A note about A/C sytem pressures

[AaronM]

Now that summer is here and I'm seeing a bunch of posts about the A/C, I wanted to post a reminder that these cars ran lower system pressures than most and that charging to "normal" pressures (like those listed on Clark's Garage) can leave the systems over-charged.

The factory shop manual gives the following:

At 2,000 RPM and 68 degrees F ambient (20 degrees centigrade) after 10 minutes operation.

Low Side: 7.25 - 29 PSI (0.5 - 2.0 bar)

High Side: 145 - 290 PSI (10 - 20 bar)

Vent Temp (center vent): 35 - 39 degrees Fahrenheit (2 - 4 degrees centigrade)

Now, these are for R12. R134a may run slightly higher pressures with slightly higher vent temps depending on the conditions, but these still remain on the very low end of the range. If you're one of the risk-takers out there with Duracool or something similar, your pressures may even be slightly lower than this.

The low-side gauges that come with the DIY refill kits typically show anything below 25 PSI as being undercharged and anything between 25 PSI and 45 PSI on the low side as "filled". This is just plain not correct for our cars. In fact, the high-pressure cut-out switch seems to be triggered at about 40 PSI on the low side, which will cause the compressor to cycle on and off, reducing cooling. Remember, these are expansion valve systems and the compressor should not cycle but rather remain on all the time during normal operation.

I think that slightly overcharged systems are the cause of many of the complaints about "bad A/C" in the 944 series. Even some shops doing retrofits get it wrong. I had my A/C converted a few years ago and was massively disappointed until I double-checked the system pressures and found that the A/C service people had overcharged the system. I took the car back and had them recover all the refrigerant (since I don't have the required machinery to recover refrigerant) and double-check the oil amount, then charged the system myself to the pressures in the factory manual.

The result now is that at 2,000 RPM at about 80 degrees I have low-side pressures of 15 PSI, high side pressures of 150 PSI, and vent temps of 42 degrees Fahrenheit when sitting still. On the freeway with better airflow over the condenser the vent temps will drop into the 30s. These cars want low pressures in the A/C system and running "normal" system pressures will result in inadequate cooling.

[onedroppr]

Will try this week.

I received a 10p15e from International Auto Air Parts in Hong Kong. Looks like a new made compressor, cost ran around $200 shipped.

Also will be flushing the system and replace drier. Will take note about this pressures and will try to test them.

Will let members know about my progress.

[John_AZ]

But...

It is also important to know the amount of R134a is less than R12.

My 924S holds 30 Oz of R12 and I use 24 Oz of R134a.

A/C Question

It has also been brought to my attention that repeated "topping off" of R134a is not a good idea. It is impossible to "add" a correct amount of R134a over a couple years and know how much R134a is actually in the AC system. The pressures may show "correct??" but you may have 25, 28, 30, 35 OZ of R134a.

After a few years, +-, it may be best to just vacuum the entire system and install 24OZ R134a again.

YMMV

J_AZ

[mikepellegrini]

Ha! I was thinking that might be the case.

I had my NA converted to Duracool maybe 3 years ago and charged to the Duracool specs (for R12 equivalents) It worked ok. I don't recall what the pressures were, but I was kinda disappointed.

This year, the A/C is just killer! With a 75F ambient temp, I'm seeing vent temps (fan on low) of the 20's and 30's.

The only thing I could think was that enough leaked out (from an over-charge) that it was running at the perfect level. Thanks for confirming it.

I'll have to hook my gauges up just to see precisely what the pressures are. Thanks.

[gtgt__bangbang]

thanks great post Aaron, from now on you are Mr O.B1 in my book.

I thought I recently saw a formula to correlate ambient temps to proper pressure, somewhere on pelican. Heck if it was 68 F out, I would just roll the windows down and hang my head out. How about 105 ? Its not even 68 at 4am here lately.

BTW, interesting to me anyway, having been 'corrected' recently, its not precisely an expansion valve system , but a THERMAL expansion valve system ... the expansion valve bleed rate is modulated by diaphragm & plunger, diaphragm controlled by a capillary-tube temperature sensor, and the sensing end of the sensor MUST be intimately connected to the LOWER SIDE of the evaporator exit tube (which I guess is the potential spill path of liquid-state refrigerant, should unexpanded liquid, aka a useless waste, be exiting the evap). In fact there is a small scalloped rest, brazed on to the evaporator tube, just for the thermal pick up (a small tight coil) plus a clip that holds it tightly in place, and the pick up needs to be there and only there.

sensor's capillary is even filled with refrigerant, ideally the same mat'l as in the system, which would transfer an open / closed signal to the TEX valve orifice, as per the liquid/vapor state existing at the time in the evap.

