Yet another A/C pressure question.
 

I did a search with little info other than I should I replace my Nippendenso compressor.

With engine running at ~2000+ rpms I borrowed a friend of a friends pressure gauge and I found my low side pressure immediately runs ~25 then it creeps back up to ~45 psi. Is this a bad seal(s) in the compressor or is it not enough R12?

With the engine lid closed, and running ~2000 RPMS the sight glass appears to be half liquid and half air. The ball is on the bottom.

As far as I can tell, everything appears to be original. R12 to the hoses to the compressor.

Thanks in advance.

If your car is all original, and you haven't charged the system recently, chances are that you're low on freon. The original hoses were designed to leak, and the system requires topping-off periodically. If you're environmentally conscious, the thing to do is replace all of your old hoses with new barrier hoses, which will not leak R12 into the atmosphere. Then recharge your system.

OK so the head pressure drops with constant RPM is still an indication of low freon/charge?

And not a bad compressor/seal?

That's what I was hoping for. I don't want to add more Freon just to find out that my compressor is bad and have to evacuate and add new Freon.

Look at the underside of your rear deck lid, over near the compressor. If you see oil and grime on the deck lid, then the front seal of the compressor is shot.

Thanks for the replies.

I don't see any there. However, it hasn't been running in a while.

Are there rebuild kits? Or is this a matter of taking it to someone, or buying a rebuild?

I've heard you can get rebuild kits here. If you want to buy a rebuilt, you might try the same company, or try Compressors International in Van Nuys, California. I bought mine there, and the prices were VERY good.

The sight glass should be clear and look like there's nothing in it. If you see bubbles (what a sentence! :D) you are low on refrigerant. The pressure rising and falling could be the expansion valve opening and closing. You normally would not have that big a swing of pressure but when you're low on refrigerant it might happen. Bad seals in the compressor will not raise or lower your pressure but "could" be the source of a leak.

OK, I will try six more ounces and see where the glass is at.

Thanks.

OK, added another 6 oz. The sight glass is 3/4 full with liquid, yet my cold air output has fallen. My average cold air output is ~60 degrees. :(

MY low pressure side is ~25 - 30psi at 2000 rpms. And it no longer changes. Do I need another 6 oz?

Is seems to have been working better with less refrigerant.

The high pressure side, seems hotter, though I haven't measured it yet.

Suggestions?

Thanks.

I was talking to Ron Maxwell at ProCooler yesterday, and he said that it's better to err on the side of a bit less refrigerant rather than too much. Given the cost of R12, I'd put in another ounce or two. If the output keeps going up, then you'll have to remove some.

Should it "leak" out the high pressure side then?

I keep wondering though, because the glass is "empty" when it idles.

Also, When I first turn the A/C on it gets cold for the first five minutes or so, then it all slowly warms back up. :(

Sounds like you have too much water in your system. I would predict your system needs to have the R-12 removed, a new receiver/drier installed, the system evacuated and R-12 reinstalled or convert the system over to R-134a. Cheers, Jim

NNNOOOOOOOOOOOOOooooooooooooo

:D

Could be anything else???

I suppose that's better than needing a new compressor.

Have you considered asking these questions over at the forum at www.ackits.com. They seem to know their stuff

I didn't want to register... :D

I will do that now.

Thanks.

I'd suggest following Jim's advice. You want to add the correct amount of refrigerant. Too much is no good. Just right is the correct amount. Take it to a pro shop and have them evacuate and check the system before filling the system.

Sherwood Lee
http://members.rennlist.org/911pcars

What is the high pressure?
That will help determine whether you are getting close to the correct charge.
With a correctly charged and operating system and engine at 2K, fan on high, thermostat set at coolest setting, engine lid as close to closed as possible you should have a low pressure of 25-30 with a slight movement of pressure as the expansion valve opens and closes. The sight glass should be clear with no bubbles.
It is unlikely that moisture has gotten into the system unless the system has lost its complete charge or has been recharged with contaminated refrigerant.
Don't think your system has a bad expansion valve as it now seems to stablize around 25#.
If your deck lid is open you will have extremely high head pressure and some of the problems you have described such as bubbles in the sight glass. Even with correct charge with the deck open you could see bubbles.

Don is right. With the deck lid open, the head high pressure goes up and can screw up the readings. Set a box fan on top of the deck lid to cool the rear condenser while your working with the compresser.

Don, Jim: With the decklid closed, engine at 2,000 rpm, switch on high, what should the high pressure reading be (with low of 25-30)?

Thanks.

Depends upon the ambient temperature and the type of Freon; pressures go up as the outside air temperature increases. R-134a pressures are higher than R-12 for a given air temperature. Ambient air temperature is measured two inches or so in front of the intake of the condenser. Jim

I'm using R12.

I don't have a high pressure gauge.

I will get a fan on the rear condensor. But why are my vent temps still high? :(

Thanks...

Don't have my tables with me but in general terms high side pressures over 250 psi will hamper cooling and 325 psi or greater indicates the system is overheating (lack of cooling in the condenser). Jim

Typical high side pressures (without adjustment for altitude):

For R-12
80 F ambient 170 to 210 psi
85 F ambient 180 to 220 psi
90 F ambient 190 to 230 psi
95 F ambient 205 to 250 psi
100 F ambient 220 to 270 psi

For R-134a
70 to 80 F ambient 115 to 200 psi
80 to 90 F ambient 140 to 235 psi
90 to 100 F ambient 165 to 270 psi
100 to 110 F ambient 210 to 310 psi

Jim: GREAT information! Do you have a similar table for low side pressures, or do they remain somewhat constant regardless of ambient temperature and/or altitude?

Thanks much!

Typical low side pressures (not adjusted for altitude):

For R-12
32 F Evaporator temp 30 psi
36 F Evaporator temp 35 psi
42 F Evaporator temp 40 psi
48 F Evaporator temp 45 psi
53 F Evaporator temp 50 psi

For 134a
33-50 F Evaporator temp 16 to 29 psi
33-60 F Evaporator temp 19 to 39 psi
40-65 F Evaporator temp 25 to 43 psi
48-65 F Evaporator temp 37 to 51 psi