AC Compressor not engaging in defrost or AC mode

[3JFamily]

History:

I picked up my 06 Suburban 1500 last year around June (?) 2020.

Freon was low due to a leaking low side schrader valve. Replaced that.

Had the AC evacuated /filled. The first time I had it done the AC wasn't working quite right so we replaced the low pressure cycling switch and the high pressure transducer switch in the high side line.
Evac'd and refilled again.

AC worked great for the rest of the summer. I haven't used the AC since last October or so.

During the winter I realized the defrost wasn't blowing hot air. The compressor doesn't engage when in defrost mode. Didn't really need the defrost here in the desert.

The heater was working intermittently, figured that was a sticking blend door. Heat has been fine past few months with a clicking sound sometimes on full hot temp. Again, desert so not that bad.

So here's my dilemma:

Today was 80+ degrees and I tried my AC during my lunch break. No compressor engagement.

I tried swapping relays, checked the AC fuse, tried jumping the switch on the accumulator, unplugged and reconnected the high side connector and the power connector at the compressor. Nothing. No engagement.

Next step is checking power to the compressor but I need to pick up a new multimeter at work.
I will get my AC gauges on there to check static pressures.

Living in Las Vegas I need to have this squared away before summer.

I know this is a long winded post but I am trying to get all the info out.

 

[3JFamily]

I had time to take a quick look at my AC system today.

waiting on a new multimeter but I did pull the AC Comp relay and jump the two forward connections and My compressor engaged.
so it would be fair to say the compressor is good.




put a set of gauges on and took a quick static pressure reading, valves closed. Seem low? Ambient air temp 85-90 degrees.

 

[OR VietVet]

Your pressures and system are low. You have another leak. Charge it and should be fine till enough leaks out again. Need to likely do the dye test for leaks. If you check ALL of your connecting points, hose ends-threaded couplings-front compressor seal-back of compressor-switch attachment, if the leak is there you will see a/c oil come out as well and leave a oily/dirty trace.

By the way, your defrost heat problem is not related to the a/c compressor. The compressor cycles during defrost to help remove moisture in the air inside the vehicle. The heat source is a whole different thing. May well be a blend door.

 

[3JFamily]

I'll add some freon and see what happens.

 

[3JFamily]

I had the system evac'd and recovered only about .22 lbs of freon.

Obviously there is a slow leak somewhere. AC worked all last summer, 3 months plus so it is a slow leak.
No obvious leak traces around underhood fittings or hoses. Could be at the front of the compressor or somewhere within the rear AC lines/system.
System now has some UV dye in it.
Filled with 2.3 lbs of freon and will top off as needed until summer is over.

I can then look for the leak with UV light without having to deal with the heat.

 

[OR VietVet]

If the under the dash Evaporator has a leak, you will not find it with dye unless it leaks at the exposed lines at the firewall. There was a time when GM evaporators had some leak problems.

 

[3JFamily]

So...Update.......Summer is here with 110+ record temps and my AC is acting up again.
Started with cool air stopping when at a light idling. Next came no cool air.
I added a can of freon with dye/sealer. Worked for a minute then no more. Added more freon. Accidently overchared.
I vented off some but still no compressor lock up.
No cold air. As much as I hated to, I dropped off the Burb at a local shop that gets parts from us.
I guess he couldn't get the compressor to stay engaged. Short cycling, which is exactly where I was at.
Being 110-116 degrees for the next week or more we decided to just get it functioning by "hotwiring" the compressor with a power switch. Not optimal and I hated to do it. He thinks it may be the PCM. I plan on talking with the shop to see if they did a scan on it.
I wish I had a GM Tech 2 scanner so I could check/manipulate permissions with the HVAC.
I don't have the time so I am going to bite the bullet and drop it at another shop where I know the Master Tech.

Anyway...I took a static reading tonite. Valves double checked closed. 100+ degrees ambient temp.
As you can see. The low side reading is pegged. WTH?

 

[rockola1971]

Im assuming when you say static pressure, the reading was taken with compressor disengaged. If so then at 100deg ambient temp the pressure for R134a would be right around 124PSI. Remember the Pressure is directly related to temperature with refrigerant. I would definitely get the truck to someone that knows what they are doing instead of this last guy "thinks it may be the PCM"? Instead of thinking all he had to do was check for the control signal coming from the PCM.
I suspect the cause of your "shortcycling" is actually hi pressure cutoff on the high side. This can be verified with gauges. You likely have a restriction and id be willing to bet with all the pumping down of the system after repairs that you sucked a bunch of crap out of drier into the orifice tube. Replace the orifice tube!

I literally just fixed this problem yesterday on my 03 Tahoe after having a compressor pulley bearing failure so I just replaced the whole compressor and got your symptoms. Orifice tube was plugged up with some grayish crud causing a restriction. Likely the original orifice tube! My High side would get up to 500psi and enable the popoff valve at the compressor and vent r134a.

And order up a ****** clone of the tech 2! Mine has saved me so many times it isnt funny. (I have two 03 tahoes and a 05 Yukon Denali). It has paid for itself so many times over.

 

[OR VietVet]

I am with @rockola1971 on this one or maybe even a clutch going out. I am looking forward to the outcome of this.

 

[corvette744]

You definitely need a different mechanic.Hot wiring the compressor lol.Sounds like an orifice tube to me also-curious if its crude or packed with metal shavings??You also need to fix the leak if its not leaking from evap you should be albe to detect dye even with a week of driving with the uv light.I have seen large leaks from evap with uv light from the drain hole.