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Q: A/C starts blowing cold then gets hot randomly.

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When i first start my truck and begin driving the a/c works great. But then after maybe 15 minutes it will stop blowing cold. Sometimes it will run for an hour with no problems though. It is very sporadic. When it does cut out, the air stays cool for a few minutes slowly warming up to ambient temp. I have not tried filling the coolant, and i have not changed the a/c switch either. I have changed the (forgive me if i call it wrong) damper that switches from heating to cooling under the dash, but that was because it wouldnt switch to heating in the past. I dont want to condemn the compressor just yet since they are so expensive.

Do you think this problem could be caused from simply high coolant pressure and needs more added? Or would it have to be the compressor? I dont want to waste money on the wrong path.

Thank you for your help!

My car has 170000 miles.
My car has an automatic transmission.

A: A possibility is an electrical problem such...

A possibility is an electrical problem such as a faulty clutch cycling switch. Also, check to make sure that the electric condenser fans are operational, and cycling properly, as the compressor kicks on and off. Faulty condenser fans, or the fan controls, will cause fluctuations in the cooling capacity. As far as the state of refrigerant charge, the level can be "inferred" from high and low side pressure readings on your system (don't try attaching gauges yourself unless you are trained - liquid refrigerant blown in your eyes will blind you). It could be that you have a combination of issues and it is also possible that the charge is just low enough to intermittently set the low pressure cut off switch. The best initial step to diagnose this issue would be to have a certified technician perform an inspection of your AC and heating system.

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