When going down the highway the air conditioning blows fairly cold when the temperature outside is below 90 degrees. I notice when it is hotter than that, the compressor will kick off for 1 or 2 minutes then will come back on for several minutes then kick off again over and over. While sitting in the driveway the compressor stays on. It does not cycle on and off like it is low on freon. Not sure if this is a separate issue but this car has never blown cold air in stop and go traffic. It will only blow cold air on the highway. Everything accept the compressor has been replaced on the car within the last 8 years (condensor, evaporator, orrifice tube, drier, pressostats). I have heard mention about a thermostat running to the evaporator and a thermal cutoff on the compressor. Could these have anything to do with the problem? I am also wondering about a high pressure cutoff??

DJ,

I'm no ac expert, but I think 134a cars are quite sensitive to over charging with freon, and that the compressor cutting out in high ambient temps is one of the overcharge symptoms.

Charley

Okay,

I releived some of the freon (twice),clutch on compressor stays engaged at idle and up to about 2300 RPM, but after that it cycles on and off quite frequently. It seems the more I let out, the warmer air it blows. What to do?

Thanks,

D.J.

DJ,

I am under the impression that the only true way to get the proper amount of Freon in the system is to start over. That is, go through the pulling a vacuum procedure, and then add the Freon by weight. There should be a spec for your car, which will have a + / - tolerance. Because newer systems tend to use less Freon, the + / - tolerance is less, which makes a good guess harder.

Below is a quote from an ac certification study guide.

Charley

Charging Systems with Small Cans or by Attempting
to Use Pressure Readings
Part of this one’s already been mentioned, but If you’re
charging with small cans, or even using 30 pound cylinders,
but trying to charge a system going by gauge readings, or
using any other method that does not involve accurately
weighing the amount of refrigerant entering the system,
STOP!
Pressure gauges are a diagnostic tool, but you can’t use
them to meter refrigerant into a system, and no vehicle or
system manufacturer provides exact pressure specs for a
precise charge. Even if you have reliable pressure readings,
as we’ve already pointed out, there isn’t enough of a pressure
difference to indicate over or undercharge.
System pressure readings, whether the system is operating
or off, will not identify the amount of refrigerant in a
system. The only way to assure that any given system’s refrigerant
charge is correct is to recover all of the existing
refrigerant, perform a thorough evacuation and install a
known amount. If you have a recovery/recycling only machine,
consider purchasing an accurate digital scale, perhaps
one with a solenoid-type dispensing valve.

While that long verbage may be correct for an indoor shop with the right (expensive) gear, shady tree mechanics have learned to install or top to the correct amount of freon without worrying about weight. My method is to "load" the system with fan on high and windows or doors open on a warm day. I gradually add until short cycling just stops. I also monitor the temp of the suction line with my hand. From there its a bit of trial and error to find the sweet spot.
--
David Hunter

This could be your problem:
http://www.youtube.com/watch?v=Wx_nOq1OXUU

That makes sense. Last year while going down the road I heard a popoff valve let go of some freon and it was very hot out. Any suggestions on how much freon to let out?

I'd guess you have cooling fan issues or the condenser airflow is blocked.
http://www.brickboard.com/FAQ/700-900/HeatingAirConditioning.htm#PoorACPerformanceDiagnosis