AC Pressure

[PSU_Crash]

Static pressure checks out. 75 psi on the high and low sides.
It was approximately 78°F at the time of testing. Tomorrow I'll grab the compressor model and report back.

75 psi static on both sides at about 78F is a good sign your gauge hookups are correct and there’s at least some refrigerant in the system.

Given that, the earlier running readings of roughly 0 psi low and only about 80 psi high, with no change when you raise RPM, really does point toward “compressor not pumping” or “no refrigerant flow.” If it were a classic restriction, you’d typically see the high side climb well above static while the low side drops, because the compressor is still moving mass and stacking pressure on the high side. Your high side staying near static suggests the compressor isn’t building differential pressure.

Next time you test it running, it helps to watch the gauges from the moment you turn the A/C on. If the clutch engages and both needles basically sit near static, that’s consistent with a dead compressor (internals) or a sheared/slipping drive situation even though the clutch face is turning.

When you grab the compressor model, also note whether the center hub is turning with the pulley when engaged, and whether the belt is tight and not glazing. If it’s a Sanden-style unit, internal failure is unfortunately pretty common when they’ve run low on charge/oil.

Since you already have a new drier and seals and can vacuum/charge by weight, I’d still do the drier and a proper evac before condemning parts, but if the compressor can’t create a pressure split, a fresh charge won’t make it cold.

Post the compressor make/model and, if you can, a quick description of what the low and high do immediately after engagement at 1,500–2,000 rpm, and we can call the next step with more confidence.

I decided to just give it some charge to see if it helped and also if it holds pressure. After approximately 16oz of 134a the gauges are 25 psi on the low side and 180psi on the high at idle. Bringing the revs up to 2k RPM, the high side reads 220 psi and the low falls to 15ish. I would have given it a little more charge, but my second can of 134a was being very finicky.
I plan to drive the car to work tomorrow, so I'll have a report in the evening on the system functionality. Once charged, the compressor is no longer making any strange sounds either.

[CarBot]

I decided to just give it some charge to see if it helped and also if it holds pressure. After approximately 16oz of 134a the gauges are 25 psi on the low side and 180psi on the high at idle. Bringing the revs up to 2k RPM, the high side reads 220 psi and the low falls to 15ish. I would have given it a little more charge, but my second can of 134a was being very finicky.
I plan to drive the car to work tomorrow, so I'll have a report in the evening on the system functionality. Once charged, the compressor is no longer making any strange sounds either.

Those numbers look a lot more like a functioning system now. At roughly 78F ambient, 25 psi low and 180 psi high at idle, then about 15 psi low and 220 psi high at 2k, says the compressor is pumping and the condenser is rejecting heat. The low side dropping into the mid-teens at 2k is on the low side of normal, which can happen if the charge is still a bit low, if airflow across the evaporator is high, or if the expansion valve is feeding a bit aggressively, but it’s not an immediate red flag by itself if vent temps are good and the evaporator isn’t icing.

A couple practical cautions. Since you don’t know what was in there before and you had some unintended release earlier, the charge amount is now a bit of an unknown. If it cools well but starts fading again on a longer drive, I’d stop “topping off” and do the proper reset you already planned: new drier, evacuate, vacuum hold test, then charge by weight to the correct spec for your conversion. That’s the only way to know you’re not chasing your tail with air/moisture or an over/under charge.

On tomorrow’s commute, pay attention to whether it stays cold at highway speed and at idle after you get off the highway. If it’s cold cruising but warms up sitting still, that points back to condenser airflow/fans. If it starts cold and then gradually warms no matter what, that points more toward charge level, moisture/freezing at the expansion valve, or evaporator icing.

If you can, grab vent temp at the center vent after 10 minutes of driving, and note ambient temp and whether the compressor is cycling. That’ll make the next step pretty clear.

[Tom]

I would add the importance of pulling a vacuum before charging. If the refrigerant ran low (sounds like) then air probably got in, and air in the system is --in a word-- bad. Harbor freight sells a cheap vacuum pump I've used maybe a dozen times now and it's good enough to get a nice deep vacuum and good results. (Not deep space vacuum like dr. bob gets, but good enough to keep you nice and cool.) Once you've pulled a deep vacuum for an hour or three and confirm the system holds it with no leaks, I start with about 29 oz of r134a (plus oil for replaced parts) and that usually puts the pressures within range of the charts and makes for ice cold vent temps.

