Ac questions

[98sub2500leg]

Model: 98 OBW, 2.5L Manual Trans.

 

I replaced my AC condenser and took it in to a shop to have the AC recharged. I am a bit weary that he actually did the job right because he did the job under an hour, he said 45m-1 hr.

the AC doesn't seem to get as cool as it used to. I drove it back after driving down the road and testing it, he said he added he added 2oz of oil since it was a new condenser and 2lbs of freon, which he said was spec.

The Haynes manual says 2lbs of freon but the guys at ackits.com told me that it should take 1.5lbs of freon max for that make and model. I put a set of gauges on it and measured 22PSI(low), and 200PSI high at 1500rpm.

 

The service manager at the dealer said generally Subaru's should have a 35 deg. temp. differential from center vent to ambient. I am getting 22 deg differential which is a 13 deg. off of what it should be. Also, the Ac seems to cycle on and off too much (every 10-12 seconds). The fans cycle on when the compressor turn on and off when it is off. If ackits.com is right, then it is overcharged. They got their data from a Michell manual which is supposed to be an accurate compiled software that shows actual individual ac for any make and model.

 

Does anyone know what kind of real pressures I should be getting on this from the actual factory service manual (not a Haynes or any other manual or general ac data)?

Edited September 16, 2009 by 98sub2500leg
TYPO

[Scoobywagon]

As a rule, you should see a 10:1 relationship between high and low. You should also see about 35-40 psi on the low side. So your pressures look correct with respect to each other, but both are low. That, combined with the fact that the compressor is cycling a lot, tells me that you are low on refrigerant. I'll look up your car tomorrow. We'll see what it should take.

[98sub2500leg]

As a rule, you should see a 10:1 relationship between high and low. You should also see about 35-40 psi on the low side. So your pressures look correct with respect to each other, but both are low. That, combined with the fact that the compressor is cycling a lot, tells me that you are low on refrigerant. I'll look up your car tomorrow. We'll see what it should take.

 

Thanks Scooby,

 

There seems to be a lot of conflicting info I am getting regarding this issue and that may be the problem. Also, whats strange I have found is that there isn't much info out there to confirm gauge readings or pressures at either static no load (engine off condition) or more importantly, at load condition and/or at during cycling. It seems like the pressures are relative to each other and output temperature differential is what seems to be used by a lot of mechanics with generally using between 1.5-2 lbs of R134a freon. In other words, some mechanics use 1.5lbs or 16oz. (max), others use 2lbs or 24oz.

 

Our 2003 Dodge caravan is not the best to drive, but I appriciate the AC as it really chills the air down. I used that as a measure since it works so well and used a type K digital thermocouple and measured 33.8 deg F.

I know that is pushing it, but with the constant cycling on the Suby, you can actually feel the temp and humidity immediately rise when it cycle every 10-12 sec. even if the doors are left open. All I know is I drove it for 11 years and this condition is not normal. Also, flustered after being quoted $79.95 which he said included freon and oil but was charged $147.

He also claims he put the right amount in. I greatly appreciate the help.

 

Bryan

[OB99W]

I replaced my AC condenser and took it in to a shop to have the AC recharged. I am a bit weary that he actually did the job right because he did the job under an hour, he said 45m-1 hr.

If I remember correctly, a hole in the condenser necessitated its replacement. Since the other work on the car took a bit of time, I assume that the A/C system was exposed to atmosphere for that period. Air and moisture can significantly impact A/C performance, so at the very least a vacuum should have been pulled for about as long as the tech said the entire job took. It's even possible that the desiccant in the drier got saturated, and the receiver/drier should have been replaced.

 

 

[...] I put a set of gauges on it and measured 22PSI(low), and 200PSI high at 1500rpm.

 

[...]Also, the Ac seems to cycle on and off too much (every 10-12 seconds). [...]

Around 22 PSI, you're likely below the low pressure cut-off switch trip point, which could explain the short cycling.

 

 

 

 

As a rule, you should see a 10:1 relationship between high and low. You should also see about 35-40 psi on the low side. [...]

Based on a 10:1 ratio and 35-40 PSI on the low side, it implies a high side pressure of 350-400 PSI. That would be excessive. Under typical operating conditions with R134a, I'd expect the low side to be closer to 28-30 PSI. The high-low ratio, as a rough ''rule'', should be perhaps 7:1-8:1.

 

 

 

 

[...] Also, whats strange I have found is that there isn't much info out there to confirm gauge readings or pressures at either static no load (engine off condition) or more importantly, at load condition and/or at during cycling. [...]

It's important to keep in mind that the temperature and pressure of the refrigerant directly influence each other, so the ''correct'' pressure varies with temperature (among other factors). Static pressure for R134a, as a ''rule of thumb'' at typical operating ambient temps, is in PSI about equal to the temp in degrees F plus 10%. For example, at 80 deg F, R134a vapor pressure is about 88 PSI.

