AC Question

67z28

Original poster
Member
Jul 27, 2012
5
I've got a 2004 Trailblazer, which has been an excellant vehicle. I have looked through many pages of a/c problems, but haven't seen any like mine. We can be driving the TB and the a\c working good. Stop somewhere where the TB is shut off, come back and the a\c doesn't cool anymore. Sometimes can stop again and restart the TB and it'll work and sometimes it will not work. It always works the next morning though. The pressure is good in the system and the fan always blows as it should. Anyone else have this problem? I don't want to just start changing parts that don't need to be changed trying to find the problem.

Thanks for any help
Ray
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
67z28 said:
I've got a 2004 Trailblazer, which has been an excellant vehicle. I have looked through many pages of a/c problems, but haven't seen any like mine. We can be driving the TB and the a\c working good. Stop somewhere where the TB is shut off, come back and the a\c doesn't cool anymore. Sometimes can stop again and restart the TB and it'll work and sometimes it will not work. It always works the next morning though. The pressure is good in the system and the fan always blows as it should. Anyone else have this problem? I don't want to just start changing parts that don't need to be changed trying to find the problem.

Thanks for any help
Ray

If it was me the first thing that I would do is see if the clutch is getting the 12volts that it is suppose to be getting. From there then I would start looking at the pressure switches if the clutch is not getting the power. You could also try swapping out the relay, maybe a faulty relay.
 

neelskit

Member
Dec 7, 2011
69
anthonyl79 said:
If it was me the first thing that I would do is see if the clutch is getting the 12volts that it is suppose to be getting. From there then I would start looking at the pressure switches if the clutch is not getting the power. You could also try swapping out the relay, maybe a faulty relay.

:iagree: Low-side Pressure Switch has a high failure rate.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Apologies if this isn't a problem for you, but I see many folks asking about AC issues, and they never seem to tell us if the compressor is engaging. When it ISN'T working, it's important to check the compressor clutch to see if it's even spinning the compressor. Look at the center part of the pulley in this picture. It should cycle on and off if the system is calling for cooling.

View attachment 22091
 

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67z28

Original poster
Member
Jul 27, 2012
5
Ok I've got more info on my A/C problem. I drove the car today and after about 10-15 minutes the a/c stopped working. After the car sits for a few hours the a/c will work again. A friend and I checked the low and high presure switchs while it wasn't working and neither had power to them. The clutch isn't spinning and the blower motor blows like it should at all speed settings. The heat works good when turned on. I thought maybe ambeiant air sensor, but I'm not sure my TB has one, I do not have auto air. The a/c light on the dash switch is on and the knobs to change where the air exits the dash work also.
 

Mooseman

Moderator
Dec 4, 2011
25,472
Ottawa, ON
So what readings do you get when it doesn't work? If you have a fairly high static pressure equal on both sides, then I would suspect the low pressure switch. To test this, pull the plug from it (it's on the receiver/dryer canister) and short the two connectors. If the compressor comes on and your pressures are as they should be, that's your problem. If it still doesn't work, it could be your high side switch. Try shorting that one. If it now comes on, one of two possibilities. Either it's faulty or your high side pressure is too high, which would mean either it's overcharged or you have blockage in the system.
 

c good

Member
Dec 8, 2011
541
push on the front of the compressor with the end of a broomstick when the compressor kicks off. Be careful not to get the stick tangled up in the serpentine belt. If the clutch engages then you have too much "air gap" and/or a weak clutch coil assembly. I know you mentioned your friend said there was no power at the high and low pressure switches but without knowing his skill level and how he probed them, I would question the validity of this test. These Sanden compressors are notarious for having bad clutch coil assemblies. HTH....c good
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The low pressure switch is a switch. The high pressure sensor is not a switch, and needs to be read by a scan tool capable of reading PCM parameters like AC pressure. If you checked the high pressure sensor as if it was a switch, you really need to get the schematics, read the manual, and understand how the system works before poking at it with sharp objects like voltmeter probes.

Pressures are best checked by gauges. Do you have a set?
 

67z28

Original poster
Member
Jul 27, 2012
5
c good said:
push on the front of the compressor with the end of a broomstick when the compressor kicks off. Be careful not to get the stick tangled up in the serpentine belt. If the clutch engages then you have too much "air gap" and/or a weak clutch coil assembly. I know you mentioned your friend said there was no power at the high and low pressure switches but without knowing his skill level and how he probed them, I would question the validity of this test. These Sanden compressors are notarious for having bad clutch coil assemblies. HTH....c good

I will try pushing on the front of the compressor this afternoon when I get off. Andy, my friend, owns a shop and restores muscle cars also so I feel pretty confident with his knowledge.
 

67z28

Original poster
Member
Jul 27, 2012
5
the roadie said:
The low pressure switch is a switch. The high pressure sensor is not a switch, and needs to be read by a scan tool capable of reading PCM parameters like AC pressure. If you checked the high pressure sensor as if it was a switch, you really need to get the schematics, read the manual, and understand how the system works before poking at it with sharp objects like voltmeter probes.

Pressures are best checked by gauges. Do you have a set?

Yes we do have a set of gauges.
 

67z28

Original poster
Member
Jul 27, 2012
5
Got home today and the a/c worked fine with nothing changed or done. I drove it for awhile and the a/c compressor shut off again. Seems like driving for a little bit the compressor shut off and then after sitting for several hours it ill start working again without anything chaning other than the engine\compressor cooling off.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
How hot has it been there?
I don't remember the details, but there has been a problem reported with the compressor (clutch coils?) when the outside temp was high.
Some one will remember. I hope.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
67z28 said:
Got home today and the a/c worked fine with nothing changed or done. I drove it for awhile and the a/c compressor shut off again. Seems like driving for a little bit the compressor shut off and then after sitting for several hours it ill start working again without anything chaning other than the engine\compressor cooling off.
So did you try the broomstick test or check the pressures or swap the relay? Seems like nothing's been done from your first post, so I'm not surprised the behavior's the same.
 

c good

Member
Dec 8, 2011
541
the roadie said:
So did you try the broomstick test or check the pressures or swap the relay? Seems like nothing's been done from your first post, so I'm not surprised the behavior's the same.

This.....also, what has the outside temperature been lately?
 

xj2202009

Member
Mar 27, 2012
105
I have seen this happen before in chevy's: the ac cooling fan in front of the car is connected in series with the clutch and the pressure sensors, sometimes the electrical brushes in the fan motor ware out and loses contact catching sometimes and losing it others. The ac was working hit a pot hole, ac stopped working, wiggling the car around would make it kick on again.

If yours is like mine: you don't have an electrical ac cooling fan, I wonder if in our trucks this function is done at the fan clutch. at good test is roll a couple of sheets of news paper and lightly stick it in the fan blades to see how much force you have.

Mr. Roadie could you confirm if the fan clutch works in series with the ac system?
 

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