Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 924/944/968 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Question AC servicing not fully successful

I serviced my AC system, and have some questions to AC experts.

Here is what I did:
remanufactured the compressor (shaft seal, large and small o-rings, internal gaskets), replaced most o-rings, added 4 oz of esther oil to the compressor, put r134a low and high fittings (after removing the shredder valves), and installed 1" shorter AC belt for clearance.

To check for leaks, I used a "vacuum pump" (essentially an air compressor run backwards) that pumps down to 20 inch Hg. After ~24 hrs the pressure stabilized completely at 19 inch Hg, making me believe I had no leaks.

Took the car to a shop for final (1 hrs) evacuation, and a few hours later added r134a from cans.

here is the PROBLEM:
I managed to add only about 1.5 cans of the refrigerant (12 oz of pure r134a and 1/2 can of r134a mixed with oil) , i.e. ~20 oz, within about an hour of running the engine. The low-side pressure was reaching 45 psi (EDITED), and I didn't want to overcharge it (the charging kit came with the low-side gauge).
The compressor was cycling on /off.
I know I should have added about 30 oz, so I didn't manage to put enough !!!!

The system works (it chills), but it doesn't get very cold.

here is the QUESTION:
Because I didn't manage to put the recommended amount of gas, I think I had air in the system, not vacuum.
As I wrote earlier, I tested for leaks, and it's OK.
Now I'm questioning the evacuation procedure at the shop: I might be wrong, but I think they let air in while disconnecting the hoses.
At the end the guy did something to the machine, and I heard 'pssssss', i.e. the sound of venting the system (what did he vent?). Then he disconnected the high side and low side hoses. I didn't hear any short 'pss' while he was disconnecting the hoses. I don't know how the hose couplings are constructed, so could he have let the air in when he had one of the hoses disconnected?

What do you think?
Thanks, Stan
__________________
1983 red Porsche 944

Last edited by Stan944; 06-15-2004 at 09:36 AM..
Old 06-15-2004, 07:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
Texas944's Avatar
 
Join Date: May 2004
Location: Dallas, Texas
Posts: 164
any pump should pump down to atleast 24 inches of mercury within the first half hour. If you sucked it down for 24 hours with no sign of leakage, then that should have been more than enough. Believe it or not, i have had more success with quicker suck down times than longer ones. When i got my first vacuum pump i would let it evacuate a system for hours. I have found that with shorter evacuation times i get colder A/C. When i recently did my blazer, i only sucked it down for an hour - the oven meat gauge i have stuck in the vent shows 38 degrees on high. When you say 45 inches of mercury on the low side - what psi is that - you should have between 25 and 35 psi there. No more than 225 on the high side. As for the couplings - when they come off they dont usually let any air in, but you dont normally dissconnect them until after you have charged it either.
__________________
Brett, Dallas, Texas (Originally from Sydney, Australia)
1986 944, Zermatt Silver / Black Leather
1994 Full size Blazer, Midnight Blue
1989 IROC, Arrest-me-Red
1966 Corvette Roadster 427 4spd, Silver / Black

DRIVE THEM LIKE YOU JUST STOLE THEM!
Old 06-15-2004, 09:26 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
1) my mistake with units - yes, it was 45 psi, let me edit the message...
2) FYI: the pump I used for leak testing was a DIY pump, NOT a pump you would use for final evacuation - it goes only down to 20 inch Hg, not 30 inch Hg.
3) well, I guess the way I had the evacuation done separately from charging is not standard. It probably would be hard to guess whether the guy let the air into the system or not...
4) I don't have a high side gauge. Is it fair to say that if the compressor was cycling on/off, the compressor works OK?
__________________
1983 red Porsche 944
Old 06-15-2004, 09:36 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Registered User
 
