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Air Conditioning on the 911
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Pelican Technical Article:

Air Conditioning on the 911


$0 to $750




Air conditioning pressure gauge

Applicable Models:

Porsche 911 (1965-89)
Porsche 912 (1965-69)
Porsche 930 Turbo (1976-89)

Parts Required:

Freon, or R134 Upgrade kit

Hot Tip:

Replace your system with R134 if you have to spend a lot of money on a new or rebuilt compressor

Performance Gain:

Better cooling during the summer months

Complementary Modification:

Replace your blower motors
101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

The Porsche 911 has never been renowned for its air conditioning systems. At best, most of the systems on the 911 can be described as marginally cool. Part of the problem stems from the fact that the air conditioning systems on the early cars were a dealer-installed option, and never fully adopted by the factory. As it happened, the A/C systems were basically patched into the car, and weren't really well integrated into the 911.

On any car, the A/C system is a complicated beast. This project is not intended to be a repair manual for your A/C system, but to serve more as a guide on how the system works, and the maintenance involved with its upkeep.

Almost all air conditioning systems work on the theories of thermodynamics, whereby heat flows from a warmer surface to a colder one. Heat from inside the car is transferred to the cold metal fins of the evaporator. The refrigerant in the system picks up the heat from the evaporator and takes it to the compressor. The gas is then pressurized which concentrates the heat by raising the temperature of the refrigerant gas. The gas is then sent to the condensers, which are located in the front and rear of the 911. These condensers cool the refrigerant and turn it back into a liquid from a gas. The liquid is then sent to the receiver-dryer, where any water vapor that may have formed in the system is removed. The receiver-dryer also acts as a storage container for unused fluid. From the receiver-dryer, the liquid flows into the expansion valve, which meters it into the evaporator located inside the car. Here the liquid absorbs heat, and becomes a cold low-pressure gas. This evaporation, or boiling of the refrigerant, absorbs heat just like a boiling pot of water absorbs heat from the stove. As heat is absorbed, the evaporator is cooled. A fan blows air through the evaporator and into the cockpit of the car, providing the cooling effect.

The compressor pumps the refrigerant through the entire system. An electromagnetic clutch on the compressor turns the A/C system on and off. In addition to cooling the car, the system also removes water vapor from the ambient air via the cooling process. It is not uncommon to find a small puddle of water underneath your car from the condensation of the air conditioning system. A thermostat control on the evaporator keeps the condensation in the evaporator from freezing.

The late model factory A/C systems from 1978 used a dual condenser system to try to remove more heat from the system. It is important to check that the blower motor in the front of the front trunk is working in order to achieve the maximum cooling from the system.

So what can be done to maintain and protect the system from deterioration? First and foremost, the air conditioning system should be operated at least once a week, if the outside temperature is above 50 degrees Fahrenheit. This will circulate the refrigerant in the system, and help to keep all the seals in the system from drying out. Most failures are caused by refrigerant leaking out of the system and can be prevented by making sure that the system is run frequently.

A belt that runs off of the main crankshaft operates the A/C compressor. Make sure that you don't tighten this belt too tightly, or you may place undue pressure on the bearings inside the compressor. If you think that you might be having problems with your compressor, then check the belt first. Turn on the system, and check to make sure that the electromagnetic clutch is engaging. If not, then you may need to replace it. Check the power connection to make sure it is live before replacing. Sometimes the A/C system will not turn on the compressor if the system is not charged with refrigerant.

Finally, if the rest of the A/C system appears to be functioning correctly, and you are still not getting cold air output, then you might have to replace the compressor. The compressor contains a piston with seals that may deteriorate if the system is not run for a while. Also make sure that when you are running the A/C system you keep the engine lid closed. You can open it to perform pressure tests on the valves, but at all other times keep the lid closed.

On the receiver-dryer, there is what is commonly called a sight-glass. The sight glass is an excellent indicator of the condition of the A/C system. Find the sight glass and clean it. After running the system at maximum for about 5 minutes, increase the rpm to 2000, make sure the engine lid is closed, and check the sight glass. If the glass is completely clear, the system is either fully charged or completely empty. Have an assistant turn the system on and off while you watch the glass. If the system emits cool air, or if bubbles appear when the system is off, then it is fully charged.

