A/C PSI Jumping around on 04 Trailblazer

[blackhawk375]

Hello! I am a new member here and I look forward to hearing from you.

My problem is this: My AC is blowing warm air so I went out and bought a 134a DIY recharge kit. When I turned my vehicle on and had the AC on max I connected the PSI gauge. It continually bounced between 35 and 70 PSI. I could hear a click every time it got to 70 and then it would gradually drop back down to 35. There would then be a much quieter click and then it would jump back up to 70. I have no idea where to go from here and am hoping you wonderful folks can give me some assistance.

 

[The_Roadie]

Welcome!

Not all ac problems respond to a blind application of more refrigerant. There's a lively discussion going on about how risky and potentially expensive a DIY ac repair can be.

That said, cycling is entirely usual behavior. First off, is your receiver-drier (the silver can on the passenger side firewall) at all chilly?

 

[NJTB]

+1 what Roadie said.
The click is the AC compressor turning on and off. When the pressure gauge is dropping, it's on, pressure rising off. The on/off cycle is controlled by a switch, and I don't remember (in this platform) if the switch is on the accumulator (silver can, 2 larger hoses going to it) or the low pressure line.
The fact that the compressor is cycling is a good sign, it means there's enough freon to close the switch, but maybe not enough to get cold.
After the AC runs for a minute or two, the accumulator should be getting cold, put you're hand on it to check.

 

[The_Roadie]

And in case it's not obvious, the flowchart next should be like:

a) Accumulator chilly -> The HVAC actuators in the dash are mixing in hot air later and stealing your cold air.

b) Accumulator frosty or iced up -> bad refrigerant level or controls and you're icing up your evaporator

c) Accumulator warm -> your refrigerant isn't working due to the wrong amount being present (too little or too much) or the compressor is hosed

 

[blackhawk375]

Thanks for the responses! The accumulator does not get cold. It stays warm. I will go take a picture as well so you can see if the switch is connected to it or not. It does have an electrical connector coming in behind the low pressure check point.
Also, I haven't actually put any more refrigerant in the vehicle yet. I just connected the pressure gauge to see where it was at and it wasn't where it should be to add more, so I just stopped at that point and came here hoping to get some help.

 

[NJTB]

If I remember correctly (please someone chime in if I'm wrong), the low side pressure should be around 25 to 30 (compressor off), and 40 to 45 (compressor on). This is with the proper 2.2 lb. fill of 134A, and is ambient temperature sensitive. They will be higher in hot weather.
Some of the cans of refrigerant at auto zone, etc have a gauge on them to check it.
Do be aware if it's overfilled too much, there is a chance the compressor can be 'slugged', which means liquid freon in the vapor line to the compressor locks a piston in the compressor. It's kind of like trying to start a car with water in the engine, very bad.
With that said, the backyard method is to start the car with the ac on, hook up the can, watch the gauge and add the freon. Hold your hand on the accumulator, you will feel it getting cold as the refrigerant is added. Stop if you hear any odd noises. You may not need the entire can.
Best of luck.

 

[Mooseman]

70 PSI when the compressor is off sounds about right depending on outside temperature but it could still be low. When the compressor is running, it should drop to about 30PSI and then the low pressure switch would turn it off until pressure comes back up to about 40-45PSI. If it's really hot outside, it may not drop down low enough to cycle the clutch off.

If yours is cycling too quickly, i.e. the compressor only runs for a second or two at a time, it is likely low but you have to find where you are losing refrigerant, which means a trip to a pro or buying a leak detector. Look around the coolant lines, compressor and connections for an oily film. If you have an ultraviolet (black) light, you could check with it as there is usually a tracer dye added to help with leak detection. Just adding more refrigerant will not help as it will just get worse over time.

 

[meerschm]

first of all, you really need a set of low and high gauges.

then you need the chart for your vehicle, and some way to measure or estimate the ambient temp.

I sold my manual, so this is from memory.

with outside temp around 70 and not too humid, run the ac on full cold, full fan, with the windows open (out of the full sun, so this is a shade operation)

after ten minutes, should have a steady high and low side pressure. low side should be within the indicated range on the chart, ( I want to say 25 to 30)

unless it is quite a bit colder than that , the low side limit switch should not cycle on and off.

sounds like you are low, but again, I would want to have low and high pressure gauges to monitor. I watched my neighbor add a bit too much, and a hose blew.

 

[meerschm]

so i found a few photos. (I had them posted, but they seem to have dropped off the latest site update)

here is the page from the book. note that there was another chart for the long wheelbase. this if for the short. (may be specific to I6, I do not remember)

you would have to figure out ambient temp and humidity, and follow across for pressure range

Then hook up a set of gauges yellow hose goes to a can or vacuum pump.

this (above) is with the engine off. reading is vapor pressure of R134a. varies with temperature, but as long as there is any repeat any liquid in the system, this will be about right. note both high and low read the same.

I think it was in the low 80s when I did this. you can see it reads around 34 low and 160 high. within range for around 80 degrees mid humidity. mine was operating at the low side of high for a given cold range. I did not want to push it, so this worked fine.

if you run on high fan, lowest temp, and have doors or at least windows open, if it is anywhere above mid 70s, the compressor should just operate (and not cycle on and off)

if it is getting a little cold, and cycling on and off from the low pressure switch (which you can feel and hear click, if I recall) there is not enough R134a.

in normal operation, with the doors closed, and mid range temps, the compressor does cycle on and off. but configured for the test, max cold temp request, high fan, (not recirc) and the doors and/or windows wide open, it should give you a steady pressure reading.


all this being said, the best way is to get to a shop.

the GWP (global warming potential) of a single 12 oz can of R134a is around the same using a couple tanks of gas. (which is another good reason to see a shop which can recover the stuff for recycling.)

 

[JerryIrons]

blackhawk375,

If you want to fix your own AC, you can, but accept the fact that you will have to invest in some tools to do it. First of which is an ac manifold gauge set pictured above. Between that and the shop manual you can accurately determine how much 134a to add, because adding too much is bad also. You can buy 134a that has some UV dye in it, to help troubleshoot this stuff as well. Not all stores carry it so you may need to check a few.

Do not add the AC "stop leak" refrigerant. That may fix a leak, but it will also plug up other small tubes and orifices, only leading to a major repair bill down the road.

You probably have a leak, and probably adding a can of 134a will "fix" it for now. You really want to do this without having to use the word probably like I just did. This is a closed loop system that isn't designed to lose refrigerant, so if you are low, you have a leak that needs to be fixed. Plus there are components on this system that have high pressure to them, so safety needs to be considered as well. I fix my own AC issues, so it can be done. For some reason I find fixing my own AC problems immensely satisfying, there is just something about turning on that air after a repair and getting ice cold AC that makes me smile.

 

[blackhawk375]

Thank you all very much! What a helpful group of guys. My cousin has manifold gauge set and is going to come by tomorrow to help me with this. I really appreciate all of the information and will keep you all updated.

 

[meerschm]

There is a typo in post #9 ( and I cannot edit)

high pressure in the last photo is around 160, not 60.

 

[IllogicTC]

There is a typo in post #9 ( and I cannot edit)

high pressure in the last photo is around 160, not 60.

Fixed

 

[blackhawk375]

My cousin just came by with his manifold gauge set and we were able to determine that my levels were in fact low and that was causing the compressor to kick off and on trying to run. We added the appropriate weight to the system and it is now working again. Thank you to everyone for your help.