Texas944's Avatar
 
Join Date: May 2004
Location: Dallas, Texas
Posts: 164
that's a good question - usually when it cycles on and off like that, it means that there isnt enough freon in the system. Having said that, 45 psi is too high for the low side indicating too much freon.
Try this - on the low side (with the engine running and a/c on high), push the shraeder valve in and let some air out - down to about 20 psi. Then, put another can in and see what that does. Actually did this one on a friends S10 pickup the other night and have done it in the past with good results. As far as your DIY pump - if you can get 20 inches of mercury on a suck down with no leaks, that's good enough. My A/C pump will only pump down to 25/26 "hg - i dont htink your a/c system would no the difference!
Does the porsche system have an orrifice tube or an expansion valve, and did you change them along with the drier?
__________________
Brett, Dallas, Texas (Originally from Sydney, Australia)
1986 944, Zermatt Silver / Black Leather
1994 Full size Blazer, Midnight Blue
1989 IROC, Arrest-me-Red
1966 Corvette Roadster 427 4spd, Silver / Black

DRIVE THEM LIKE YOU JUST STOLE THEM!
Old 06-15-2004, 10:27 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Thanks for the reply.
1) My former cars either had no working AC, or it was so long ago that I don't remember. So should the compressor be cycling on/off, or it should be steady ON? Does the answer depend on engine RPM?
2) The compressor started cycling on/off around the end of the first can, so I would have to remove a lot of r134a. Keep in mind I'm in Canada, and one can't buy r134a in the stores (I "imported" it during the last visit in US). So I would prefer to release it only if absolutely determined this is THE problem. I kept monitoring the AC vent temps during the process - by feel, :-), and it was approximately same after the first 1/2 can.
3) Following your comment, I would have probably done better if I used my DIY pump instead of taking it to the shop for evacuation. I think the technician vented (intentionally or not) my AC system prior to disconnecting the hoses.
4) I didn't change the orfice tube /expansion valve (I forgot which one it is), nor the dryier. I know I should have, but was trying to save some $$. I think the new drier would keep the moisture out in the long term, but I don't think the old unit would create problems immediately if the system was evacuated properly. I might be wrong though, :-)

So do you think the system 'was' / 'was NOT' air-free before I started charging. or not enough information to answer this question?
__________________
1983 red Porsche 944
Old 06-15-2004, 10:49 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
Texas944's Avatar
 
Join Date: May 2004
Location: Dallas, Texas
Posts: 164
Stan,
as soon as the cycling switch activates, the compressor clutch will start to cycle on and off. As you approach the correct amount of 134, the clutch will stay on longer and longer (i have never actually timed it, but maybe a minute?).
it was a mistake not changing the expansion valve or orrifice tube - they can block up with small debris over a period of time and could be the cause of the high pressure. The decadent bag in the drier can also clog some-what if it is old enough.
As for air-free - put it this way - i have seen people do their A/C systems without evacuating the air and have it still blow ice cold air - actually - i have a friend that retrofitted his r12 to 134a - all he did was let all the r12 out (lookout ozone), put the 134a fittings on, and then throw in a couple of cans of 134a - you can almost see the ice cubes jumping out of the vents it gets that cold - so i dont think the evacuation is as critical as ensuring that there is a clear path for the freon to travel!
Holla if you need another can - i will shoot one to you in the mail!
__________________
Brett, Dallas, Texas (Originally from Sydney, Australia)
1986 944, Zermatt Silver / Black Leather
1994 Full size Blazer, Midnight Blue
1989 IROC, Arrest-me-Red
1966 Corvette Roadster 427 4spd, Silver / Black

DRIVE THEM LIKE YOU JUST STOLE THEM!
Old 06-15-2004, 11:44 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Thanks for the reply.

It's definitely surprising that lack of air evacuation didn't create problems for your friend. But if this is the case, then that's what it is.

My system works now: it's not blowing extremely cold air, but it definitely cools it. I should get a thermometer to get a reading.
And a 1 minute 'on' cycle sounds comparable to what I get.

What you say about the orfice tube makes sense - I should have put a new one.

