"The A/C on my '88 S4 is putting out about 50% of cold capacity in both the front and rear (seat) systems. I have the necessary equipment to charge it (gages and tank of R-12) but it's been too long since I've done it. Is there a web site that shows the high and low pressure connecting points along with the procedure for using the recharging equipment?"

A good resource is www.aircondition.com , but you won't find specific car data there. I'm sure it's available elsewhere, but haven't had to look since I have the workshop manuals. Since I just checked mine (88 S4), here is what I remember:

High pressure fitting is on the drier which is starboard side under the hood, just in front of the condenser. You will find the connector cap readily accessible and facing up.

Low pressure fitting is right behind the high side fitting on the other side (toward firewall) of the condenser/radiator. You will have to move the upper radiator hose and possibly a water pump hose to get to the fitting, which should obviously also be capped.

After hooking up, you should bleed a little gas from both lines up at the gauge head to prevent air from entering the system. Start engine and run A/C. You probably know the drill by now. You should look for something between 225 and 300 psi on the high side. I know that's a wide margin, but there are so many variables that influence the readings you will get. Low side should definitely be below 35 or 40, but not below 15.

I usually watch the sight glass in the drier and check the cabin expansion valve (driver side between windshield and fire wall under the plastic cowl cover) for sweat. Sweat is a good thing. If you need to charge, hook up and bleed the connection hose to your tank and crack open only the low side (suction) on your gauge.

The only proper way to do this job is to evacuate the system and refill with the appropriate amount of Freon. I normally don't do that, but just beware doing this by gauge makes it impossible to know exactly how much Freon is in the system and makes it easy to under or over fill.

Hope that helps.

JP Rodkey


Subject: A/C Oil Capacity Nippondenso 10PA20C
From: barryorlando@hotmail.com
Date: Wed, 14 Jul 2004 22:02:15 -0400
X-Message-Number: 28

I have some conflicting information about how much oil to add to the A/C compressor. Anyone know what’s the correct amount of oil to add?

Won’t excessive oil accumulate in the drier and result in a lower refrigerant capacity? System has been flushed, resealed, and is ready for vacuum and recharge.

I have a few references, including the Factory Work Shop manual, a Factory Technical Specifications booklet, and the Haynes A/C Techbook.

Shop manual says 4 oz. (120 c.c.)
Technical Specs says 9.3 oz. (280 c.c.)
Haynes Techbook says 9.3 oz. (280 c.c.)

Which is correct?

The compressor on the car (1990 S4 w/rear A/C) is a Nippondenso 10PA20C.
The earlier models used at 7E171 with 9.3 oz of oil, but the later models, including 93’ with R-134a, used the 10PA20C.

Since the 7E171 has a square bottom and the 10PA20C has a rounded body, I’d think the amount of refrigerant oil is much less for the 10PA20C. Is this true?

The factory work shop manual lists for the 10PA20C with R-134a an oil capacity of 160 c.c. (5.3 oz.). If the work shop manual is correct at 120 cc for R-12, does 160 c.c. seems reasonable for R-134a on the same model car and compressor type?

Here's some of my reference notes on oil capacity:

- Porsche Technical Specifications, 90-91 S4/GT, 92-93 GTS, says:
Page 74, air conditioning, refrigerant oil in compressor: 280 +- 20 cm3 Densoil 6

- Porsche Work Shop Manual, Volume 4, page 87-90 says:
Refrigerant oil in Compressor: Type 6E171 280 +- 20 c.c. Densoil 6 Type 10PA20C 120 +- 20 c.c. Densoil 6

- Page 87-129 says:
As of MY ‘93, For R134a, Compressor type 10PA20C, Refrigerant oil in
compressor: 160 +- 20 c.c. Densoil 8

Haynes A/C Techbook,
Refrigerant oil capacity tables, 1990 Porsche 928: 9.3 oz

When I removed the compressor from my 90' (with rear A/C), I only drained out about 2 oz of mineral oil from the compressor gravity drain. I'd guess that an additional 1 oz was trapped inside.

