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The air conditioning does not operate on my 89 Bentley. I have performed most of the test procedures as set out in the Work Shop Manuel (TSD 4700) using a multi-meter and I have not uncovered anything that indicates a problem. In doing the test for the fans control circuit I can faintly hear the relays energize, however the fans do not operate. The manual does not indicate whether or not the fans should operate when 12 volts is applied or that the relays should only energize during the test. Flying Spares has a test box for rent but it appears that all of the test procedures can be made using a multi meter. Do the mirco-processor boards become defective over time due to age and temperature conditions? Thanks for any insight you may be able to provide. Tom Greasel
 

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There are many components to an A/C system.

First..Can you verify the A/C compressor clutch clicks on and locks?
Does the system come on but not cold?
A lack of refrigerant will shutoff of the compressor
 

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I have come across the occasional board failure but not that often in my experience. Have you pulled the plug off the 44° sensor? That is a common fault that prevents operation. Also check that the ground wire under the carpet near the ACU has good contact.
 

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Discussion Starter #4
There are many components to an A/C system.

First..Can you verify the A/C compressor clutch clicks on and locks?
Does the system come on but not cold?
A lack of refrigerant will shutoff of the compressor
The compressor does not engage at all.
I have come across the occasional board failure but not that often in my experience. Have you pulled the plug off the 44° sensor? That is a common fault that prevents operation. Also check that the ground wire under the carpet near the ACU has good contact.
I do not know where the 44 degree sensor is located. The ground wire is making good contact.
 

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Thermostat housing, plug with two wires. The one with one wire is for temperature gauge so not that one. Just pull off the plugs with two wires to test. This topic has been covered many times in the past and a search should show diagrams which will pinpoint the exact sensor. It's actually just an on/off switch, normally closed, that opens at 44°C allowing operation of the ACU.
 
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