I think I'll let it work for a while without doing anything, or at least use it sporadically for now.
Right now I think any further progress requires getting a proper high side gauge. I'm a bit tired of working without proper tools (e.g. had to improvise with hose connectors that don't open the shredder valve, etc). Especially that my GF is complaining that I spend so much time in the garage.

And thanks a lot for the offer on the cans. I'll ask if I need to.

Thanks, Stan
__________________
1983 red Porsche 944
Old 06-15-2004, 12:16 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
Join Date: Feb 2000
Location: Kingsport, TN
Posts: 2,925
A purpose of the vacuum pull down is to check for leaks. Don't know how good a check that is because the pressure difference is much greater when the system is charged. It might not leak holding vacuum but it could with 10X the pressure difference under operating conditions.

The real purpose of the vacuum pull down is to remove all the oxygen and other extraneous gases from the system. The oxygen can combine with refrigerant and eat the system from the inside out. Without oxygen the chemical reactions cannot occur and your system tends to last longer. This is the real purpose of the initial vacuum.
__________________
Lawrence
1986 951
2002 SLK32 AMG
1987 328GTS
2011 528i
Old 06-15-2004, 01:38 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Moderator
 
AFJuvat's Avatar
 
Join Date: Feb 2001
Location: Viera FL
Posts: 5,643
You should be able to suck and hold 30 inhg in a leak free system.

AFJuvat
__________________
Es geht nicht darum wie schnell man faehrt, sondern wie gut man schnell fahren kann.

Ihr Brunnen der nutzlosen Porsche Information
Old 06-15-2004, 06:42 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
Wolf1's Avatar
 
Join Date: Jan 2003
Location: Idaho Falls, ID
Posts: 1,199
I see a few problems or irregularities here:

1. The evacute process also helps to remove moisture from the system. The boiling point of water is very low at 20-30 in hg, it vaporizes and some of it is removed this way.

2. Check to make sure your cooling fans are working correctly and your condensor is in good condition. R134 requires more cooling than R12 to work properly.

3. I believe your system also contains way too much oil. You mentioned putting 4 ounces in the compressor plus the 134 oil charge. You did not flush the system to remove the original oil, therefore you are overcharged with oil. This will cause high pressures and poor cooling, as well as higher compressor temperatures.

4. Also, you now have a mixture of Esther and Pag oils...
__________________
75 914 - Undecided.
80 931 - Gone, but not forgotten.
72 914 - old toy- sold.
And a whole bunch of German scrap metal shaped like 924's.


Old enough to know better, and stupid enough to do it anyway!
Old 06-15-2004, 06:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
Join Date: Feb 2000
Location: Kingsport, TN
Posts: 2,925
Yep, I should have mentioned getting the moisture out too. I forgot. I think it also can react with the refrigerant.
__________________
Lawrence
1986 951
2002 SLK32 AMG
1987 328GTS
2011 528i
Old 06-16-2004, 03:39 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Thanks for the replies guys:
- I might not have made it clear, but during the rebuild of the compressor, I drained old oil from it (about 3 oz). I added 4 oz of oil, so I think my oil load should be close to the specs.
- I acknowledge that testing for vacuum leaks not necessarily is valid for leaks under pressure.
- The fans work fine.

- I guess the next step should be replacing the EXPANSION VALVE.

- INTERESTING FACT: I just did a temperature comparison between my 944 and Jeep YJ (2003): the "hand thermometer" feels almost identical in both cars.

