Strat_928

Hey all,

My A/C was working cool prior to my recent engine pull. But afterwards, it would only keep up if you went from a cooler garage into this sweltering heat we have had here lately. If you stopped in the sun and left it and then started back up, no way could it cool the car down.

I checked the low side pressure and it was less than 30 PSI. (like 1/2)
So I added one can of coolant (has been converted to 134a previously). It was my first time to doing this and I neglected to watch the pressure as it went in. The whole can went in fine. And cold air blows immediately now.

Afterwards I checked it again after a complete disconnection and test run. The low side now reads 40 PSI. Is that too much, or am I ok at this reading? I suppose I could bleed some out if needed.

By the way - the Low side for an 83 is on the passenger side below the fender, pointing towards the fuel rail. I had no cap on mine and the high side had no color on the cap (in front of the radiator near the dryer). The Low side is a larger tube and had obviously been disturbed (hence the cap missing!)

Anyway any comments welcome.

UncleMaz

I thought low side pressure s/b between 20 and 30. So, I would say your problem was elsewhere. What is your high side reading and how long did it take to equalize without the compressor running (slow equals possible blockage).

Strat_928

Thanks for the reply, well the problem was that I only had one gauge and hose setup for the low side that I borrowed from my son-in-law. it would not fit on the High side connector...too big. (Unless I am doofus of course, which is plausible).

The other thing I found is that the connector on the hose sometimes sticks so it does not always release all the pressure without a little jostling. Unfortunately I found that out AFTER I put the can in.

So the short story is I really do not know.

So I may verify the high side fitting connection and get an accurate reading from both places, with/without the compressor running. Then I will have more info to post. I might invest in a dual gauge setup since this will likely not be my last A/C issue... HA

In the meantime I got some advice from WallyP - see below......

The temperature of the evaporator (and thus the lowest possible temp for the cold air) is determined by the low-side pressure. Approx 30 psig is the usual target, but for R-134a, 28 psig is about 32 deg F, 30 psig is about 35 deg F, and 40 psig is about 45 deg F. Goals include having the desired low pressure, but still having enough refrigerant and refrigerant flow to keep the return line to the compressor cold.

Having non-condensibles (air, water vapor, etc.) in the system lowers efficiency, as does too much oil.

WallyP

joejoe

If I remember correctly the high side should be around 2.5 times the ambient temperature. So if outside is 90 degree it should (high side) be around 225 lbs. This is just an estimate as sitting while checking pressures will cause heat soak and higher pressures as well. Your low side pressure may be a bit high, but really need readings from both.

When I do work on my systems I will use a 24" industrial fan in front of car to simulate air flow like when driving. There is a noticeable difference in pressure with fan and without (higher without). As for connections the low side 134 is smaller than high side. You cannot connect wrong.

Strat_928

Thanks JoeJoe, good info there! I do have a shop fan I use to keep me from dying out there, will change the position and give that a try as well....also makes sense why the low side connector would not fit the high side. I only have a 1-hose, 1-gauge setup right now.

dprantl

You can't really tell much with a reading from only one side. You need to tell us what both the low and high side pressures are and also the ambient temp. R134a high-side temperatures can approach 300psi when it is over 95 deg F out and the air flow through the condenser is marginal. But if I had to guess, if you ran the system for a while and the low side did not get below 40psi when before adding the charge it was below 30, there's a good chance that you now have an overcharged system. Remember that one can of refrigerant is 12oz which almost exactly 33% of the total charge of a 928 system with rear A/C, and almost 40% of total withour rear A/C.

Dan
'91 928GT S/C 475hp/460lb.ft

j.kenzie@sbcglobal.net

You know the correct procedure is to have the system evacuated, and charged by weight to 80-85% of the R-12 weight. That's about the only way to get back to normal operation. If the system was open for any length of time you should probably install a new receiver-drier.
Good luck,
Dave

dr bob

So... When you added more gas, did you purge the air out of the hose first? Connect hose and can, with can valve clodsed. Connect gose to car. Loosen the hose connection at the can end, until refrigenerant vents. Tighten the hose, then add refrigerant.

Low side pressure is a function of in-car temp, at the floor. Then ambient temp, and how the compressor is doing. You --really-- need to see high-side pressure at the same time to really know if there's too much refrigerant in there.

For conversion cars, especially pre-S4 cars with tired cooling fans, you will do well to swap in a dual-pressure safety switch. The factory R-134a cars from late '93 models have a switch that includes under-pressure protection for when the refrigerant leaks out again, and also has overpressure protection for the time when the system is overcharged or the condenser is cooled well enough.

Strat_928

Thanks guys, I have already decided to go pick up a full pressure gauge kit this weekend. It does make sense to see the whole picture first before doing anything else (dumb... You need to know exactly where you are before you can figure out where you are going... !

Dr.Bob, I did that sequence as follows - 1- yes, 2 - yes, 3- yes, 4 and 5 - NO, just started adding refrigerant. FYI When I connect it p to the low fitting and the car is not running, the pressure reads 81. Once I turn it on and turn the A/C and fan on full, it drops to 39-40 PSI.

So I will get a proper gauge set and see where we are. What are the figures (close range) of what I should see on the high side and low side when the compressor and all is on full bore? I would say the garage temperature is at least in mid 80's, as I wait until evening to try and do anything. The car will be stone cold when I test.

Thanks for the input.

j.kenzie@sbcglobal.net

Evacuate and recharge by weight. You have air and water vapor in your system. You have no idea how much refrigerant is in your system. I have in the past tried to top off systems until finally something goes bad such as blowing out a high pressure hose on a hot day when I have topped off a little too much. If you don't have an ongoing leak, you should only have to properly fill your system once, and the cost should be under $200, assuming you need to pay someone who has the equipment needed.
Good luck,
Dave

UncleMaz

In addition to above, flush the system with ac flush or mineral spirits. Be sure to EXCLUDE flushing the expansion valve, compressor and receiver/drier. Blow out the lines, evap and condenser with compressed air.

dr bob

The challenge with any readings is knowing how much of the pressure is refrigerant and how much is air. "just a little air" wreaks havoc with the system's ability to cool the car, and obviously interferes with our ability to judge system health looking solely at pressures and temps.

Dave is correct when he says to evacuate all the refrigerant and air out, then charge by weight. As it is now, there's no way to know how much air and how much refrigerant is in the system. Gauges will help you see what the partial pressure of the air is. Unfortunately, once air is allowed in, it's virtually impossible to get it out without sacrificing the refrigerant charge. Good recovery machines can get a lot of it by recirculating the old refrigerant through a cold setion and bleeding the air out of the refrigerant.

j.kenzie@sbcglobal.net

Unless his compressor grenaded, I don't think flushing is mandatory, and would involve dis-assembly of most of the system. This should be done in proper conversion of R12 to R134,along with o-ring replacement, but I am assuming it was done in the past (?).
Dave

UncleMaz

After wrestling with this recently on another car, I am impracticably paranoid about contamination. So, Dave is right and my suggestion is probably overkill if this work was done.

Kevin in Atlanta

Start with the basics. Feel the inlet and outlet pipes on the expansion valve near the windshield. They should be so hot or cold so as to prevent you from holding them for more than a few seconds. If the cold side is not too cold to touch then you might have a stuck expansion valve. All this moot without pressure readings.