If you have any leaks, which is probably why your servicing the A/C, you really don't know how much mineral oil was left in the system from the orignial fill. So, the only way to be sure is the do a complete flush of the system, then fill the specified amount.

Last year on mine, I replace all the o-rings, hoses, filter/drier, performed flushes, then refilled the system with mineral oil, pulled a vacuum of 600 microns, and refilled the system with R12.

Just wondering if you have pulled the cover off the rear a/c to see if the hoses have leaks (oil residue)?

90’ S4, Slate Metallic

Barry and Kevin,

I tripped over all of the misleading and inaccurate info in the shop manual and tech spec books when I started doing 928 AC repairs. I will try to clear it up.

1. I believe you both missed page 87-103 in the shop manual. It is no wonder, it is so poorly organized. On a totally new system with no oil, the compressor is fitted with the total oil capacity of the complete system.The compressor will distribute the oil through the system and you end up with 40% in the compressor, 35% evaporator, 15% condenser, 10% receiver/dryer. If you replace any component, add oil to the component.

2. Page 87-90 says "Refrigerant in compressor". That should read Refrigerant OIL in compressor. Or better yet, Total system oil capacity.

3. Page 87-90 Type 10 PA 20 C 120 cc or 4.23 oz.
Page 87-129Type 10 PA 20 C 160 cc or 5.6 oz I think they discovered that 4 oz was not enough and changed the spec or possibly the difference between R12 and R134a.
I would use 5.6 oz total system capacity.

That means you end up with 2.24 oz in the compressor.

Kevin, If you did not flush any oil out of the system and only replaced the compressor, then you added too much oil. If you replaced the receiver/dryer at the same time without adding oil to it, you may be OK.
Did you measure the oil removed from the compressor?
Kevin, If your '86 has the original 6E171 compressor then the system has
9.5 oz of oil, so ~4 oz in the compressor is correct.

4. It appears that the '90 and later tech spec book is wrong and quotes the 6E171 numbers.
The back of my book says "1st edition - Subject to errors and changes". CYA?

Keep in mind that many factors affect the amount of oil that you drain from a compressor, mainly affected by how the compressor was operating just before shutdown. 1 oz could be correct. I drained 1 oz out of a
6E171 that had a bad shaft seal but was mechanically perfect. Usually 6E171s have about 3 oz in them.

I have no 10 PA experience. My '91GT had no oil in it and was totally destroyed. I installed a 6E171 that I had.

Hope that clears up some of the mystery.

BTW, I do NOT recommend converting R12 systems to R134a.
I have started converting them back to R12.

If you have a '93 or later that came from the factory with R134a then stay with it but be sure to use ND8 oil. There are many different PAG oils and the are not compatible.

Earl Gillstrom


Barry said:
On the flushing with mineral spirits. I saw a thread on one of the a/c
boards that mentioned the low vapor pressure of mineral spirits prevents it from boiling off during evacuation using standard commercial a/c vacuum pumps that typically  only pump down to 20 microns. At 20 microns, water is easily boiled off, so I see no reason not to use dry air to blow down the flushing solvent. I used a $20 filter/drier that I bought at HFT and a 11 gallon air tank that I filled up at the local gas station.

There are several reasons that air is not used: air, moisture and compressor oil in the air. That is why nitrogen is used.

Plus, I noticed the drier only comes shipped with loosely fit caps; they must expect it to dry during evacuation.
If the drier has loose caps, don't install it, get one with tight caps.
Here's the thread that I read:

That 2000 rpm for 10 minutes, I interpret as a way to stabilize the system. Since it says "after", I assume the readings are at idle. The curves are dated in the WSM, "1986", but I believe that besides the compressor, the other components are the same, so I'd expect the curves to be the same for cars thru 1991 using R-12.

I interpret it to mean at 2000 since all other manufactures call for "high idle" or 1500 or 2000.

Greg said:
OK, no need to replace the expansion valves, drier, O-rings?

