get a bigger hammer 

[chris b]

tried that! 

when I connect a 12V supply direct from battery, the clutch engages (yipee!), but it won't run on AUTO, even though I can detect a weak 12V signal through the cable when on AUTO....  is there another electrical switch or signal that commands the clutch to run?

[alang]

Not sure if this helps but the wiring diagram shows a relay (in car) with a feed from fuse 11 which pulls in the rad fan and the AC compressor.  It is pulled in by a feed from fuse 12 which looks as if it can be interupted by an anti freeze stat on the evaporator and the rad fan pressure switch on the accumulator/dryer.   (I think that is right, always have problems following these diagrams through).  I seem to remember hearing of problems with the dryer pressure switch holding the comressor out.

[chris b]

Fixed it!  Got it regassed tonight (it was down to around 400g from the 950'ish it should be at), but the compressor still did not run when AUTO selected.  Then took the air con relay out (relay box next to the battery).  Opened up the tabs to take the top off the relay unit and it is a simple 2-switch relay.  The lower switch was almost atomised away and not making when the relay closed.  When the contacts were held closed by hand, the compressor ran.  Have bent the lower contact slightly to reduce the gap so that the contact makes when the relay is energised, and now it works fine.  It's a temporary solution until I get another relay.  Hope this is helpful to anyone else with compressor woes...

[sgould]

Well Done!

[chris b]

thanks to everyone for their advice!