- A FUNDAMENTAL QUESTION: Although I acknowledge, I don't uderstand why you guys claim that (short of removing moisture and testing for leaks) vacuum is not needed before charging the system with r134a? If you leave air in, you end up with a mixture of air and r134a. The amount of air in such case is not negligible. This reduces the actual amount of the refrigerant gas.
I the extreme (hypothetical) case, I don't think charging with compressed air would allow AC to cool, although I don't exactly know why. I imagine the temperature of the liduid-gas transition is different, and you can't get proper condensation /evaporation when the system needs it?
__________________
1983 red Porsche 944
Old 06-16-2004, 06:15 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
 
Registered User
 
Texas944's Avatar
 
Join Date: May 2004
Location: Dallas, Texas
Posts: 164
Stan - you hit the nail on the head - you dont get proper liquid to gas transition unless you evacuate the system - and regardless of what anyone says, the only reason you evacuate a system is because there is not supposed to be anything in there except for freon and oil (incidently, the oil that was originally in there was mineral oil, not pag).
After you charge a system with 1 can of 134 (enough to cycle the clutch), then you check for leaks with a sniffer (if you dont have one then you just hope for the best - you will know within 24 hrs if you have a leak or not).
Having said that, every once in a while (especially when you sell parts and deal with a lot of people), you meet someone that knows nothing about A/C (that thinks he does) that does everything wrong, and yet still ends up with ice cold air! "Throw all the theories out the window when that happens and just go with what you know"!
I think that for your first A/C job, the simple fact that you got coldER air to come out is awesome! Next, you need to get a good set of gauges and maybe an o-ring kit, and then start charging people to fix their systems - that's how I started 4 years ago - it's easy, and I have never had any come-backs - just referrals!
Bit of advice for your next A/C job - always use a new compressor and change the orifice tube/expansion valve and drier - especially if it is for a customer! Cant beat that warrenty!
Regards,
Brett.
__________________
Brett, Dallas, Texas (Originally from Sydney, Australia)
1986 944, Zermatt Silver / Black Leather
1994 Full size Blazer, Midnight Blue
1989 IROC, Arrest-me-Red
1966 Corvette Roadster 427 4spd, Silver / Black

DRIVE THEM LIKE YOU JUST STOLE THEM!
Old 06-16-2004, 07:01 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Thanks for the nice word, Brett.
__________________
1983 red Porsche 944
Old 06-16-2004, 08:51 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
Join Date: Feb 2000
Location: Kingsport, TN
Posts: 2,925
Reason why charging with compressed air will not cool much is because there is no phase change. When the refrigerant goes from one phase to another it needs a lot of heat or to expel a lot of heat. Air at reasonable pressures will not do that. You do get some cooling by expanding air. Look at the exhaust on a jack hammer. You can see it because it is quite cool. Gases cool when expanded but the change in energy is nothing like what happens when there is a phase change. All the refrigerant does is move the heat from the cabin in your car to the air passing through the condenser that is in the front of your car. It is merely a transfer medium so you want it carry as much energy (heat) as it possibly can. It can store a lot of energy due to its phase change. For instance to change saturated liquid freon 12 from 10 F to 20 F with no change to vapor, about 2.2 Btu are needed per pound of the liquid. To change the liquid freon R12 from a liquid to a vapor at 15 F(without any temperature change), you need to supply about 67 Btu per pound. That is why is moves so much heat.
__________________
Lawrence
1986 951
2002 SLK32 AMG
1987 328GTS
2011 528i
Old 06-16-2004, 10:22 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
Registered User
 
Stan944's Avatar
 
Join Date: Apr 2004
Location: Vancouver area, Canada
Posts: 69
Thanks Lawrence, it make even more sense now.

On a funny note, I guess you could use AC filled with compressed air if you wanted to cool from the ambient temperature around 80 K (liquid nitrogen, i.e. the main component of air, vaporizes at 77 K). Hmmm, I guess that would be usefull mostly for polar bears in the Arctics :-)
__________________
1983 red Porsche 944
Old 06-16-2004, 11:01 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Registered User
 
Join Date: Feb 2000
Location: Kingsport, TN
Posts: 2,925
Good luck with your AC. I have never recharged or repaired one with R134. The ones I diddle with are R12 or a substitute. Fortunately my 11 year old car with R134 still works well so I have not had to work with it.
__________________
Lawrence
1986 951
2002 SLK32 AMG
1987 328GTS
2011 528i
Old 06-16-2004, 12:26 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #17 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 11:31 AM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.