It depends on how it was converted to r134a. If the o-rings and expansion valve were replaced and there are no leaks, then they should be OK. The drier should always be replaced when the system is opened. If changing refrigerants the drier MUST be replaced

Jay said:
It is possible that the system may have the Schraeder valves on it still. Most conversions I see including my own have an adapter in the system that screws onto the Schraeder valve. For those unfamiliar with that name it is the same exact fitting that is on your tire to put air in.

I should have said Schraeder valve CORES. The core is removed from the
R12 fittings and the cores in the r134a fittings are used. When you remove the r134a fittings, you must reinstall the R12 valve cores.

When Earl says never recycle R12 what I think he means is never put Reclaimed R12 in your system. Please do go to a shop and have your Rxx removed before doing any work. If a shop knows you and knows you are going to do your own work they should evacuate and reclaim whatever is in there. Take the belt off when you undo the system so you don't run your compressor dry. Then do the work and replace whatever you need to replace if you are doing a rebuild and then go back and have the system re-oiled and recharged. Most times if you are going to release the pressure in your AC system there are things that can be done like replacing o-rings and such. Take the opportunity to do those simple things.


Watching Louie Ott go up in flames was an eye-opener for a lot of us. The discussion that ensued was also an eye-opener. I did not know that the
R134 derivatives were combustible!!! Most likely I will be going back to
R12 as soon as is practical although my system is working pretty well on R134a it didn't keep up in 101 degree Wichita with it's high humidity.

Louie was using "Enviro-Safe". It is mostly or all propane. It is probably safe for the environment, but seems deadly to cars and people.
The MSDS says: " do not try to put the fire out unless you can stop the leak". Us dummies didn't know that and used 6 fire extinguishers. Maybe we were lucky we didn't get killed.

Jay said:
Re: expansion valves, o-rings, drier, my Audi guy has been changing systems over to R134a for a bunch of years without doing anything to the system. There have been no issues whatsoever. Old hoses that have been under pressure with R12 for a long time tend to get better sealed. The older barrier hoses and the R12 blend somehow eventually infuse and plug the permeation path. These old systems seem to hold better than new OEM hoses until the hoses get old and crack. So it is worth a shot in my opinion to just recharge older systems. However there is a special oil that is cross compatible with both R134a and R12 so you don't have a chemical reaction and leaving systems open to air might corrode the gas paths internally. If you are going to store a used compressor or condenser put it in a sealed bag and put some compatible oil in it.
Flushing with nitrogen and compatible oil is a luxury and correct but not practical for the home wrench.

79 (back from the big round trip)

No comment.

Greg said:
Thanks Jay, I'm in no rush to convert back, though I may do so at some point. I did the conversion to R134a a few years back, will be nice if it does not require replacing all those O-rings again. I have a slight leak, I believe from the center seam of the compressor (occasional oil drip), and there is a split in the outer sheathing of one of the hoses that goes to the compressor, but that is not leaking and i put a few tie wraps around it and it has not gotten worse.
The system works OK, it's cool but not cold.

You should not have to replace the o-rings again. The compressor can be resealed.

Bill said:
I think what Louie had was R12a, not R134a. R12a is basically propane.
R134a is not that combustible but the R12a is barbecue gas, as you saw.

See http://befreetech.com/esmsds.htm

Envirosafe is "auto combustible at 1585 F". r134a is ~1300 F.

Jay said:
I don't think what Louie had was R12a either. Think more Bic lighter juice blended with something to make it even more dangerous I can't remember what he called it Ecotane or something like that... maybe refrigablast? I know a bunch of volunteer fireman skipatrollers. Their attitude toward bottled gasses is to just back away quickly. If a trailer park is already involved or a motor home or something they sorta just wait for the explosion so they know the fuel is exhausted before going in.

See above.

Jay said:
It was subsequent discussions following Louie's episode that went in the direction of R134a being combustible as well. I trust Cap'n Earl to not offer that tidbit unless he was sure. Earl is as adamant about not using anything other than R12 as I am about not using Teflon brake hoses on the street. Just bad juju. Would be nice to hear from an air conditioning professional about new alternates that are on the market.
Carrier is touting a new refrigerant called Puron which is Chlorine free and is supposed to be better than the residential R22...
Must be a similar grass roots campaign going on in the automotive world by now...

