Expert: Paul Evans/ www.chillywillys.org - 2/12/2011

Question
After replacing the compressor with a used one I vacuumed for 15 minutes without noting the amount of vacuum.  The clutch clicks on and off every couple of seconds.  I am wondering if there is a high pressure shutoff causing this due possibly to a defective expansion valve or receiver dryer (which I didn't replace) or possibly insufficient vacuuming?

Answer
The usual reason for hasty compressor clutches is too much gas.
To diagnose this, note what the high pressure reading is doing when it cuts out.
If the high pressure switch is active the gauge will go round to about 28 bar then shut off
This can also happen if the condenser fan is not working, observe that it is.
A worn compressor clutch that is drawn in by an electromagnet will also disengage quite quickly if the magnet is weak.

You don't say what happened to the old compressor, if it was burnt out the the following applies before fitting a new one;
Remove compressor, expansion valve and drier. Dispose of by an approved and legal method
Pressure flush open remaining components with an approved flushing agent. This is not brake cleaner, a soap based product or similar.

Replace Compressor, expansion valve and drier with new.
Vacuum for 1 hour then recharge with OE specification weight and recommended PAG oil

You describe your vehicle as '90 Mazda' if this means registered in 1990 the it will be the old system outdated in 1993.
If the compressor you have used is from the same era it will not like the new gas without a conversion oil. If for example the old recharge amount was say 1kg the conversion amount should only be 70% of that.The drier and expansion valve will be geared to the old gas but should work better than they are.
I hope this gives you a few idea's to be getting on with, 15 minutes is never an acceptable vacuum but is unlikely to be the cause of the problem.

Please write back when you have tried these things if they don't work.
It would be useful to have the year of manufacture, the age of the new compressor and what happened to the old one.
If I had to guess from the information I have, the compressor clutch sounds worn, it is not engaged long enough to allow the high pressure side to reach 28 bar, however, there is an outside chance that the pipe leaving the compressor on the high side is blocked, mind your fingers but if it almost too hot to touch with the system running and the condenser is cold then a problem exists there. A blockage/ restriction on the high side with be very hot before it and very cold over it

Let me know