a/c blows cold then hot then cold etc.

[hvacjo]

2000 blazer a/c blowing cold then hot in a/c mode. Refrigerant pressures ok, compressor cycling normally, pressure switch good. Acting like it's modulating between a/c and heat mode.

[mcmalloy]

if it has automatic climit control , the sensor might be gone

[old_master]

To properly diagnose it, we need more info from you.
Automatic climate control or manual AC?
What is:
Ambient temp?
Relative humidity?
With engine at 1500 RPM, windows & doors closed, AC mode switch on MAX setting, what is:
Hi side pressure when compressor shuts off?
Lo side pressure when compressor shuts off?
Lo side pressure when compressor turns on?
What is compressor ON time?
What is total cycle time?
What is the temp at the center register?

[hvacjo]

Auto climate control
ambient temp 103 F
humidity 65%
220 psi high side compressor running
110 psi compressor off high side
110 psi compressor off low side

[old_master]

R134a refrigerant is not nearly as efficient as the refrigerant that was used before, (R12). With R134a, on a good day, you can only expect a 20 degree drop from ambient temp. With an ambient temp of 103F, you can reasonably expect 83F from the registers, anything cooler, consider it a bonus. The 110psi reading you have when the compressor is off, is slightly higher than ambient temp, which is inherent with R134a and R12 refrigerants. They will have approximately the same pressure as the ambient temperature that surrounds the system. ie 103 degrees = 103psi. Heat radiating from the engine will increase pressure/temp slightly.

As for the pressures and cycle times, let me re-word it

With engine at 1500 RPM, windows & doors closed, AC mode switch on MAX setting, what is:
Hi side pressure just as the compressor shuts off?
Lo side pressure just as compressor shuts off?
Lo side pressure just as the compressor turns on?
What is the compressor ON time?
What is the total cycle time?
What is the temp at the center register?

[hvacjo]

Don't think I have a refrigerant issue. Compressor cycling as advertised, not cycling off prematurely. Supply air temp pulling down thru 80 F until hot water circulates thru heater core. After about 15 seconds cool air starts again and the whole cycle repeats.

[blazee]

It sounds like a blend door problem. You may just need to recalibrate it. Have you recently had the battery disconnected or recently replaced any fuses by chance?

Remove the RDO/BATT fuse for a few minutes. Put it back in and then turn the key on. It should cycle through and relearn its position. Post back the results.

[laxman21]

Originally Posted by old_master

R134a refrigerant is not nearly as efficient as the refrigerant that was used before, (R12). With R134a, on a good day, you can only expect a 20 degree drop from ambient temp. With an ambient temp of 103F, you can reasonably expect 83F from the registers, anything cooler, consider it a bonus.

Really? Mine gets way colder tha that when we had 100+ temps.