There are 15 or 20 "drop in" replacements for R12. It has been proven that none of them can adequately replace R12. Most can't adequately transport oil in the system, some are flammable. Some or all cost more than r134a and NO professional AC shop will service the system. If you have to, convert to r134a.

If you have to go away from R12, then use r134a.
If your car came from the factory with r134a then stick with it.
If your car came with R12, the stick with it.


Cost of freon is NOT a reason to switch. The cost of R12 has settled to ~$15 lb wholesale.
The freedom from problems and colder vent temperatures will justify the slight cost difference.

There is one thing that you can do to absolutely DESTROY YOUR AC SYSTEM.
Install sealer.
Then when it fails, you can replace the COMPLETE system to fix it. Also, ALL professional AC shops have sealant identifiers. That is the first thing they check. If you have sealant in your system, they send you down the road rather than risk damage to their $5,000 AC service system. DID I SAY NEVER NEVER PUT SEALER IN YOUR AC SYSTEM?

Put sealer in your tires, then when they leak, you only piss off the tire jock.

Greg said:
I knew I had a technical bulletin covering A/C conversion; FWIW here's what bulletin 9501 "Retrofitting R12 A/C System to Refrigerant R134"
has to say:

Under "928 S4, 928 GT - Model Years 1987 to 1989":

| ND-Oil 8 Refrigerant Oil
| Capacity 300-320 ml (270 ml for R12)
| Refrigerant R134a
| Capacity 860 g, 1030 g with rear AC

So I guess it's the same with or without rear A/C.

I doubt it but don't know how to find out.
That bulletin was written before the hazards of retrofitting to r134a and using PAG oil were known.
Pag oil is used in new cars. Ester oil is used in retrofits to r134a.(ND8 oil is PAG)
I have not put all of my reasons for my convictions in this email. Some day I will document them when I have time.

Earl Gillstrom '91GT


Hi all,
Since its almost summer its time to use the AC more frequently, trying to keep cool.
I noticed the AC runs ok, works ok, or just maybe less efficient than last year. Checking things, I saw the low-pressure return line had a small leak. When the engine is hot, I even see small bubbles coming out from the rubber-metal coupling. That seems like a leak to me.
Maybe I can have the rubber replaced without ordering a new Porsche part.

The car has R12 in, and I was not planning to replace the R12. Am I stuck now, and need to swallow the bill of retrofitting R134a? How bad are the effects of downgrading to R134a in real life experience? Its said that R134a leaks out pretty quickly where R12 tends to stay in a lot longer due to larger molecule structure... Any experience on alternatives like R431 (I think Graham has it in the 928GT)?

1988 928s4 cherry red
The Netherlands

> The car has R12 in, and I was not planning to replace the R12. Am I
> stuck now, and need to swallow the bill of retrofitting R134a?

I don't know what the pricing structures of refrigerants are in your part of the world, but here in the US, the price of 134a has jumped dramatically, not much of a savings over R-12.

How bad
> are the effects of downgrading to R134a in real life experience? Its said that R134a leaks out pretty quickly where R12 tends to stay in a
> lot longer due to larger molecule structure...

On a retrofit, 134a will cool ok, but not as well as R12. Newer cars (from 1994/95 on I think) with factory 134a work great because of other changes to the technology, condenser, etc.

Some compressor seals are not meant for 134a, leaks out quickly. Several years ago I purchased a "Genuine Porsche" compressor (stupid, stupid, stupid, bigtime stupid) and changed to 134a at the time, a few weeks later, no more cooling, refrigerant had leaked out at the seal. Sent it back under warranty, and it was replaced, along with the warning that if I used 134a, there would be no warranty. So then I switched back to R12. The compressor was under warranty, but all the shipping, R12/134a/R12 changeover, refrigerant, labor, etc all came out of my pocket, a very costly affair, one I will not soon forget.

