Another boring AC thread - installing a 993 condenser in a 3.2 Carrera

[DaveMcKenz]

Thanks, Griff. I barely understand this stuff on a good day. When I charge my system, both the high and low pressures rise together. It seems in the past when I have overcharged it, the low side pressure would go up into the 40-45 PSI range. I am not arguing, I just wish I understood.
Dave

[Iciclehead]

If it is of any help, my Retroaire system takes about 18 - 20 oz to get pressures in the range....

Dennis

[Bob Kontak]

Originally Posted by kuehl

In a perfect world, at idle:
R12 at 81F running, high side target would be 194.4 psi.
For R134a it would be close to that of R12, say 204 psi

I use twice ambient (F) +30 FOR R12 and twice F +50 for R134a for ballpark high range. This is meaningless (I think?) as it's not really the topic and I don't mess with (yet) other than regular cars - factory stock.

I do have one question - this is full-on noob ignorance. To assess the low side pressure, is the blue valve turned off to negate the 70 or so lbs pressure from the 30 lb tank?

[Ronnie's.930]

Nathan, looking good, and yes, I would consider your pressure a bit high for the ambient conditions (I would defer to Griff for anything on that subject, however).

Bob, yes, you have to stop the flow of refrigerant from the tank/bottle to get an accurate, lowside pressure reading - the number is way different with the tap open versus closed.

[ganun]

Nathan, looks like you got some cold!
Enjoy it and if it were a bit over so what, it can't be that far off optimal. IMO
Even if you wanted to take some out how would you measure it?

Originally Posted by Iciclehead

If it is of any help, my Retroaire system takes about 18 - 20 oz to get pressures in the range....

Dennis

Dennis, I put 22oz and have vents of 39F in my Retro system, when I went to 24 oz the temp went to 41. Thats with the stock front blower and condenser. But I dont know what it would be with the 20 oz you say, you got any numbers?

[Iciclehead]

Just did mine, low pressure was 45, high was 240....temperature at vent after a short drive was 40F. Ambient 85 degrees.

Total put in was 20 oz as per can weight, I suspect I have about 19 oz in the system what with leakage/stuff in hose etc.

Dennis

[Ronnie's.930]

Just for the sake of comparison, and possibly added confusion , at about 92-95 ambient, I had 30 lowside, 220 highside and vent temp of 27-28 degrees. This was when I re-charged about two weeks ago. I got the same results when I charged in almost identical temps last summer.

[ganun]

How much did put in Ronnie?

[Ronnie's.930]

George, no idea on the amount of refrigerant - I didn't weigh the tank. I've never charged any vehicle via weight/amount of refrigerant (never even look that info up), but just use the vent temp + pressures method. I'm not saying that is how it should be done, but it's always worked for me.

[NathanR]

Ronnie, do you use Griff's formula for your pressures based on ambient T?

[NathanR]

The problem with vent temps as a yardstick is, as you cabin cools off, the vent temp falls whether your ac is cooling it more, or less. And what if some other variable changes?

[SilberUrS6]

Originally Posted by NathanR

The problem with vent temps as a yardstick is, as you cabin cools off, the vent temp falls whether your ac is cooling it more, or less. And what if some other variable changes?

I'm sure Griff has a good reason for doing it this way. You could ask him directly...

[wwest]

Originally Posted by NathanR

The problem with vent temps as a yardstick is, as you cabin cools off, the vent temp falls whether your ac is cooling it more, or less. And what if some other variable changes?

That's been my question, concern, ever since I came across this closed cabin topping off procedure.

Chasing a moving target, it seems to me.

Absent: charge, measure, turn off A/C, wait for IAT to rise back to OAT, repeat ad infinitum...

[Ronnie's.930]

Originally Posted by NathanR

Ronnie, do you use Griff's formula for your pressures based on ambient T?

Sort of, but the thing is, I don't think I've ever charged a system when the temps are below mid 90s, (are that way around here from the end of May to well into October), so I'm usually always shooting for about the same pressures whenever I charge.

And regarding the vent temp yardstick - I always try to determine what is the lowest vent temperature the system is capable of delivering (takes experimentation & experience, obviously) and try to synchronize that with the pressures I am looking for - no changing variables that way, as far as I see it, buttofcourse (that's for Bob, buttofcourse).

