Automotive Air Conditioning Information Forum (Archives)

GnatGoSplat on Wed June 06, 2012 4:07 PM

Year: 1990
Make: Pontiac
Model: Grand Prix
Engine Size: 3.1L
Refrigerant Type: R-134a
Ambient Temp: 80
Pressure Low: 35
Pressure High: 105
Country of Origin: United States

1990 Pontiac Grand Prix has been converted to R-134a. Compressor OFF pressure is 75psi both low and high side. Sounds pretty normal according to the service manual chart.
With AC on, compressor running, low side pressure is 35psi, high side 105psi. Air is just slightly cool, definitely not cold.
Service manual troubleshooting charts seem to point to the control valve. What is the likelihood that the control valve is the problem?

I thought the AC worked when I tested it last a few months ago, but I'm not sure. This is a sitting car I want to put back on the road this summer and last I checked it was 40-50 degrees in mild winter weather so maybe that just gave the illusion that the AC was cold enough (I didn't check pressures at the time).

GM Tech on Wed June 06, 2012 5:10 PM

Control valves are responsible for a/c failures 1% of the time while 65% of the time a system is low on refrigerant....so what do you think? I'd go with the percentages---you are low on refrigerant- find and fix your leak...

-------------------------
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......

GnatGoSplat on Thu June 07, 2012 11:35 AM

For some reason, the system won't accept any more refrigerant. I tried adding more, but none is coming out of the can. I even let it sit above the engine awhile till the can got very warm and nothing. The troubleshooting chart in the service manual seems to indicate that 75psi with the system off on both high and low sides is a normal charge, or close to it. Unless I am misunderstanding it, which is possible.

GM Tech on Thu June 07, 2012 12:16 PM

75 psi static pressure will be the norm for 1/2 lb , 2 lb, or 20 lb in your system-- you can't tell the amount of refrigerant is in the system by the pressure--it IS NOT a tire-- refrigerant is a condensible, not a compressable....the only way to know how much is in there is to take it out and weigh it.

You need help in recharging you vehicle- if you can't add refrigerant, then you are doing something wrong and could cause harn to yourself and others...

-------------------------
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......

GnatGoSplat on Sat June 09, 2012 2:44 AM

I see. The service manual flowchart said >50psi static pressure to continue down the flow as though refrigerant is fine and below to that to charge the system so that threw me off.

Oddly, I've never needed help charging the system in the past and I've done this probably a hundred times over the past couple decades. Low side pressure is pretty low so when I thought about it, it makes no sense it wouldn't go in. So I opened the can tap valve and also opened the manifold gauge low side valve with it off the car to see if anything would come out. Nothing came out. I pulled the can w/tap off entirely and turned the valve handle several turns and nothing came out. If I turn it as far as it will go, some refrigerant does come out of the can tap, but mostly out the stem of the handle. Looks like my can tap valve was the problem all along.

buickwagon on Sat June 09, 2012 9:03 AM

GM Tech is right on the money. System pressures can tell you a lot about what's going on in the system, but they cannot tell you how much refrigerant is in the system. Since the refrigerant carries the oil, an undercharge can starve the compressor of oil. An overcharge could be just as bad or worse, since liquid could then be sucked into the compressor -- the liquid cannot compress further, so that's the immovable object meeting the irresistible force.

There needs to be just enough liquid so that the last remaining liquid is vapourizing as it exits the evaporator and enters the accumulator. Nothing but vapour should exit the accumulator. The only way to be certain of that is to evacuate the system and charge the empty system by weight.

If you are unwilling to either purchase the right equipment or take it to a professional, then charge by temperature of the evaporator lines. Using a pair of good quality temperature probes, measure the temperature of the inlet and outlet at the firewall. Put some foam plumbing insulation around the line and probe for greater accuracy.

The outlet should be just a few degrees warmer than the inlet (since it's mostly vapour) -- IF everything else in the system is working properly, engine RPM >1500, good airflow through the condenser (or water mist on the condenser) and IF you haven't introduced any air into the system fiddling around with a bad can tap.