> experience on
> alternatives like R431

Don't know what that one is, but the
propane/hydrocarbon inspired alternatives, while costing less, are an accident waiting to happen.

You don't put the cheapest, non-approved experimental tires and brakes on your car, why would you want to do that with refrigerant? If you do, please send us "before" and "after" pictures...


I am converting the A/C on my 90GT (with rear A/C) from R-12 to R-134a. I found a very useful step by step on doing this at http://members.rennlist.com/pirtle/svc_ac.html

In the procedure is says "Pull the compressor out, turn it over and empty the oil. Refill with ounces of the new oil"

In the shop manual it says the whole system should have 12 oz of oil and 8oz of that should be in the compressor. Should I put 4oz or 8oz of oil
in the compressor?

Dan Scheper
90GT - Maui

There is a difference between a "used" and new compressor. If you use a new compressor that never has had oil in it, I'd put 8 oz of oil. For a used compressor, all of the oil doesn't drain out.

I suggest that for a used compressor, you measure the amount of oil that comes out and use that as the guide for how much of the "new" oil you use for replacement.

'93 GTS
'87 S4
'79 Euro track beast


How to successfully recharge the A/C of the Porsche 911 that use rotary compressors such as SANDEN or similar and R134A gas.

The procedure is based on using the following equipment from Interdynamics, Brooklyn NY 11232.
1) Interdaynamics R-134a Service kit that comes with dispenser device and lowside pressure gauge.
2) A 12 oz can of R-134A/oil charge can (contains 2oz of oil)
3) A 12 oz can of R-134a charge can
4) A/C test thermometer

Primis charge cans can also be used ( http://www.iqproducts.com )

A/C pressure reading check

1) locate service low pressure service port on the compressor. The low pressure service port is usually identified by a blue cap. If it cannot be identified by the cap color, it can be identified by tracing the low pressure line that connects the evaporator to the compressor.

2) start the engine and set A/C dashboard controls to max cooling.

3) remove blue cap from service port. Connect the pressure gauge to the low pressure service port through the connecting hose. If the gauge reading is below 25 psi the A/C needs to be charged.

A/C charge procedure

4) Attach the dispensing device to the threaded area on top of the can containing the recharging gas.

5) Attach the dispensing device to the compressor’s low pressure service port through the dispensing hose.

6) Hold the can in upright position and start opening the valve of the dispensing device.

7) When charging procedure is completed, close the valve of the dispensing device by turning the handle in clockwise position till stopped.

8) Remove charging valve and hose from can empty and discard.

After the charge procedure, the A/C should be checked again for the operative pressure (steps 1 to 3). If the operative pressure is not reached the A/C charge procedure should be repeated.

The A/C is fully loaded when:
1) the gauge reading shows a pressure between 25 and 45 psi.
2) the dryer shows the floating ball centered in the reading window.

A/C operative temperature check.

Place a thermometer on the main vent opening in the car dashboard. Turn on the A/C at max cooling.
After 5 minutes the thermometer reading should be between 45 and 60 F.

Charging tips:

It is important to not overcharge the A./C system. Refrigerant should not be added when the gauge reading is more than 45 psi.
The all capacity of the A/C is about 25 oz therefore no more than 2cans should be used.


>As of yesterday my 928's air conditioning was operating correctly, except a lack of refrigerant made it nearly useless. Just as I started  adding refrigerant this evening, the compressor stopped running!! It confused the hell out of me for quite some time--if I disconnect the wire going to the compressor clutch, I see +12V on it, but when I hook it up, I see zero voltage at that junction. What's up with that? Is there a big resistance in the pressure switch or something such that a totally shorted-out compressor clutch will cause this behavior, rather than blowing a fuse?
>Please CC responses to joseph.elliott@boeing.com  so I can distract myself with this at work tomorrow.
>Thanks again,
>Joe Elliott

One possibility is that you slugged the compressor - that is, you may have let liquid refrigerant get into the compressor, rather than the required gaseous form. This will stop the compressor, and the clutch may be slipping. You can check that by seeing if you can turn the compressor compressor clutch center by hand (engine off, obviously!).

