[Vwdiesel] Manual AC cycling

LBaird119 at aol.com LBaird119 at aol.com
Sun Jul 19 01:25:27 PDT 2009

In a message dated 7/18/2009 1:36:50 PM Pacific Daylight Time, 
tadc at europa.com writes:

> Now I've never heard a technical explanation as to *why* this is, but I
> assume it's something like this: when you turn off the unit, it's still 
> got
> a full head of pressure in the system.  If you were to turn it back on
> immediately, the electric motor driving the compressor would have too much
> load and risk overheating or overloading or whatever.  Waiting 3 minutes
> allows the system pressure to drop.

  Yup, that's why electrical compressors have a load/heat bi-metal 
cutout on the body of the compressor.  Too much load or too hot 
and it clicks open.  You have to wait for the pressure to equalize 
so the load is reduced.

> Why do I ask?  Because I enjoy the benefits of AC, but I don't like the 
> penalty.  I sometimes try manually cycling it so it only runs during
> MPG-optimal times, such as when decellerating, braking or coasting 
> downhill,
> but I wonder about the wear-and-tear penalty on the system.

 Automotive systems don't see quite the same load. Load comes 
from pressure diferential and volume.  Most electrical systems (other 
than whole house) tend to equalize more slowly due to the lesser 
volume of refrigerant cycled through the system over a given period 
of time.  More volume, more btu's transferred.  Also many systems 
you're familiar with are for freezing rather than cooling, thus a higher 
pressure differential.
  R-12 automotive systems run about 30psi on the low side and 
120 to 140 on the high.  The needles move about 10 degrees/second 
as soon as the clutch disengages.  The pressures equalize rather 
rapidly.  The closer they get the slower they equalize but it's still 
  Cycling mostly puts wear on the clutch.  It's rare to ever see 
the friction material worn out on one though.  

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