 

If static pressure at the ambient temp read significantly lower than vapor pressure shown on a refrigerant chart, you could probably assume that the system was undercharged. However, if the reading is as high as expected, that doesn't ensure sufficient refrigerant. The system could have only vapor in it, or anything up to the correct charge (and beyond) with vapor plus a liquid component, and the static pressure would measure the same. (The vapor reaches an equilibrium pressure for the particular ambient temp.)

 

Two questions:

1) After it's been off for at least several minutes, does the system short-cycle very soon after turning it on, or does it run longer at first and then get worse?

2) After running and short-cycling for a while, when the system is shut off how long does it take for the static pressure to reach equilibrium (that is, when both low and high side gauges read the same)?

[98sub2500leg]

Thanks for the informative post OBW,

 

Yes, I did replace the condenser, unfortunately not the drier on recommendation from another shop. That guy told me that since it was sealed from the factory charge it should be ok w/o replacing. The guy who did the job later quoted quite a bit lower and was recommended by the machinist who rebuilt my cylinder head so I took it to him. He originally told me that it would take 2-3 hours, but when I got there told me it should take 45min-1hr. I thought that didn't sound right. I now know the correct way is to vacuum for about an hour. Stop for 1/2 hr, check for leaks, then revac for another 1/2 hr then charge.

 

My static pressures I measured at 70 deg ambient was 25PSI low and 75PSI high. Although my gauges scale at the bottom shows deg C. on both gauges.

I have read a few posts showing the high side should be 4x higher than the low side as a general rule, not sure if this has merit.

Unfortunately I don't have a vapor pressure chart. So 70deg F should read 77 or 21.2 deg C which is the scaling of my gauges.

 

When you said the high side should be 7:1-8:1, is that relative again to ambient? If it is, then the numbers all show it to be good because again my gauges read in deg C.

 

I will stick the gauges on again this morning and get back to you with some numbers. Thanks for the post OB.

[98sub2500leg]

Ok, here are the readings:

 

Outside ambient is 62deg F

 

Initial static pressures with engine off and system not run since last night: Low=68

high=65

 

AC compressor time cycle:

Cycling first ran for 50 sec on then 5 sec off then 8 sec on and stayed at the 8 sec on and off for the rest of the cycling.

 

AC compressor cycling on readings:

Low=22

high= 105

It seems to stay at steady state with these numbers at 8 seconds on too short of a on cycle to say with any accuracy. It seems to stay at steady state after 6 seconds on and seems to stay there for the last 2 seconds.

 

Compressor cycling off readings:

Low=45

high=90

[98sub2500leg]

Sorry, I just checked it again and the the answer to question #2 is it reaches equilibrium with in about 5 sec.

 

The on cycle reads as following:

1st 50 sec cycle(on):

Low=26

High=115

 

off for 8 sec then on

 

cycle2 readings:

Low=20

High=110

 

off 8 sec, then on

cycle 3 readings:

Low=20

High=105

 

off 8 sec, then on

cycle 4 readings same as cycle 3 and the numbers are still slowly dropping when the 8 seconds is up and the compressor hits it off cycle.

[Scoobywagon]

Again...low refrigerant.

[Scoobywagon]

ok. Looked up your car. It calls for 1.3 lbs R134a. Most vehicles call for a specific amount +/- ~4 oz. This one does NOT. Having said that, I'll bet that the same +/- thing applies, so 1.5lbs is a bit high, but probably just fine. 2lbs is WAY high. But I don't think that's your problem.

 

Rapid compressor cycles indicate low refrigerant, especially when combined with low system pressures. So one of 2 things has happened here. Either someone DIDN'T put in the amount of refrigerant they said, or you've got a leak somewhere. Since the system hasn't gone flat over the last couple of days, I'd guess the first thing is your problem. Not trying to throw anyone under a bus, but its the only explanation I can come up with.

[OB99W]

Thanks for the informative post OBW,

You're welcome.

 

 

Yes, I did replace the condenser, unfortunately not the drier on recommendation from another shop. That guy told me that since it was sealed from the factory charge it should be ok w/o replacing.[...]

 

If the system was still under charge, then the drier should be fine. However, if the system was left open to atmosphere for a sufficient time, then a long vacuum application and possible drier replacement could be needed.

 

 

My static pressures I measured at 70 deg ambient was 25PSI low and 75PSI high. Although my gauges scale at the bottom shows deg C. on both gauges.

Unfortunately I don't have a vapor pressure chart. So 70deg F should read 77 or 21.2 deg C which is the scaling of my gauges.

Static pressure on low and high sides should be equal, so I assume the low also read around 75 PSI (not 25 PSI). Manifold gauges for use with particular refrigerants have corresponding temp scales that can be read to determine saturation temp for the pressure (and vice-versa). Your gauge set apparently is set up for C degrees, so of course you'd have to convert to/from F deg, but you don't need a chart if the temp scale is there. Just to make life easier, I found a chart online with both C and F temps versus vapor pressures for R134a and other refrigerants at http://www.chillers.com/PT%20charts%20for%20refrigerants.htm . The ''rule of thumb'' I gave earlier of course results in a ''ballpark'' figure -- actually, for 70F/21.1C the chart says R134a sat vapor pressure is 71.2 PSI(G).