[PSU_Crash]

I would add the importance of pulling a vacuum before charging. If the refrigerant ran low (sounds like) then air probably got in, and air in the system is --in a word-- bad. Harbor freight sells a cheap vacuum pump I've used maybe a dozen times now and it's good enough to get a nice deep vacuum and good results. (Not deep space vacuum like dr. bob gets, but good enough to keep you nice and cool.) Once you've pulled a deep vacuum for an hour or three and confirm the system holds it with no leaks, I start with about 29 oz of r134a (plus oil for replaced parts) and that usually puts the pressures within range of the charts and makes for ice cold vent temps.

Copy that Tom. I just couldn't help myself
Now that I have a good feel for the gauges and charging I'll be changing out parts next weekend and do it right. I'm fairly confident at least the high side Schrader valve leaks. I get a little psshh when I remove the cap. Either way it's all getting replaced soon!

[dr bob]

FWIW, I target the pressures you report, to get the lowest practical vent temps. Goal was coldest possible to help recover a black heat-soaked car in L.A. basin. It doesn't offer the best total heat 'removal' capacity, but it does offer the coldest vent temps with the freeze switch disabled.

There are some handy little tools that let you swap the shraeder valves in the service ports with the system still under pressure. Not at all expensive. You can buy this one https://www.amazon.com/R134A-Valve-Remo ... pln=0&th=1 from Jeff B's garage for $20.

On my car there are service ports on the compressor hose manifold plate. These are in addition to the ones in the lines in the engine bay. If your car has those, it's certainly worthwhile upgrading them to R-134a capable valves, especially if you still have the original R12 valves in them. Swap new green o-rings into the cover caps at the same time. The PAG, and to a lesser extent the POE oil, are death on the original seals in those valves. For your regular valves in the engine bay, the original valves need to be out before the port adapters are installed.

[PSU_Crash]

FWIW, I target the pressures you report, to get the lowest practical vent temps. Goal was coldest possible to help recover a black heat-soaked car in L.A. basin. It doesn't offer the best total heat 'removal' capacity, but it does offer the coldest vent temps with the freeze switch disabled.

There are some handy little tools that let you swap the shraeder valves in the service ports with the system still under pressure. Not at all expensive. You can buy this one https://www.amazon.com/R134A-Valve-Remo ... pln=0&th=1 from Jeff B's garage for $20.

On my car there are service ports on the compressor hose manifold plate. These are in addition to the ones in the lines in the engine bay. If your car has those, it's certainly worthwhile upgrading them to R-134a capable valves, especially if you still have the original R12 valves in them. Swap new green o-rings into the cover caps at the same time. The PAG, and to a lesser extent the POE oil, are death on the original seals in those valves. For your regular valves in the engine bay, the original valves need to be out before the port adapters are installed.

Appreciate the info, as always!

I have looked at those tools to change out the valve without loosing charge. Handy little tool that pays for itself in the cost of refrigerant with just one use. I'm certain my car has the screw on adapters over the original R12 valves. I would also bet the system was never properly converted, rather just adapters put on and recharged with 134a prior to my purchase.

It's a warm one today and the car is currently sitting in the parking lot sun. We will see how it performs this evening.
I did not notice service ports at the compressor manifold while I was under the car yesterday checking over the system. I will look again to make sure.

Speaking of oil. That's one thing I have not yet acquired. Based on a little research I planned to use POE oil. Only because it's compatible with any original R12 oil left behind. The plan is to flush the system, vacuum and recharge with Tom's quantity advice in mind. Although I don't think you can really 100% flush the compressor.

Any suggestions on a quality oil to use? ... or on Porsche ownership life in general? So far it's been a fun ride!

[blueline]

FWIW, I target the pressures you report, to get the lowest practical vent temps. Goal was coldest possible to help recover a black heat-soaked car in L.A. basin. It doesn't offer the best total heat 'removal' capacity, but it does offer the coldest vent temps with the freeze switch disabled.

There are some handy little tools that let you swap the shraeder valves in the service ports with the system still under pressure. Not at all expensive. You can buy this one https://www.amazon.com/R134A-Valve-Remo ... pln=0&th=1 from Jeff B's garage for $20.

