Good Morning! (Or Afternoon or ??)
I have a 2000 SIerra 2500 (New body style) & have a question about the Air Conditioner I'm hoping some of you have had experience with.

When the truck is stopped at one of the 6 bazillion train tracks or at traffic jams, the A/C won't run. If I'm at idle, no A/C. If I'm actually driving, even at 30 or 40 mph, it runs fine.
Anytime it is holding still, no matter the RPM, the A/C will not run. I can use normal or Max A/C- Same results.

Normally, this would be something I'd ignore, but adding to the "need" for A/C- In my job, sometimes I have to "babysit" temporary power jobs for hours. My escape is a quick truck visit, with A/C on- of course. (Hopefully this isn't bad for the engine- 10 or so minutes of idling.) Pardon the wording "need for A/C" Funny how I "need" a luxury. Spoiled, huh?

After reading may posts here about possible causes (Cycle Switch, Low Pressure Safety), I tried bypassing the Low Pressure safety (at idle) & the A/C stil didn't come on. I also, while in "Park", increased the RPM to see if the clutch would pull in. It didn't.

I don't know where to find the cycle switch, just in case that is the problem. I also would not know how to diagnose it. Where is that switch? Is it diagnosable? Or, am I looking at the wrong component?

Also, if low refrigerant is suspected, what should my pressures be on a correctly charged R-134a auto system be?

I almost forgot- Around here there are lots of free A/C check places. I stopped in at three & was told three different things- Compressor clutch is bad Control module is bad & GM vehicles have no A/C unless it is in gear.

What do you folks think is going on & what's my next diagnostic step?
Thank you all very much for helping! I appreciate your guidance.
Paul

 

Is this something that just started or was it this way when you bought the truck? I can tell you GM trucks do have AC whether they are in gear or not.

 

Thanks for replying, TMW
It has been this way since I bought the truck- But I bought it used about 3 years ago, so who the heck knows when it started.

Paul

 

This might sound weird, but it could possibly be as simple as a cooling fan issue. Your cooling fan is supposed to kick on whenever your A/C is engaged to help cool your condenser. Does your truck have electric cooling fans? If so, you might look into that.

Here's one way I test the A/C system sometimes. Start the truck, turn the A/C on, and grab a garden hose and spray the front radiator (Actually the condenser, not the coolant radiator). Just literally stand there and spray all over the front condenser and see if your A/C kicks on.

The idea is that your refrigerant needs to cool down when it's in the condenser, otherwise pressures get too high, and the A/C high pressure switch will disengage the clutch.

If your A/C works when you spray water on the condenser, then you need to figure out why your cooling fan isn't turning on when you turn on your A/C.

 

I don't think it is a bad compressor clutch since it works while driving. It could be the HVAC control module, power train control module or the AC high pressure switch. According to my Chilton manual the A/C request comes from the HVAC control module thru the A/C high pressure switch to the powertrain control module which applies a ground to the A/C compressor clutch relay to turn on the compressor clutch.

 

Thanks for the tips NullHead & TMW
I tired the cold water on the condenser idea. I didn't get any luck having the clutch kick in.
For fun, I tried jumping the high & low pressure switches, to rule out low or high pressure problems- Still no go at idle. The radiator/condenser fan does run anytime the control is set for AC.

I'm now leaning toward the control module of powertrain control module, as TMW suggested. I noticed another oddity that makes me thing something isn't right with it: If the control is set to "recirculate" the AC comes on no matter if the AC button is pressed or not. (But only when driving- not sitting still.) Goofy.

I also noticed if I'm driving & stop for a light or something, the unit will shut down. In my wife's Chevy (HHR), the compressor short cycles, as have other cars I've owned. But this truck = Off.

I'm not sure what my next step should be to try to figure this one out. Any suggestions will be greatly appreciated. It is certainly not a critical issue, but now it's a challenge!
Paul

 

Subscribed for outcome. Same issue with my 05.

 

Hi Folks-
Here's the latest on the problem-
I took off every single connector that had anything to do with the heating & cooling system. (Well, at least those I could find and access.)

I cleaned all terminals & made sure they seated well. Lots of them were corroded. I believe, from other things I found in the past, that my truck went swimming at some point in its life. (That's how it got it's name: Nemo)

Now, the truck does not have to be moving to get the A/C to come on. It only has to have the engine RPM above about 1100 on the dashboard tachometer.

If I can't think of anything else to try, I probably will evacuate the system, leave a vacuum pump on it for several hours, then weigh in the correct charge. If nothing else, this should rule out under or over charged system. (Assuming there is no wax build up making the label charge now an over charge. I remember this from tiny R-12 capillary tube systems. I'd have to guess the accumulator would void any effect of wax- if PAG oil even waxes.)

