[DML] Re: Air Condition at Highway Speeds

["Dave Swingle" <swingle@xxxxxxxxxxx>]

It's based on pressure in the system. Depending on the vintage of the 
car, there are either one or two pressure switches in the freon 
system. All cars have a low pressure switch (the one on the bottle), 
cars after a certain point (not sure when but later than 3000?) have 
a high limit switch too. The cycling is caused by the low side of the 
system going low, or the high side of the system (condenser)going too 
high. Normal operation is the first case. The length of the cycle 
will depend on how hot it is outside and how high your blower (inside 
the car) fan is running. If it's exceptionally hot out the cycles 
will be very long, when it's below about 70 out it seems to be as 
short as 8-10 seconds. 

There is no relay running the compressor, it is run right off the 
limit switches. The clicking relays that people are talking about are 
the fan relays that are slaved via a diode from the limit switches.

If you don't have sufficient air flow thru the condenser (highly 
likely if you are having engine overheat problems too) your system 
pressure may be too high (bad for the compressor). In the later cars 
this will hit the high limit also causing the compressor to cycle, in 
earlier cars if it's bad enough you'll blow freon thru the relief 
valve (or worse - blow up hoses). You probably ought to put a 
pressure gauge on the system and see what the high and low pressures 
are at the compressor ports. 

What's the VIN on your car? Are you sure both fans are working, 
blowing in the right direction, and moving enough air? 

Dave Swingle


--- In dmcnews@xxxxxxxxxxxxxxx, "Videobob Moseley" <videobob@xxxx> 
wrote:
> Please wllow me to hijack this thread for a moment....
> My AC compressor seems like it is on all the time (when the AC is 
on)
> the clutch doesn't cycle on and off, except when I turn the AC off.
> Is this normal?
> What tells the clutch to release on and off?
> What should I check?
> Is there a relay or sensor that operates this?
> Thanks.
> - VB
> 
> >From: "John Macias" <jdm5@xxxx>
> >Reply-To: dmcnews@xxxxxxxxxxxxxxx
> >To: dmcnews@xxxxxxxxxxxxxxx
> >Subject: [DML] Re: Air Condition at Highway Speeds
> >Date: Sun, 24 Jul 2005 22:58:18 -0000
> >
> >Clicking is good, it means that the compressor is cycling on and
> >off.
> >Cycling on one second intervals usually means that you are low on
> >freon. I just had mine serviced (cycled every five seconds or so),
> >and now no more cycling.
> >
> >John
> >#7009
> >
> 
> >







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