TrailBlazer AC Clutch not engaging.

2004 TrailBlazer LT, 124K miles, 4.2L, Automatic Dual Zone Climate Control, all stock.

This truck has been very good to me, without many problems at all.

However, now that the weather is warm, I have found out that the AC clutch is not engaging most of the time. Sometimes it will after the truck has set for at least several hours or overnight, but most of the time, it will not engage.

I have checked the charge with my R134a gauge set, and it is full, and blows ice-cold air on the times the clutch is working.

I have switched the micro-relay with others in the fuse box under the hood, but it does not engage. All fuses are good.

I pulled the cover off of one of the relays and plugged it in. When the AC is switched on, the relay snaps on, but the clutch does not engage. If I unplug the low side cutoff switch, the relay snaps off. When I re-plug the low side cutoff switch, the relay snaps back on, but still no clutch.

So, do you think that the clutch coil is shot? Can it be replaced without removing the compressor and thus, evacuating the system and then needing to refill it with PAG oil and refrigerant?

Any ideas are appreciated.


The ac relay to the compressor only supplies the 12 volt power to the compressor through the clutch coil, when it is pulled in by the HVAC control module. It show that it is a light green wire coming from the HVAC control module, section "B-12" on my schematic. If you don't get power from section B-12 to the clutch coil, the compressor coil will not pull in to activate the 12 volt power.. Being that it is intermittent, I would suspect the HVAC control module not supplying power to the clutch coil of the compressor while the relay is activated.

That is the way it looks to me, maybe someone else may read it differently.


Sounds like you did almost everything except the most obvious and common cause of this. Check the clutch gap. Been a problem with GM AC compressors ever since some nitwit elected to use an interference fit for that clutch plate. It either closes the gap so the compressor is running all the time or just getting red hot from friction or more common widens so the magnetic field that decreases by the square of the distance can't pull it in.

If that isn't bad enough, need a special clutch plate remover and installer tool, wouldn't be so bad if they left that same, have three different ones already, keep on changing the threads. Understand places like Autozone loan these out, either buy or make my own.

Reason why you need these tools besides setting the gap, I prefer 20 mils. (0.020") is to remove that clutch plate so you can get at that cheap idler pulley bearing, never had problems with these when GM was putting the AC compressor on the top of the engine and even using a retainer ring. To add to the misery, they are peening that bearing in now. Never have good success in repeening it with an older idler pulley, that cast iron like glass hardens with age, chips off instead of forming a new peen. So end up using flat head screws countersunk to catch the outer race of the bearing.

Another reason was with all this new EPA BS, GM quit using a perfectly good ceramic seal and switch to a neoprene single lip seal, finally went to a double lip seal sometime in the late 90's, that could also be changed with the compressor on the vehicle. But mostly on R-12 systems. With R-134A and PAG, better off to remove everything, drain and flush the compressor with PAG, drain that out, flush out the entire rest of the system, replace the dryer, draw a deep vacuum, and then inject the PAG, but only from a tin can. If the slightest bit of moisture gets in there, forms acid, then you really have major problems.

Correct amount of oil started in the early 60's when GM dumped the York compressor that had an oil sight glass on it, one glance and you were done. Only way to do it right is to flush and drain and put the correct amount in. Lot of other guys add a couple of ounces, I don't do work like that, just guessing. Too little and your compressor will seize, too much, reduce cooling capacity or even oil slug the compressor and wreck it.

Also had to get myself 608/609 EPA certified so I can buy a can of R-12, R-22, or even R-134a in Wisconsin. Not a damn thing about AC, just proper recovery and disposal, but with the crap on the market today, never had to recover, when I see it, all that refrigerant is already gone. EPA has zero restrictions on the manufacturers of this crap.

Not ranting, just telling you the way it is. If a car is decent, will spend 600-700 bucks and buy all new stuff, can't even flush a parallel condenser anymore. If the car is not worth it, just roll down the window and get the last few miles out of it.

Too even further complicate this field, EPA permitted the sale of all these substitute refrigerants, not because they are good for your system, but because they they meet their ozone depletion standards, this flooded the market so now you need a refrigerant identifier to learn what kind of crap was put in there. Ozone depletion, what a farce, even NASA quietly backed off this issue.

One thing for sure, repairing an AC system is no longer a five minute job.