Another possibility is that the little blue relay in the A/C control head has gotten burned, and is letting 12 vdc thru, but not enough Amps to pull the clutch in. You can use a jumper wire to hook the wire to the compressor directly to the jump start terminal, and see if the compressor works.

If the compressor works with the jumper wire, the relay is the first suspect. If the compressor turns by hand, but still doesn't work with the jumper wire, either the clutch or the ground is likely to be faulty.

Of course, since you are carrying the car to Mo anyway, he can probably help...

Wally Plumley
928 Specialists

High side pressures are greatly dependent upon the ambient temperature, the efficiency of the condensor, and the air flow over the condensor, and may be anywhere from 150 psi to 300 psi. With good airflow, I would expect to see 175 - 250 psi at 1500 rpm.

Low side pressures at 1500 rpm should be around 30 psi, but will vary up and down a bit from there.

Higher than normal pressures on both sides may indicate air in the system or overcharging.

Wally Plumley
928 Specialists


At 11:59 AM 9/21/2005, Bob Foster wrote:
>I have a container of R-12 as well as the recharging hoses, valves,
>gage mechanism but it's been too long since I have been through this
>process, consequently, I would greatly appreciate guidance on the topic.

Wear eye protection and be careful! You can freeze your skin or eyes with Freon. Never open the high side (red) valve with the tank attached and the engine running.

>The car is my '88 S4 auto which has some R-12 in the system but has
>been depleted enough that it requires 'topping off'. Bubbles are
>visible in the receiver sight glass. My questions are:
>1. Where are the Schrader valves to which I connect the high and low
>pressure hoses?

High side (red) is on the receiver dryer, between the condensor and the front bumper.
Low side (blue) is under the left-hand coil - up under the radiator hose, and a pain (literally) to get to. Long sleeves will help with the heat...

>2. At what point do I start the engine and run it at what RPM?
>3. When do I open the valve on the R-12 canister?
>4. When and which valves do I open/close on the hose, gage, valve mechanism?

Close both valves on the gauges.
Attach all three hoses:
- Yellow or White to the refrigerant tank.
- Red to the high side.
- Blue to the low side.
Open the tank valve.
Crack (slightly open) the high side valve (red). Slowly and carefully loosen the high side (red) hose at the receiver/dryer slightly until you get a small amount of refrigerant leakage from the hose fitting. Quickly tighten the hose fitting. Close the high side valve. This removes any air from the hose. Be certain to close the high side valve!
Repeat with the low side (blue). Be certain to close the low side valve.
Crank the engine. In hot conditions, a fan on the condensor helps a great deal.
Turn the A/C on, temp lever at the coldest setting, fan on high speed. If you have rear air, turn it on, with the fan at high speed.
Open the low side valve slightly, and slowly add refrigerant thru the low side port. Never open the high side with the engine running! Watch the sight glass - a flashlight helps. When the stream of foam disappears, add 8
- 12 more ounces of R-12.
The high side pressures will depend greatly upon the ambient temp and the airflow over the condensor, usually somewhere around175 psig to 275 psig at 1500 rpm. The low side pressures should be somewhere near 30 psig at 1500 rpm.
Close the low side valve. Close the tank valve. Shut off the engine.
Remove the low side hose from the port. You will burn your arm. There will be some spray of refrigerant - avoid letting this freeze your skin. Install the port cap. There must be a cap on the port.
Remove the high side hose from the port. There will be more spray of refrigerant. This is normal - it is the refrigerant trapped in the hose at high pressure.

>Any other suggestions will be appreciated.

You can not properly charge the A/C with this method, as you can not get the exact charge that is needed - the best that you can do is a guess.

The proper way to do it is to use a recovery system to empty and pump down the system to a good vacuum, then recharge by weight.

Good luck!

Wally Plumley
928 Specialists