No offense intended, but I think you would have noticed the bad can tap earlier if you had purged the charge hose of air before connecting it. I too had charged a few systems over the years before I learned how (and more importantly: why) to do it properly. They generally cooled the air when I was done so I blamed the eventual failures on worn out parts, not realizing it was my own ignorance that caused the "worn" parts.

Be very careful with FSM flow charts. The FSM is an invaluable source of information but they are written for the trained mechanic and sometimes assume a certain background knowledge and understanding. GM FSM's are particularly irksome since they make you jump around from section to section and have a lousy index, but if you skip a reference you might miss something important.

-------------------------
I've saved hundreds on service by spending thousands on tools.

Edited: Sat June 09, 2012 at 9:16 AM by buickwagon

sanderson231 on Sun June 10, 2012 3:28 PM

Given the era of the car (and the fact that the clutch is not recycling) I'm guessing this is is a variable displacement V-5 compressor and probably a fixed size orifice tube. With the variable displacement compressor it will reduce compressor capacity to maintain the low side pressure at a constant value even with little freon in the system. While it is true that pressures can not tell you the exact amount of freon in the system they do tell how much freon is circulating if there is a fixed size orifice. So a system with 200 psig high side and 40 psig low side is circulating more freon than one one with 105 psig high side and 40 psig low side. And with the low circulation rate the evaporator outlet is likely above what temp-press charts show for the low side pressure. As freon is added to the variable displacement compressor system the low side pressure should remain the same and the high side pressure will climb. Where it lands depends on ambient conditions, air flow across the condenser and the cleanliness of the condenser.

Never add more than the specified amount of freon. I'm strictly a DIYer but I have worked on a couple of cars where I could not get a full charge in without having the low side compressor increase. I've attributed that to the compressor being a little weak. If the high side pressure is decent like 200 psig (to ensure enough freon to lubricate the compressor), I have elected to live with the cooling I was getting and not go after a compressor replacement.

If you are charging vapor from 12 oz cans (i.e. can in upright position), the can will autorefrigerate as the freon vaporizes and the charging rate will be very slow - at least 30 minutes for a can even with 90 'F ambient. If I charge vapor, I will put the bottom half of the can in warm water
I fairly confident that if you recover freon from your system you will find that it is undercharged. My recommendation would be to vacuum test and make sure you get down to at least 29" Hg vacuum. Shut off pump and close valves on manifold and watch the pressure for at least 30 minutes. Any movement in the gauge is the sign of a leak which needs to be located and fixed. I'd start with a visual inspection for oil and if that doesn't show anything, I'd add 1 can of freon w/ dye. Run the car for 10 minutes then go searching with the UV light and special glasses. You can buy the UV test kit at any major auto parts place. With freon in the system you can also soaps suds joints to identify leak. If leak is in evaporator or condenser the only way to locate it may be an electronic detector and then you're pretty much at the mercy of a professional shop to find it.

GnatGoSplat on Mon June 11, 2012 4:12 PM

Quote
Originally posted by: buickwagon

No offense intended, but I think you would have noticed the bad can tap earlier if you had purged the charge hose of air before connecting it. I too had charged a few systems over the years before I learned how (and more importantly: why) to do it properly. They generally cooled the air when I was done so I blamed the eventual failures on worn out parts, not realizing it was my own ignorance that caused the "worn" parts.


Actually, I didn't learn how to purge the charge hose of air till I read your reply and Googled it. I actually have thought about the air in the hoses and how to get it out, but it just never clicked that's what the schrader on the gauge set is for. Now I know. A little embarrassed, but nothing new there.

I did get it charged up successfully and seemingly working quite well, at least temperature-wise. Seeing pressures of 35/200.

I always add UV dye because my cars are old and leaks aren't uncommon. The leak was from the cap pressing in on the low side valve. I had already put an O-ring as a spacer in the cap since it was bad enough to hiss initially. I thought the O-ring was enough, because I didn't hear a hiss afterwards, but when I checked it recently after it already mostly leaked out, there there was a dot of UV dye inside the cap where the valve was touching it. I bought the cheapest R134a fitting kit from eBay and I guess I got what I paid for.

Thanks for all the help.


Edited: Mon June 11, 2012 at 4:16 PM by GnatGoSplat