A/C Recharge 2000 Pontiac Montana Rear A/C

Joined
Feb 11, 2007
Messages
360
Location
NorCal. USA
1627391805400.jpg




I thought I better start a new thread, don't mean to hijack. This is my question copy&pasted from mrlawnguy thread on 7/27/21

My compressor stopped working I assumed due to low pressure. I pushed the Schrader valve in, still had a tiny bit of pressure, so I assumed the system hadn't sucked in any air/moisture. I bought the can pictured here, noting it had oil and seal swell, didn't notice the sealer. The system was last charged 10 years ago, so it's a good assumption just normal loss from seepage had occurred.

So I used the whole 20oz can, and low pressure side was barely out of the "low" white area (around 25#). The gauge is reusable, so I returned the empty can for my $10 deposit (CA requires a $10 deposit to ensure proper return/recycling of the can), and bought two of the 12oz cans. I put in one 12oz can and the low side is now not quite to the middle of the green area around 35#. Revving the engine dropped the pressure to 25 so I did that to get the most out of the can, then at idle got the 35# reading. You can tell when the system is not taking any more, as the can stops feeling cold.

This is the 2000 Pontiac Montana minivan with rear A/C, which takes 2.88lbs (46oz). So far I have put in 32oz, and have the reading around 35# on the low side at idle.

Two questions: (1) The normal seepage of refrigerant over 10 years would not deplete much oil, so the oil contained in the first 20oz can should be enough so adding the 12oz can(s) with no oil would be ok? (2) Knowing there was very little refrigerant left in the system, would it be wise to add the second 12oz can to bring total added refrigerant to 44oz?

Thoughts?
 
Last edited:
Joined
Mar 20, 2008
Messages
14,124
Location
USA
Do not add the second can if you don't need it. Once it is cold, then you have enough in there. Many cans have a small amount of oil in them for the reason you mention.

If you want, you can get a small can of oil to add.
 

JC1

Joined
Nov 29, 2008
Messages
6,735
Location
Oshawa, Ontario Canada
Just adding my 2 cents.

You should get a hold of a proper gauge set. Harbor freight sells one, or maybe the local loan a tool at the auto parts store has one you can borrow.


There are many youtube videos on how to use the gauge sets. You need to know the high and low side pressures are for your Vehicle. Without knowing that you will probably add the wrong amount and have issues. The Pag oil can be bought in separate cans and added if needed. That's usually needed if you have to replace the compressor or lines etc.

Did you check the Montana for leaks on the rear lines?

Here is a chart for oil capacities.

I found this post which you can read for some more tips. This was a 2004 model.

"normal pressure should be 20 to 35 on the low side and 150 to 200 on the hi side."

 

garageman402

Thread starter
Joined
Feb 11, 2007
Messages
360
Location
NorCal. USA
Just adding my 2 cents.

You should get a hold of a proper gauge set. Harbor freight sells one, or maybe the local loan a tool at the auto parts store has one you can borrow.


There are many youtube videos on how to use the gauge sets. You need to know the high and low side pressures are for your Vehicle. Without knowing that you will probably add the wrong amount and have issues. The Pag oil can be bought in separate cans and added if needed. That's usually needed if you have to replace the compressor or lines etc.

Did you check the Montana for leaks on the rear lines?

Here is a chart for oil capacities.

I found this post which you can read for some more tips. This was a 2004 model.

"normal pressure should be 20 to 35 on the low side and 150 to 200 on the hi side."

Thanks for the informative reply. The system had last been charged 10 years ago, so I attribute the loss to normal seepage. I pushed the low side Schrader valve in, still had a tiny bit of pressure, so I assume there was no moisture introduced. It was so low, the compressor would not engage. I also assume no oil was lost, and the 20oz can pictured (slightly different from the can I purchased) has "super synthetic" oil, so I assume there is enough oil in the system.

There was a change between 2000 and 2001, lower capacity. The label on the core support says 2.88lbs for rear A/C equipped. So that's about 46oz. Exactly what the chart you linked says. I did find that chart before I started, but thanks for posting it, I forgot which search result it was.

I did not get the pressure manifold gauge set I just used the gauge for the low side that came with the 20oz can. It does not have the temp adjustment, I searched pictures of it, and it seems that at 70ºF 35psi is the max. I have 33-35psi on the low side at 70º and used all the cans for a total of 44oz. I also assume I will never get the entire contents of each can, although I did rev it to get the low side down to 25psi so it would empty the cans more.

Time will tell, I suppose. The compressor was a little noisy when it first engaged, but with more refrigerant it seems to have quieted down. Maybe the "super synthetic" pag oil is really working, LOL. Or maybe that's just from circulating it more thoroughly.
 
Joined
Jan 2, 2004
Messages
9,800
Location
California
I’m not a fan of that AC Pro stuff but it does have its place. As long as the high/low side pressures are within spec and there’s no weirdness like the clutch cycling rapidly or abnormal pressures which indicate a blocked TXV/orifice tube or signs of a failing compressor(I think GM did phase in variable displacement compressors in that Montana), I wouldn’t worry too much. You might have some air in your system but that’s an easily-prevented problem - purge the hose by letting a little refrigerant out. That’s another reason for the gauge set.

There’s a few ounces of oil with seal swell agent and sealer. The 20oz cans have 3oz of it, and the 12oz cans have 2oz of it. If you used the 20oz and 2 12oz can, you added in 27oz of R134a and 5 oz of oil. If you need to add more, I’d get a can of virgin R134a. Unlike the older GM/Ford systems that can hold excess oil in the accumulator, modern HVAC systems don’t take much oil and too much can cause issues. I think today’s AC systems hold 80-160cc/ml of oil. Which works out to 2.7-5.5 Oz of oil.
 
Last edited:
Joined
Jan 2, 2004
Messages
9,800
Location
California
Also, by purge the hose, I meant the charging hose. On a gauge set, there’s a Schrader valve just for that. On the DIY charge hose like AC Pro, quickly hit the button or trigger for less than a second for a little puff of refrigerant before connecting to your system.
 

garageman402

Thread starter
Joined
Feb 11, 2007
Messages
360
Location
NorCal. USA
Also, by purge the hose, I meant the charging hose. On a gauge set, there’s a Schrader valve just for that. On the DIY charge hose like AC Pro, quickly hit the button or trigger for less than a second for a little puff of refrigerant before connecting to your system.
Thank you, yes I figured that, but the A/C Pro hose won’t discharge unless it’s connected. Probably more consumer protection. The valve going to the can is one-way as well, so I couldn’t back flush, though the system was practically empty anyway. So I probably introduced a little air.
 
Joined
Jun 5, 2003
Messages
25,557
Location
Apple Valley, California
The low side gauge is a gimmick. It means nothing without a high side reading.

I really wish they would outlaw that gauge on cans! The proper way to charge a system is by weight. Pressures are mostly for diagnosis
 
Top