Do any of you have a better plan or some other ideas?
Thanks! Your help is genuinely appreciated.
Paul
PS: I see that AtLarge has the same problem. And to add another; one of my friends has a 2000 Silverado whose A/C won't come on unless driving either. Perhaps this is more common than I realized.

 

It appears that TMW's suggestion of the control module possibly being the culprit is correct.

Here's Why I Believe TMW Was Correct In Suggesting The Module-
Days ago, I cleaned every terminal connection. the A/C would run at over about 1100 RPM only.
I removed the charge, vacuumed the system & weighed in the correct charge (Details are at the end of this post) The A/C now ran at idle. (ca 500 rpm)

So happy that the A/C was now behaving, I shut down the truck.
Later, I re-tested. No A/C!
What the heck?

At Idle or Moving:
If I put the control to "Recirculate" with or without the A/C button pushed, the A/C compressor clutch engages & the system starts cooling.

If I release the "Recirculate" button in favor of the "Fresh Air" button, the compressor clutch de-energizes. If I drive with "Fresh Air" or run the RPM to about 1100, it re-engages. (Air Direction = At Dash Vents)

The control must have a bug in its brain. I doubt GM designed it so "Recirculate" is required when stopped. You with working A/C would know better than I would.

Status:
"Recirculate" gives cooling no matter what any of the other controls are set at. No matter the engine RPM.
A/C button is ignored. Always cooling, even set a "Floor" & A/C button off.
"Fresh Air" gives no cooling until the idle is above about 1100 rpm. (A/C button on)

"Defrost" (Which should engage the compressor clutch for drying purposes) = A/C runs at idle. Defaults to "Fresh Air" (By design, I believe. Makes sense.)
"Part Defrost Part Floor" = Same as Full Defrost

The Defrost thing is what convinces me the control is goofy. If Defrost can run the A/C at idle & "Fresh Air", why not idle A/C any other setting with "Fresh Air"?


Boring Step-By-Step of My Morning:

Background- I have experience only with 100 ton & up chiller units, using 'olden days" refrigerants such as R-22, R-500, R-502 & the evil R-717. I don't have much experience with small units, such as vehicles & refrigerators, which are far more difficult for me to service & charge correctly.

The label calls for 1.8 lbs of refrigerant 134a.
I recovered 1.9lbs from the system. (Surprised me. I've never touched it in the 3 years I've owned the vehicle. Maybe it has no leaks??)
The vacuum pump was left on the system overnight at just above 29mm hg.
I replaced the removed refrigerant, stopping at 1.8lbs. (Hoping any oil which was removed is now replaced.)
The crazy thing was working at idle. Yipee!! Maybe I cleared a restriction?
Results-
Ambient = 79F
Suction Pressure = 34 PSIG (Right? Wrong? I think I'm OK)
Discharge Pressure (at service valve) = 182 PSIG (Not a great indicator. I'd assume it would change as the vehicle moves & more air crosses the condenser.)
Center Vent Temperature = 40.5F (Recirculate, Max Fan, All Vents Open, Clean Cabin Filter)
The kicker, as I mentioned above, is the switch has to be at 'Recirculate" or "Defrost" for any cooling at idle.

Hopefully some of this information will help others who are having this problem.
It appears that all I had to do, starting 3 years ago, was to clean up all the connections & put the control in "Recirculate". Live & Learn!

Thanks again to everyone who wrote in to help out. And, of course, if I'm way wrong or any of my reporting shows impending doom, please let everyone who reads this know!

Paul

PS: One question remains- If I follow the line from the condenser outlet to the evaporator, there are two access fittings. The "North" one is a fitting I have no idea of size or type. The "South" one has the access port for manifold gauge set lines. In between are two what look like flare nuts connected together. On the North side, the line is warm. On the South, cold & sweating. Are these two "nuts" the expansion device? It is a fixed orifice? Just curious.

 

As I read your posts about your A/C troubles, I am so grateful that I live in an area where we don't really need or use A/C. Some folks do, but I disconnected mine on my truck and plan on removing the pump. I keep the cooler for a brush guard for the radiator.

What a P.I.T.A.!!! Still, I would like to mention that I have spent many bored hours reading through the owner's manuals on company cars, and GM often reccomends using Recirc, just to reduce load. The air is already drier than the outside air and the A/C has to do less work cooling the recycled air. Defrost wants to clear your windshield as quickly as possible, so I am not suprised that it works on either setting. It is the only heater control on your rig that has to meet a federal regulation...you have to have defrosters, but not a heater!