A/C blows ice cold when driving, warm when car is stopped


Original poster
Apr 5, 2012
The A/C in my 2005 6 cyl Trailblazer with manual A/C control blows really cold when I am driving the car, but as soon as I stop at a light or something, it gets warm and muggy air. I thought that maybe the condenser fan was inop, causing no airflow over the condenser, but in looking, I don't have one! I assume the car relies on the engine driven fan to both cool the engine and produce airflow over the A/C condenser? I imagine the fan clutch is working properly, as the car doesn't overheat, even down here in the Texas heat. I got the cheap auto parts A/C gage, and it says that compensated for ambient temperature, the charge is right where it should be.

I know very little about A/C. Does anyone have any ideas on this?



Aug 26, 2012
if you pop the hood when stopped and the AC is not working, (but should be) does the compressor clutch look like it is not engaged?

common problem is that the compressor clutch is old and weak. (specifically the coil)

fixes can include, removal of clutch shim (skinny washer under the clutch) (if the gap is excessive), replacement of clutch coil (available as rebuilt ac clutch) or replacement of the compressor with the clutch.

mine started doing it last summer when it was 100. kicked off with the family on vacation in backed up traffic on I-95. not fun. I have isolated that the signals are all proper, and have the clutch to replace. just waiting for a cool day in nice weather to tear into it. ( I think I will pull the alternator to get to the compressor, unbolt and tip to access the snap rings that hold the coil and bearing pulley on the compressor.)

To really check health of system, you need a gauge set to measure high and low side, and the right table of readings.


Dec 6, 2011
You are correct that the engine fan pulls air across both the engine radiator and AC condensor. Even in the heat of summer it really doesn't take much to keep the engine cool (under light load). The AC, on the other hand, requires quite a bit of air movement from the fan.

Does the AC ever blow cold (or cooler) again while stopped or does it remain warm indefinitely? In other words, is your AC attempting to cycle back on? You may also notice a subtle change in how the engine idles.

Like meerschm suggested: with the hood popped and the AC on high, look at the AC clutch (driver's side, bottom of the engine). Does it cycle on briefly then kick back off? Stay off?

How comfortable are you with basic electrical work? If you jumper 12v to the fan clutch it will engage fully (can explain if you want to try). If the AC then cools properly you can start troubleshooting the fan clutch. If not, it's likely a problem with the AC system itself.

Finally, consider that the AC condenser may be crudded up enough and require a hosing down.

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