[Miata] Air Condx Short Cycling

[James Thorusen]

For Michael and the list:

Greetings!

   The phenomenon you are describing is known as "short cycling".   Here's a
diagnostic tip from my book on refrigeration:   "Rapid pressure rise may
occur on the low side where an expansion valve is leaking.   Short cycling
may result.   This leak will cause a frosting or sweating of the suction
line."   In other words, a bad expansion valve is a likely culprit.

   The problem you describe is almost always associated with anomalous
pressure readings, however.   I am at a loss to understand how you can have
normal refrigerant pressure readings.   A 1999 car will have R-134A as the
refrigerant.   Are you certain that you are using the correct pressures for
the type of freon in the system?

   Your comment....

"2.  If you sit still for too long  (maybe ten minutes
tops) with the AC going, then the car will rev to 2000
rpm and stay there.  AC will completely cut out.  You
have to either move or shut it off to stop it."

....is explained by the high side over-pressure cutout switch shutting off
the current to the AC clutch.   The engine control unit cannot "see" this
disconnect and so advances the throttle to compensate for the AC load on the
engine, exactly as it is supposed to do.   Since this load is not present, a
high idle results.   In any event, the cycle time of the system is
controlled by the high side over pressure switch.   For whatever reason,
this switch is opening and closing too rapidly, hence the short cycle.

   If the system pressures really are correct and there is no moisture or
air in the system, then the most likely suspect becomes the over-pressure
cutout switch itself.

Regards,
Jim T.
entropycat at verizon.net
'74   914
'90   Miata  (x3)