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My A/C in my trailblazer randomly stops blowing cool humidity free air

So I can’t figure out what is causing my a/c to randomly stop working,it’s not leaking or freezing up and it’s not the clutch or fuse and so on. What it does is basically it transitions from cool humidity free air to warmer humid air It’s been in two different shops and produces no error messages either,however when it does it if you turn the car off and start it again it starts working fine. I’ve been going crazy trying to figure out what it is! Any suggestions??

When my car had those symptoms, the issue was a short in the wiring harness on the compressor, and the compressor had to be replaced.

I suggest you find a good A/C shop and drop it off with them. Tell them the technician is free commute in your car if they are unable to reproduce the symptoms at the shop. Prepare to be without your car for at least several days, because A/C troubleshooting can take time.

I had a similar problem on my 2000 Blazer. Either the high or low pressure switch was replaced to fix the problem.

Ed B.

I second the pressure switch.

My father’s 2003 Trailblazer EXT had a similar problem recently and the A/C low pressure switch (the one attached to the accumulator) was the culprit.

I think I’ll start with the two switches first and see, I understand they are fairly cheap. If that doesn’t work then I’ll look into the wiring and what not.

yes, the switches are about $35, but it does take some labor to install them. Refrigerants have to be evacuated, switch installed, a/c system vacuumed (evacuate all air) and then refilled with refrigerants.

Unless you are comfortable with car a/c and troubleshooting, I recommend to just take it to a mechanic and have him/her do it for you. It may not be one of the pressure switches, could be a bad clutch or clutch coil or even the compressor (likely not). By the time you start replacing switches and refilling with refrigerants it will easy cost you a couple of hundred $$$. It’s not rocket science troubleshooting an a/c system, but does take experience and knowledge of gauge readings.

Replacement of either of the pressure switches on this particular vehicle (at least the 2003) doesn’t require system evacuation. They are simple remove and replace. The ports they screw onto have Schrader valves in them.

One way to test the switches is to tap on them lightly while the A/C is turned on. I did this on the low pressure switch on my father’s car and it caused the compressor clutch to randomly engage and disengage.

Clutch coil?

Well the clutch is still engaged and running when the car does it. I’ll find out if they are schrader valves and go from there.

If the compressor is operating when this happens, then the problem is not the pressure switches or the clutch or the clutch wiring.

@Salser2012, you say the system isn’t leaking. How do you know for sure? A slight undercharge (not enough to trip the low pressure switch) could cause the air to go warm.

The GM service manual contains extensive diagnostic info and procedures. In fact, in one of the charts it is asked, “Did the customer mention that the A/C system output temperatures are good at first, but then turn warm during extended drives?”

As mentioned by @Whitey and @kurtwm2010, you may need to find a good A/C technician that can perform the proper procedures for a diagnosis.

Why in th world would you do that? It’s more fun and probably more expensive to just keep guessing and throwing parts at it. On the other hand, you just may get lucky and the first part you throw will do it. Good luck with that ! !

Does it happen when you’re cruising down the road or more when you’re stopped in traffic?

What year is your trailblazer? For example, the older trailblazers had a weakness in this area due to the electro-viscous coupled fan. It required PCM reprogramming. But a weakening fan clutch has the same effect and these are know weakness with that design. Grasp the fan when the engine is off and see how easy it is to rotate it by hand…

Aside from the actual AC unit, you might be having trouble with the HVAC venting controls. Something like the mode or blend doors changing position erratically…

Looking for a short in the wiring harness would be cheaper than throwing parts at it. Just start the car, turn on the air conditioner, and then gently bend and twist the wiring harness a little to see if the compressor disengages and re-engages when you have it in different positions.

Maybe I missed it, but nowhere did I read in this thread whether or not you looked at the refrigerant level/pressure.

You might be borderline low on refrigerant, but sufficient pressure to activate the a/c clutch. Keep in mind, as you cruise around with the a/c on, the compressor will cycle off when it feels it is not needed and on again when it is. If you are low on refrigerant, during the off cycle, you might actually experience warm air coming into your car.

Before replacing sensors I would make sure the refrigerant is at the recommended level. Cheap and easy to do.

I agree with that to some extent. I’m the kind that likes trying to figure things out for himself.

I was going to add a similar comment but you beat me to it.

Back in the mid-naughts, there were some service bulletins regarding the fan clutch and a software update for the HVAC control module.

If something within the air door system gets too out of whack DTC’s should be set, but this possibility should still be addressed, especially since the OP said power cycling seemed to fix the problem (though for how long it is fixed is unknown). A GM scan tool might be required to test all of the individual components.

A crude test to see if air is being directed through the heater core when it’s not supposed to be would be to crimp one of the heater hoses to block coolant flow through the heater core and see if that makes a difference.

These are all great ideas to try and yes I love to work on my vehicles myself but this situation with the A/C is driving me crazy. As I stated before it’s been in two different shops and hooked up to diagnostics for A/C one of which is a Chevy dealership and second is a shop owned by a family friend,and neither one can find any errors or what not and it is not leaking coolant that’s one of the first things they checked. As I stated before it is random when it does it, (usually after it’s been on a while) the clutch is still engaged and spinning so I really don’t think it’s that or the compressor. I recently however noticed something funny about it when it acts up and that is a sudden change in the voltage gauge in the car, it’s always pointed straight up and hovers around the fifteen (I think it’s fifteen I forget right now) but when it happens it’s dropped in between that and nine or lower. But if you turn the car off then back on it starts working again so I really think it’s something electrical like the switches or the harness,that being said should I try the switches and wiring first? And if those don’t do the trick then what? Help!! Lol

The reason I’m on here looking for ideas is because my father in law who is an engineer and mechanic is the one with the friend that owns the shop, basically said all we can do is wait and see what it does. His knowledge of a/c units isn’t very much unfortunately but I’m the type of person to not stop which is why I need ideas! I’m not an amateur mechanic as I do all my maintenance myself (some with the help of my father in law cause he has a lift in his barn) but this has me stumped…

Yeah, it’s these types of situations that call for leaving it with a good competent a/c technician who can keep and drive the vehicle for several days. The problem is that if it’s a short, it won’t act up while the vehicle is sitting still in someone’s shop. It will only do it when the car is moving.

By the time you try all the suggestions given here you probably will be at retirement age and have tons of gray hair. Here are two more suggestions to make that happen:

Rather than the pressure sensors or actual compressor at fault, there is a chance that you have problems with the clutch coil. If it is on its way out due to burned windings or insulation it may not hold the current needed to keep the clutch hub against the pulley. There is an easy way to test this. Unplug/disconnect the coil from the compressor (probably only one hot wire) and, using a multimeter measure the ohms. A normal reading should be between 3-5 ohms, anything less or greater would indicate a problem.

Ok, another suggestions: The spacers between the clutch and pulley assembly may have worn down or the clutch may have worn down changing the air gap between the clutch and pulley. With a simple feeler gauge you can measure the gap and see if it falls within range. It should normally be between .02 - .03, but may vary from model to model. An incorrect gap may cause the clutch not to engage or even intermittently disengage because the coil may not be strong enough to pull and hold the clutch in.

Dumb question: your condenser fan is running when the a/c is on, right?

@Salser2012, what is the year of this Trailblazer?