A/C Problem 99 Suburban won't cycle off

You might take a look at the inside temperature sensor if the 99 Blazer is set up to maintain a certain temperature.

You didn't say weather or not you have a problem with insufficient cooling or not. I assume you do and that is why you are worried about the compressor cycling. If it the system is providing sufficient cooling in the cab then you probably don't have anything to worry about

So, the compressor should only cycle off when the low pressure clutch cycling switch senses the evaporator pressure is below a preset pressure, usually around 26 psi. There could be several reasons why the evaporator pressure would not reach that point. Most of the reasons are normal. A high heat load on the evaporator will keep it's pressure up. That could be due to high in cab temperatures or hot outside air getting into the evaporator case. There could be other reasons but I doubt that is what's happening.

Maybe your compressor just doesn't have enough suction to pull the low side down at idle. Try speeding up the engine a little for about 3 minutes. If it still doesn't cycle then you may not be getting enough air flow through the condenser. The fan clutch may be going out and the condenser pressure is staying so high the low side pressure is also affected. To check the fan clutch for proper operation warm the engine up to normal operating temperature and have an assistant turn off the ignition while you watch the radiator fan. If it continues to spin for a couple of seconds after the engine quits turning then, it needs to be replaced.. For proper diagnosis you would really need to hook up a manifold gage set and monitor the high and low system pressures. An experienced tech should be able to tell from the pressure readings weather the system is working properly or not.

Kevin, Thanx for the response...I know that the accepted way to determine proper level of charge today is to use an infrared thermometer to measure the difference in temp between the inlet and outlet of the evaporator....the inlet temp appears to be correct, but the outlet is much too high...does this mean that I have an undercharged condition? I am not sure if this temp difference is the same for a Suburban...

Vince

Well, using the latent heat method (sometimes referred to as the super heat) to judge refrigerant charge works well on home A/C systems, but automotive systems are different. You do a much better job of analyzing the system based on pressure readings rather than evap temperature.. Home units do not have to operate in such a wide range of operating and load conditions and do not have the same sort of control systems. Checking the inlet and outlet temp of the evap is only one small piece of the whole story. Also the latent heat difference for various refrigerants is different. Home units use R22 while most automotive applications now use r134a. Both have different boiling points and operate at different evap pressures with different latent heat values.

An interesting article on refrigeration:

Besides what is in that article there are only about a thousand other things you need to know to diagnose and repair most automotive A/C systems. That's really not a lot compared to the many thousands of things you need to know to diagnose and service some other automotive systems such as the electronic engine controls, but it is a good place to start.

the symptom sounds like its slightly low on gas.dont take much. if it hasnt been used sometimes you can loose the good bit of the gas mix and be left with the not so good and oil.this is why you stay away from blended gasses use R134A and leave the a/c on all year.

Im guessing that you checked the basics like the blend doors and recirc door,leaves in the evaporator...with these not working/adjusted radiator restricted/ cooling fan not working 100%.very basic stuff.belt tensioner-harmonic balancer.

if it has a simple system the temp idea works,you give it a rev and the suction pipe pulses with cold gas and you can look at the frosting on the evap.,with new systems we have to measure the suction hose as below freezing after 1/2 hour.as the gauge pressures are needed but not as staight forward as the older cars.

what i was wondering if its an orifice system the orifice is out the front and all the 2 pipes at the evap will be cold the hot pipe stops at the orifice.change of state gas to liquid.Why i added is that i wanted to know if this model has the control valve compressor.the later dephi compressors fitted to GM cars idle back when the low side reaches

40 psi so they run all the time but dont destroy the clutch.they use the cycling switch to prevent freezing.in low humidity they run all day.

Thanx for the reply....it's GM DA6 comp with orifice tube in liquid line, cold after orifice tube. Checked evap doors,use only Dupont Suva refrigerant as I have had trouble in the past with the "offshore" stuff.

Thanx for the reply....it's GM DA6 comp with orifice tube in liquid line, cold after orifice tube. Checked evap doors,use only Dupont Suva refrigerant as I have had trouble in the past with the "offshore" stuff.

may be that the thermostat (expansion valve) is shot, and continues to call.

why not get a cheap pencil-type gauge and check the low-side - it'll give you a reading.

with a situation like that - charging indiscriminately is a no no.

mho v=83e