 

 

When you said the high side should be 7:1-8:1, is that relative again to ambient? If it is, then the numbers all show it to be good because again my gauges read in deg C.

The ratio is a ''ballpark'' estimation of how many times greater the high side pressure should be than the low side when the system is operational. For example (only!), if the low side were 30 PSI, 8 times that would suggest a high side reading of 240 PSI. System load and other factors influence that, naturally. In case there's any confusion, the gauge temp scales, whether in C or F degs, in no way affect the pressure readings.

 

 

As to the readings you last took, the rapid (5 second) equalization time implies no significant system blockages. With a 65 deg F ambient there wouldn't be much load, but the pressures seem to be dropping more than that would explain. I'd suggest running the system again when warmer, doors closed, windows and hood open, blower on high speed, and recheck pressures. If much lower than what you said in the first post of this thread (25L/200H), it would seem to point to a leak.

 

I'd proceed by verifying whether there is a leak, and of course deal with anything found. Next, if the system had previously been open for any significant time, the refrigerant should probably be recaptured, and a vacuum pulled for long enough to ensure proper operation (need for a new drier depends on length of time system was open). With an evacuated system, adding back the proper charge should be a lot easier. A good shop will have equipment that does an air/moisture purge of the recovered refrigerant so that with recycling those contaminants don't wind up back in the system.

[98sub2500leg]

Very informative info, thanks to Scooby and OBW.

 

I had a can of freon on the shelf from Interdynamics. It has a pressure gauge with limits. Example, 0-25-low, 26-45 filled, 46-65-alert, 66-225 warning.

They have a system pressure chart as a general rule (depending on make/model).

The method you described is more accurate OB and Scooby seems to have the right amount of freon to add for my car. Yesterday was too cool to do anything later today should be ok.

I will add a bit of freon and repost. Functionally, it does cool and during the compressor cycle the evaporator cools the air inside rapidly. I can't help wonder if the sensing bulb is reaching its set point then cutting out. I doubt it though.

 

 

 

 

 

http://www.efproducts.com/oneadmin/faqsupport/home.print.php?faq_id=53&category_id=12[/url]

[98sub2500leg]

I think I got it working ok now. I charged it a bit with a 12 oz. can of r134a got the numbers up to 25 and 180 and that is very relative as the pressures are always changing and trying to equalize with the delta T.

 

The compressor stays on for about 50 seconds with the door open, about 20 sec with it closed. The outside temp. was 75 deg F. cools good but still cycles more than I expected but cooling is acceptable. One of the big problems I was having was that I was testing it at too low an ambient temperature. It seems the best way to properly charge it and test is to have a heat load on a warm day, not a cool day.

 

I also tested our Caravan which has one of the best ac systems I have ever seen it really chills. It seems to cycle about the same.

[OB99W]

I think I got it working ok now. I charged it a bit with a 12 oz. can of r134a got the numbers up to 25 and 180 and that is very relative as the pressures are always changing and trying to equalize with the delta T.

 

The low side might still be close to the low pressure cut-off switch threshold, but ...

 

The compressor stays on for about 50 seconds with the door open, about 20 sec with it closed. The outside temp. was 75 deg F. cools good but still cycles more than I expected but cooling is acceptable. One of the big problems I was having was that I was testing it at too low an ambient temperature. It seems the best way to properly charge it and test is to have a heat load on a warm day, not a cool day.

... there's no question that refrigeration is all about temperature! It's good that you got it to the point where it's not short-cycling. I agree that waiting for a day that's warmer when you can recheck the pressures and overall operation under a more typical load is a good idea.

[Luvn737s]

I replaced the O-rings on mine and recharged it with the Autozone gauge/pistol-grip device. I added 1 can and the low side pressure compressor on was about 22 but the high side was alarmingly high. I addded a second 12 oz can and the low came up to 25 and the high dropped to around 60. About a half a can of additional brought the low side up to about 33 or so and the high to about 45. Again the system seems to short cycle at idle. Output air temp is about 42F with ambient temp of 70F

 

What are the consequences of excessive R134a volume and are there other components that sense pressure that could need replacement to stop the short cycling (which occured prior to re-filling as well)?

[Scoobywagon]

Overfilling will result in higher outlet temps because the refrigerant can no longer expand properly. Additionally, it will result in higher head temps at the compressor which can eventually lead to compressor failure. In extreme cases, it is possible to lock the compressor. A little over or a little under will not tend to matter overly much.

 

As for pressure sensors, there are frequently 2 switches. One turns the compressor off for excessively low system pressure, the other turns it off for excessive high pressures. Come systems use a single switch for both conditions.