Also, Griff explained to me behind the scenes why it is a good idea to record refrigerant weight/amounts added - good reference and troubleshooting data to have for future charging - so I will do that from this point forward.

[Bob Kontak]

Originally Posted by Ronnie's.930

from this point

Check for that on top of your noggin.

I'll bet it's sharp as a pin.

Let's see.....like a .........pin head. Kinda catchy.

Ronnie is a pinhead. Yeah. Heh heh. A pinhead.

[Ronnie's.930]

Originally Posted by wwest

That's been my question, concern, ever since I came across this closed cabin topping off procedure.

Chasing a moving target, it seems to me.

Absent: charge, measure, turn off A/C, wait for IAT to rise back to OAT, repeat ad infinitum...

That "moving target" business is not how it works.

In order to see the coldest temps the system is capable to delivering, the cabin air has to be cooled (the evaporator's incoming air) via re-circulation mode - that does not take as long as you would think if the vehicle is in the shade and has not been pre-heat soaked when you are charging. Once the coldest possible temps are reached, (the thermostat comes in to play here), you note the pressures and verify that they are in your target range - if they are, then you can consider it a success, and if they are not, it's time to start troubleshooting.

[Ronnie's.930]

Originally Posted by Bob Kontak

Ronnie is a pinhead. Yeah. Heh heh. A pinhead.

Check your phone for pics to verify or rebuke yer "pin-head" theory. I advise caution regarding who is around when you view said pics, however!

[wwest]

Originally Posted by SilberUrS6

I'm sure Griff has a good reason for doing it this way. You could ask him directly...

"I'm sure Griff has a good reason.."

I thought you to be the expert in these matters.

But, a theory, if I may...

Orifice tube systems have very narrow range of refrigerant charge. With the charge just a tad low and the evaporator tends to freeze up.

TXV systems, on the other hand, due to having evaporator "condition" feedback, have a much wider range of fill levels for which they will maintain control and prevent the evaporator "charge" from reaching the freeze up zone.

But, as Charlie suggests, what if you ever so slowly charge the system just barely short of the charge level wherein the TXV is in its range of control? You then have successfully "simulated" an orifice tube system with a low refrigerant charge.

Eureka! You now have a system with the lowest possible vent temperatures.

But with a strong tendency to freeze up.

But, shouldn't the thermostatic capillary control switch prevent that?

Not if you calibrate it in accordance with Charlie's "seeming" parameters.

I can't be certain but it appears to me that Charlie's set points for the capillary sensor thermostatic switch will put the average evaporator temperature in the sub-freezing zone.

How else do you get our systems to produce sub-freezing vent outflow absent somehow over-riding BOTH control aspects..??

How do you prevent freeze ups? Pay close attention to the vent outflow air volume, or live in a very dry climate.

[Ronnie's.930]

Originally Posted by wwest

I thought you to be the expert in these matters.

But, a theory, if I may...

Eric can speak for himself, and will correct me if I'm mistaken, but I don't think I have ever seen him post about doing a/c charging, and unlike you, he does not have a track record of posting about subjects that he does not have actual experience with.

[wwest]

Originally Posted by Ronnie's.930

That "moving target" business is not how it works.

In order to see the coldest temps the system is capable to delivering, the cabin air has to be cooled (the evaporator's incoming air) via re-circulation mode - that does not take as long as you would think if the vehicle is in the shade and has not been pre-heat soaked when you are charging.

Once the coldest possible temps are reached,

Wouldn't that be once the inlet air temperature is equal to the vent outflow air temperature? Any time there is a difference between the vent outflow temperature and the inlet temperature the vent temperature cannot be at the minimum.

(the thermostat comes in to play here),

No, at max cooling setting a properly working, properly calibrated, thermostat will not allow the AVERAGE evaporator core to decline below freezing, therefore making it impossible for your system to produce sub-freezing vent temperatures.

you note the pressures and verify that they are in your target range - if they are, then you can consider it a success, and if they are not, it's time to start troubleshooting.

On the other hand many orifice systems use a thermostatic capillary switch for control and all of those appear to be subject to freeze with a sub-standard charge level.

Perhaps Charlie, by charging "just so", has figured out a method to simulate an orifice tube system. TXV outside its range of control, capillary thermostat system not capable of detecting freeze up onset.

Any other theories as to how Charlie's method gets you su-freezing vent temperature my ears are wide open.