I insulated the sensor at the evap tube, loosely tiewrapping some hot water pipe insulation sleeve ala hardware store, and I recall a insulation jacket over the valve/diaphragm assembly (still available per poorsche, like $15) that I replaced with a judicious application of Insul-Foam (used a little plastic "mold" made from the can cap around the TEX valve to keep the foam in place when expanding ...) I was going to do a evap flush write up & glossy pix but want to wait until things cool down so I dont have to eat crow.

[AaronM]

Originally Posted by John_AZ

But...

It is also important to know the amount of R134a is less than R12.

My 924S holds 30 Oz of R12 and I use 24 Oz of R134a.

A/C Question

It has also been brought to my attention that repeated "topping off" of R134a is not a good idea. It is impossible to "add" a correct amount of R134a over a couple years and know how much R134a is actually in the AC system. The pressures may show "correct??" but you may have 25, 28, 30, 35 OZ of R134a.

After a few years, +-, it may be best to just vacuum the entire system and install 24OZ R134a again.

YMMV

J_AZ

I tend to agree with this if you're dealing with a slightly leaky system. Especially since the lubricants are miscible and any leaking of refrigerant will necessarily also involve some amount of leaking oil as well, so after a while you're down on lubricant for the compressor too.

That said, I've found that the "use X% less" is a rough approximation, not necessarily a hard and fast rule. I've seen recommendations for R134a at 90%, 85%, and 80% depending on where one looks. I'd always thought 80% was the standard (which seems to jive with your recommendation), but my point is that figuring out the "correct" charge of R134a for a system designed for R12 can be a bit tricky sometimes. For example, the factory spec for the 1986 944 R12 amount is 950g (33.5oz). The factory R134a guide says 860g (30.3oz) of R134a for the same car. For the 924S, the factory recommended amount of R134a is 770g (27oz). As you've found, however, 24 ounces of R134a works better in your 924S than the "official" amount.

I still have to admit that I'm shocked at how low the system pressure specs are for even R12 though; especially on the low side. The fact that down to 0.5 bar was acceptable absolutely stunned me. The Clark's Garage table suggests a minimum of 28 PSI low side (at 60 degrees), which is a bit over 1.9 bar. If someone charged to that spec, they'd be hitting the high-pressure cut-out all the time when sitting still at idle in traffic on an even moderately warm day.

[Bradical]

The thermostat expansion valve tries to maintain a constant temperature suction line, for maximum evaporator efficiency. If the suction line warms up, it increases flow of refrigerant into the evaporator and decreases as it cools down. Its the idea that the refrigerant will boil in the center of the coil where it has the best opertunity for efficient heat exchange.

[AaronM]

Originally Posted by Bradical

The thermostat expansion valve tries to maintain a constant temperature suction line, for maximum evaporator efficiency. If the suction line warms up, it increases flow of refrigerant into the evaporator and decreases as it cools down. Its the idea that the refrigerant will boil in the center of the coil where it has the best opportunity for efficient heat exchange.

Yes, that's how expansion valves work. But it doesn't have any bearing on how to charge a system or what pressures are correct on the high/low sides.

[Bradical]

Originally Posted by AaronM

Yes, that's how expansion valves work. But it doesn't have any bearing on how to charge a system or what pressures are correct on the high/low sides.

No bearing? It has all the bearing, because it's going to try to maintain a constant evaporator condition regardless of the charge. If you want to get technical, nobody on this thread as said anything relevant to what low and high side pressures are correct because they are affected by more factors, such as outdoor ambient and evaporator return air temp, then they are refrigerant quanity. Technically, the amount of heat absorbed into the system after the evaporator before entering the compressor is the only truly accurate way to charge it, which is exactly what the expansion valve maintains.

The fact is, the only way you can charge the system to spec is to know the exact quanity of refrigerant the manufacturer specifies (which I am certain will be too much, to allow for leakage) and add only that amount. Charging to suction pressure is misleading because a 30psi suction with a high load and a 30psi suction with a low load will be two different quantities of refrigerant.

Basically what I am saying is that almost everything has more bearing than a standard pressure to charge to.

[AaronM]

Originally Posted by Bradical

Basically what I am saying is that almost everything has more bearing than a standard pressure to charge to.

At a given ambient temperature, with a proper charge, a system will run within a predefined range of pressures. Period. That's why the factory specs also give the ambient temperature at which to make the measurements.

While you cannot charge only to low-side pressures, you absolutely can use the system pressures to determine whether your system is operating properly and the charge amount is correct. For example, if your low-side pressures are exceeding 30 PSI at 68 degrees ambient, something is wrong with your system or the system is overcharged.