On my car there are service ports on the compressor hose manifold plate. These are in addition to the ones in the lines in the engine bay. If your car has those, it's certainly worthwhile upgrading them to R-134a capable valves, especially if you still have the original R12 valves in them. Swap new green o-rings into the cover caps at the same time. The PAG, and to a lesser extent the POE oil, are death on the original seals in those valves. For your regular valves in the engine bay, the original valves need to be out before the port adapters are installed.

Appreciate the info, as always!

I have looked at those tools to change out the valve without loosing charge. Handy little tool that pays for itself in the cost of refrigerant with just one use. I'm certain my car has the screw on adapters over the original R12 valves. I would also bet the system was never properly converted, rather just adapters put on and recharged with 134a prior to my purchase.

It's a warm one today and the car is currently sitting in the parking lot sun. We will see how it performs this evening.
I did not notice service ports at the compressor manifold while I was under the car yesterday checking over the system. I will look again to make sure.

Speaking of oil. That's one thing I have not yet acquired. Based on a little research I planned to use POE oil. Only because it's compatible with any original R12 oil left behind. The plan is to flush the system, vacuum and recharge with Tom's quantity advice in mind. Although I don't think you can really 100% flush the compressor.

Any suggestions on a quality oil to use? ...

or on Porsche ownership life in general? So far it's been a fun ride!

[PSU_Crash]

90+ all sun all day. Turned on the AC right after start. Immediate ice cubes. Kept the fan speed on 2 and full cold. After about 20 minutes I had to drop the fan. My frozen fingers were grateful!

Seems the system is more than capable. I'll try to find the thermometer for a real report.
Hard to play with it when it's currently working so well

[dr bob]

I did the R-134a retrofit almost 30 years ago with POE, mostly because I chose not to do a full system solvent flush. The compressor came out and was sorta-flushed (three drains, three fills with POE, cycling the compressor by hand in each effort), last fill with the oil volume that the WSM proscribed for the pieces changed.

Then charged to the pressures you report. The new dryer, like the original, has a sight glass to monitor for bubbles in the liquid stream at the receiver/dryer. So a relatively few bubbles at 2500RPM, fan on "2", about 80 degrees ambient and SoCal-low humidity. Then 20 PSI suction pressure target while charging.

Running slightly undercharged like this means we worry about maintaining proper oil circulation to lubricate the compressor. Under-oiling very slightly improves condenser performance, but also risks leaving the front seal a little dry. That allows a bit of leakage through the mechanical seal, so not a good idea in the bigger picture. So OK to slightly undercharge the refrigerant, but be sure to add enough oil.

Many seem to slightly (or more...) overcharge the systems, thing if enough is good, more must be better. The dynamics of the refrigerant and system say otherwise. Just barely enough seems to yield the coldest evaporator temps, since the compressor suction pressure is lower when the high-side pressure is lower. It's a little counterintuitive, but the lower suction pressure with just-enough system charge results in the lowest evaporation temps. Max system capacity happens with a full liquid stream at the expansion valve, but coldest evaporation is with lowest charge pressures.

[PSU_Crash]

I did the R-134a retrofit almost 30 years ago with POE, mostly because I chose not to do a full system solvent flush. The compressor came out and was sorta-flushed (three drains, three fills with POE, cycling the compressor by hand in each effort), last fill with the oil volume that the WSM proscribed for the pieces changed.

Then charged to the pressures you report. The new dryer, like the original, has a sight glass to monitor for bubbles in the liquid stream at the receiver/dryer. So a relatively few bubbles at 2500RPM, fan on "2", about 80 degrees ambient and SoCal-low humidity. Then 20 PSI suction pressure target while charging.

Running slightly undercharged like this means we worry about maintaining proper oil circulation to lubricate the compressor. Under-oiling very slightly improves condenser performance, but also risks leaving the front seal a little dry. That allows a bit of leakage through the mechanical seal, so not a good idea in the bigger picture. So OK to slightly undercharge the refrigerant, but be sure to add enough oil.

Many seem to slightly (or more...) overcharge the systems, thing if enough is good, more must be better. The dynamics of the refrigerant and system say otherwise. Just barely enough seems to yield the coldest evaporator temps, since the compressor suction pressure is lower when the high-side pressure is lower. It's a little counterintuitive, but the lower suction pressure with just-enough system charge results in the lowest evaporation temps. Max system capacity happens with a full liquid stream at the expansion valve, but coldest evaporation is with lowest charge pressures.

Thanks for the info Bob! I'll report back once I have time to do a full service on it. Hopefully this weekend.