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Confused on pressures...

rammitch on Mon April 28, 2008 12:42 AM User is offline

Year: 97
Make: Pontiac
Model: Grand Am
Engine Size: 3.1
Refrigerant Type: R-134
Ambient Temp: 65
Pressure Low: 29
Pressure High:

This is a great forum! Thanks in advance for looking at my problem. Last fall my evaporator sprung a major leak. At that time I decided to replace the compressor, the receiver/dryer, the expansion valve and flush the system. After all the maintenance was done, I pulled a vacuum, and serviced with 3 cans of r-134. I do not remember the problem I had last fall, but the cooling was not effective. Winter came and I put it up. Last week I started in on it again. After hooking up the gauges and seeing the pressures of 20-25 Low and a high that would climb to app 200, then the fan would kick on and pressure would drop to 75-100. Cooling would drop to 50-52 until the fan kicked on, then it would raise to ambient. I did my best head scratching and decided the expansion valve was bad. So, today I replaced it. vacuumed, recharged with 3 cans. Same thing. Accepted the fact that I probably got a bad compressor last year I replaced it again. You can probably guess were this is going. Almost the same figures with a new compressor, except the low side is 25-30. Here's the rundown again:

Low pressure= 25-30
High pressure=75-200 (It will immediately drop when the fan kicks on to pull air over the condenser, the vent output will rise to ambient until the fan kicks off. Then the cycle repeats)

My system is tight, held a vacuum for 45 minutes. I've got the correct amount of oil in the system. I am kinda at a loss as to what is causing my pressures to be off. Can anyone help shed some light on this? Thanks again!

Dougflas on Mon April 28, 2008 5:11 AM User is offline

65* is kinda low ambient to check an AC system. Did you change the control valve in the compresser also as I think this vehicle used a V5 compresser.

rammitch on Mon April 28, 2008 10:19 AM User is offline

I realize it's a little on the chilly side, but I've only got limited times to work on it. The vent temps do the same thing as the ambeint temp is warmer, best guess is upper 70's. I just didn't have gauges hooked up at that time. I did change the control valve, but only by replacing the entire compressor. Thanks!

mk378 on Mon April 28, 2008 11:08 AM User is offline

When you change the TXV, be sure the sensing bulb is properly attached and insulated exactly like the original one was.

You need to weigh in the proper charge before trying to diagnose a variable compressor system. Pressure readings don't mean much if you're not sure of the charge. The low side should stay 25-30 over a wide range of conditions. This means the variable compressor is working.

rammitch on Mon April 28, 2008 12:51 PM User is offline

I have it clipped to the outlet of the evap, For the second valve, the one installed now, I did not insulate it. It has made no difference either way. I'm getting pretty good at entry to the evap area. One other clue maybe, the condenser and the high pressure line by the gauge attachment get real hot to touch prior to the fan kicking on. Maybe that'll help some.

rammitch on Tue April 29, 2008 1:49 AM User is offline

Alright, after thinking about this all day I remember a bit more from last fall. I remember that I was told initially that the oil could be serviced with the r-134 after the repairs were made, I just needed to add it to the compressor and the shop would do the rest. Last Fall I did not add any oil to the evaporator or reciever/drier, just 2 ounces in the compressor. When I took it in to the shop for them to service, they told me that was not right, but they'd service it anyhow. He told me that he'd put in 6 ounces plus the dye. This is how I'm saying that my system has the correct oil charge.

On top of that, Yesterday when I changed the compressor, I put in the blue pag 150, Last fall I used the green dye pag 150. I have assumed that it's all compatable. Does anyone think that this may be related to too much/too little oil in the system? Also, Do you think that not spreading the oil out into the system initially caused any problems?

I've about talked myself into flushing the system and starting from scratch since I wasn't the one to put the oil in it initially last year. Any thoughts on this would be greatly appreciated.

rammitch on Wed April 30, 2008 1:45 PM User is offline

Just a follow up for whoever cares...

Before I flushed the system, I decided to add some oil. The first 2 ounces improved performance. The next 2 ounces fixed all my problems. My advice for the DIY'ers: Do not trust anyone but yourself to get the job done right!

TRB on Wed April 30, 2008 2:50 PM User is offlineView users profile

Glad it's working for you. Any idea how much oil is in the system now?


When considering your next auto A/C purchase, please consider the site that supports you:

rammitch on Thu May 01, 2008 2:47 AM User is offline

I'm not entirely sure. This weekend I'm going to flush it and get everything on the up and up.

Thanks to this forum and all the knowledge everyone brings....a great tool!

TRB on Thu May 01, 2008 10:23 AM User is offlineView users profile

If you have good pressure readings and the compressor sounds good. No odd noises and it's not heating up to much. I would reconsider flushing out the system. Unless you sure on the process and know you will be able to remove all the flushing agent. Why I prefer to know a known system capacity in oil and refrigerant. Sometimes when things are working it's best just to leave them alone in my opinion. I would be double checking connections for leaks at this point.

Compressor Type Harrison V5 5-Cyl.
Compressor Belt Tension (1)
System Oil Capacity (2) 9 ozs.
Refrigerant (R-134a) Capacity 36 ozs.
System Operating Pressures (3)
High Side 150-350 psi (10.5-24.6
kg/cm2 )
Low Side 25-45 psi (1.7-3.2
kg/cm2 )
(1) Equipped with automatic serpentine belt tensioner.
(2) Use PAG Refrigerant Oil (Part No. 12345923).
(3) With ambient temperature at 80°F (27°C).


When considering your next auto A/C purchase, please consider the site that supports you:

rammitch on Thu May 01, 2008 10:48 AM User is offline

It works a lot better. It tries to cool all the time now, but after getting it on the road, I noticed that it will still cycle from 46-49 deg up to 58-60. It will only stay there for a little while (
If it doesnt straighten it out, then it's time to roll the windows down and get a spray bottle of water to hose off with.

GM Tech on Thu May 01, 2008 11:17 AM User is offline

Your V-5 compressor is not designed to "cycle" when your temps change is the compressor "on" or "off"? If off- then I highly suspect high pressure cut-outs- due to either a massive restriction -overcharged, or no cooling fans running--

Run it down the road with the gage set attached- what is the high side doing when the temp climbs?

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

rammitch on Wed May 07, 2008 12:13 PM User is offline

Just a post mortem...

Flushed the system, serviced with known good quantities and she blows between 42-50 deg with 82-84 ambient temp. Thanks again to this website and all the replies, also, the previous posts that answered a lot of questions.

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