If you know the appropriate ranges for system pressures, you can accurately diagnose whether the system has been overcharged (if both low and high pressures are high, or if the low pressure is OK, but the high side is too high, though the latter could also indicate a blockage at the TXV), and even if the system has been undercharged (if the low side is low and the high side is low, or if the low side is OK but the high side is low).

As I've said already, the proper way to charge a system is to charge by refrigerant weight and then to double-check the pressures to make sure they're in line with what is expected. Since the "correct" weight of an alternate refrigerant is not an exact science (notice how John uses 24 ounces of R134a despite the official factory amount being 27 ounces), it's important to check the system pressures after charging to make sure that both high and low side fall within the defined ranges.

Most cars' systems expect higher system pressures, both on the high and low side, than the systems used by Porsche. If you use the "normal" pressure ranges to verify a charge on these cars, you'll mistakenly believe that both the high and low sides are reading low, which would indicate undercharging, leading one to believe that the conversion factor was wrong and ultimately resulting in overcharging the system.

[JD159]

I changed some o rings, bought red tek oil recharge and red tek 12A. Pretty good when moving. As for pressures I'll have to double check. Anyone have a clue about this 12A stuff and how it impacts pressures vs. R134???

[mattdavis11]

There are differences that none of you have addressed. The early 944 and 924, are different from the 85 1/2 944, 951, S, S2, 968. The early and all 924 models have an angle style externally balanced TXV, while the others have a block style internally balanced TXV. The tonnage is the same at 1.5 but the superheat is different. 4.8 for the early and 924, 4.2 for the late.

It makes a difference.

Get the oil right first, then play with it. I don't charge to the OEM specs on oil or refrigerant because I don't charge to specs, ever. I typically use less.

[AaronM]

Originally Posted by mattdavis11

There are differences that none of you have addressed. The early 944 and 924, are different from the 85 1/2 944, 951, S, S2, 968. The early and all 924 models have an angle style externally balanced TXV, while the others have a block style internally balanced TXV. The tonnage is the same at 1.5 but the superheat is different. 4.8 for the early and 924, 4.2 for the late.

It makes a difference.

Get the oil right first, then play with it. I don't charge to the OEM specs on oil or refrigerant because I don't charge to specs, ever. I typically use less.

I was under the impression that you could not charge based on superheat with a TXV system. Am I mistaken? Also, I'm assuming your superheat values are in centigrade? They seem far too low to be Fahrenheit.

I agree with getting the oil right first, and I agree that OEM refrigerant charge is not always correct in the field, especially on systems that are now 23 years old at the newest and have necessarily developed some quirks in that time.

My intent was to point out to those who were checking pressures and seeing low readings that these low readings are probably not cause for alarm as long as they fall within the factory ranges at the specified ambient temperature.

I will say that I've also occasionally noticed that the "de-icer" (the temp sensor for the evaporator that prevents evaporator freezing) is not always within spec after all these years either. Some will start switching off the compressor when vent temps are in the 40s, while others won't shut off the compressor at all, leading to lower than normal vent temps (at least until the evaporator freezes). It is my understanding that the early cars (and the 924S) lack a "de-icer," though I could be wrong.

The part in question for '85.5+ cars is 944-659-221-00 and is to some degree adjustable (turn the slotted dial clockwise if it's tripping at a temp too warm, counter-clockwise if it's not tripping and allowing the evaporator to ice up). If your system tests well otherwise, but your vent temps are warm and you hear a "click" from the driver's (left) footwell that coincides with the compressor cycling a couple times a minute, it's possible that the "de-icer" is no longer working properly and needs adjustment or replacement.

[mattdavis11]

I agree. My suction pressure is read right at the compressor, so naturally it's going to be lower than a newer 944 where the low side service port is by the strut tower. the bus I was working on this weekend had both the high and low at the compressor, so I wasn't alarmed when the low was really low, and the high was through the roof. On that system, you can't see much, checking delta t's is not possible, so I had to play with it, a lot, and 6lbs later, there's excellent vent temps. There's a 100ft of hose on that sucker, and you can only see about 6 feet of it.

[AaronM]

Even the later cars with the low side fitting near the strut tower see those low system pressures. The numbers are from a version of the shop manual that includes '87 model year cars. My own car is an '86, which has both fittings near the strut tower (they only did that for one year, which kind of sucks because it's a ridiculously convenient arrangement), and at a proper charge runs system pressures of 15/150 at 70 degrees.

The system pressure curves for R134a in the 964 and 993 also give similarly low system pressures. Overall the Porsche systems just seem to like lower system pressures than most.

[stogie25]

For the past six years my 1988 944 n/a has had mediocre at best air conditioning. I used the numbers in this thread and 860g of R134. It works. It actually blows cold like real air conditioning at a stop or driving. Thank you!!!