To check a clutch coil, I hook up a volt/ammeter power supply directly to the clutch coil, typically should show 3-4 amps at 14.5V, but leave it on for awhile, as the coil heats up, that current should go down. If it goes up, your coil windings are shorting out. Also the clutch should engage, with a variable power supply, can find what voltage the clutch will engage at, I like 10.5 volts as a safety margin.
With AC problems, yes you have AC problems, first thing I do is connect my gauges and power supply directly to the clutch coil. If like the high side skyrockets, can instantly switch it off. But that tells me if the system is mechanically okay and performing properly. With that under my belt with still problems, then its attacking electrical/electronic problems.

The latter is very long subject depending on the system. One reason why I don't care for automatic climate control, suppose to be for convenience, but not very convenient when you are having problems. Tied up like a criminal anyway while driving, with nothing else to do but to play with the temperature controls. With just occasional minor adjustments.
Darren, at a time that clutch is in "failure mode" supply 12v to proper clutch relay socket to see if the clutch will apply..."click!". If it doesn't, push on the clutch to see if it will engage..."click!". (engine not running, of course)

Nick, putting on my seatbelt makes me feel like a race car driver/fighter pilot/astronaut. It's all about attitude.
Yeah, way back in the 50's taking flying lessons, installed a seat belt in my car. All my buddies thought that was really cool. Ha, with OnStar and that spybox, even put on my safety belts when backing twenty feet out of my garage, besides that ding ding ding drives me nuts, would never knock wearing safety belts.

Just my way of saying frustrations with automatic climate control systems. OP never mention if he has ACC or not, never do, so I brought up that subject. To me, a question of control, I like being in control, not some silly-con diode.
Thanks everyone.

I was on the road from Detroit to Atlanta and back when I discovered this.

I will try jumping the clutch now that I am home (well, when I am not at work that is...) If it engages, then I know that there is a break in the wire for the clutch somewhere (would make the problem intermittent). Will also check the air gap.

Will let you know what happens when I can.

Guys, I have one question before I work on this.

I should jump to the dark green wire to see if the clutch engages, correct?

I want to jump it correctly to see if the clutch does engage before I tear into it.

Thanks again.

A dark green wire supplies power. A black wire supplies ground. A light green wire sends a signal to the HVAC module.

So, Yes. Dark green wire. But black wire must also be connected to compressor.
Just in case you need more information on how this automatic system works and how the ac clutch engages and disengages, I download the following information for you. Automatic system are a lot more complicated than manual ac.

[attachment deleted by admin]
If the clutch is okay, and the ACC works fine in the heating mode, maintains preset temperature, blower motor speed varies properly. Problem is typically in the ambient sensor mounted way up front someplace, in front of the condenser normally. Exposed to the elements. terminals corrode. infinite resistance indicating to the BCM its -100*F outside, so won't permit the compressor to switch on.

Another simple problem is just a spec of arching on the relay contacts, coil is an inductive load and arching is quite common, that can be burnished off quicker than you can run to your dealer with your credit card.

Reread your post that you do have ACC and with dual controls. Someone would have to explain that to me, yeah, your wife only sitting a few inches away can have hot air blowing on her, you, ice cold air, but what about the people sitting in the back seat? Ready to kill both of you.
OK, I'll give my $.02, based on the schematic and also the explanation of the operating system that is posted......

Old Man, you are wrong as far as the light green wire is concerned.....that is a feedback to the HVAC control head, to confirm power has been sent to the clutch from the confirms the "permission parameter".....I assume that switch in the diagram of the clutch, is the temperature switch that is internal to the compressor......

First of all, I would check to see if the "gap" is 0.020".......

If a scan tool is present(have you checked for codes in the PCM, BCM, and HVAC control head?), you need to monitor if the the permission parameter is present(when A/C is requested at the HVAC head) , and also look at the data of the low and high side sensors(to see if they are within limits)......also, when the compressor disengages, you look to see if the permission parameter is still present, and if the feedback signal is still present......if the feedback signal is not, but the permission is, then that temp switch is opening up......this has to be determined, otherwise you could be replacing a clutch for nothing....
You are correct on all accounts Gus. That is where I got thrown off. Thinking that overload was a switch and needed to be pulled in by the coil. That diagram is a bad example of a overload in the compressor. When I looked at the Alldata schematic, it show it correctly as a overload. That is why I gave him that second information as why a clutch will not engage and how it works. My apologies to the man.
Like an overlooked step child..... Gus sees it -12+ posts and noone asked

Have you started with the EASY SIMPLE things?

Any recient repairs modifications?
Any codes?
You did remove harness to AC low pressure?
CEL working- on at start @3sec then OFF?
Can you check - ECM, BCM, Control head for DTC's
Did you weigh the refrigerant charge OR... tell us?