A few bubbles in the sight glass mean that the system needs a charge, or the compressor is not functioning properly. Make sure that the clutch is on, and check the sight again. If the bubbles remain, then the system needs to be charged.

If the sight glass is foamy, or if oil streaks appear, then the system is very low on refrigerant and needs to be recharged. If the sight glass is cloudy, then this is an indication that the desiccant in the receiver-dryer is breaking down, and the unit needs to be replaced.

For environmental reasons, it is illegal in some states to perform your own A/C refrigerant recharging with the older R12 refrigerant, but you can check the overall charge (amount of refrigerant) of the entire system. The system's pressure can be checked using a pressure valve that is located on one of the hoses that is attached to the compressor. Before checking any of the valves in the system, make sure that you put on a pair of thick gloves and wear eye protection. If Freon is released from the system, it will rapidly expand, and could cause frostbite if it gets on your hands. On a similar note, the system is pressurized, and the Freon could discharge into your eyes if you are not careful.

There are two test valves on the compressor. These Schrader valves are very similar to the valves that are used to inflate your tire, and the pressure of the system should be checked with an air-conditioning pressure gauge. Make sure that you check the pressure of the system while the engine is on, and the compressor and A/C system is running. Make sure that the clutch is engaged when taking the readings, and the system is set to maximum cooling.

The compressor will have two markings on its housing where the two hoses connect to it. One is marked 'S' for suction, and the other 'D' for discharge. The valve on the suction side should read about 10-30 psi when the system is running. The discharge side should be about 140 psi if the compressor is functioning correctly. Be careful again that you wear hand and eye protection, as the refrigerant can easily give you frostbite.

Take another set of readings when the system is off. If the system gives readings that are equal, and the gauge indicates a temperature/pressure value equal to the outside air, then the system may need a recharging.

If your A/C system needs a major overhaul, this can be a difficult and time consuming process. In order to work on it, the entire system must be evacuated of any Freon. The process of recharging the system from empty is not an easy task either. In order to prevent water vapor and other impurities from entering the system, it needs to have a vacuum drawn on it for about a half an hour. Needless to say, the repair and replacement of most A/C components are beyond the average weekend mechanic.

The original Freon that was used in the older style air conditioned cars is no longer being manufactured. Around the year 1995, auto manufacturers started phasing out the use of the Freon based A/C systems, and started implementing the newer R134 systems. The cost of the replacement freon is skyrocketing as the current supplies disappear. This Freon, which was once sold to the public in do-it-yourself kits, can now only be purchased by dealers who are trained in recharging these systems. If your A/C system needs a major overhaul, it's a wise idea to upgrade to the newer R134 kits available for the 911. Particularly if your compressor is broken, it doesn't make much sense to replace it with a rebuilt original one. Although the R134 system is not as efficient and doesn't cool as well as the original system, the refrigerant can be purchased inexpensively at your local auto parts stores. The typical non-factory installed 911 A/C system uses about 28 oz of R12 freon. The factory A/C systems with both a front and rear condenser uses about 39-41 oz of freon.

The workhorse of the system is the compressor.
Figure 1

The workhorse of the system is the compressor. The white arrow points to the high port side of the compressor. This line comes out of the compressor and goes to the condenser, which is located on the inside of the engine grille. The low part of the system is located directly beneath. The valve for testing the pressure in the system is covered with a small black cap that needs to be removed.

The receiver/dryer is commonly located in the inside rear of the front left fender.
Figure 2

The receiver/dryer is commonly located in the inside rear of the front left fender. There should be a small cap covering the sight glass, which contains a small white ball. This ball helps to tell you whether or not your system is full. When adding refrigerant to the system, simply check the level of the ball. When the ball rises to the top of the glass, you should have more than enough fluid in your system.

The A/C blower motor located in the front trunk is an important part of the system that occasionally fails.
Figure 3

The A/C blower motor located in the front trunk is an important part of the system that occasionally fails. The blower motor helps the front condenser cool the refrigerant in the system by blowing air over it. Check the proper operation of the motor when the system is running, and replace it if necessary.

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Comments and Suggestions:
Bob Comments: Can anyone tell me a good source to have the compressor rebuilt?
August 19, 2015
  Followup from the Pelican Staff: I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
robo333 Comments: where do I find a replacement a/c radiator for a 1973 911T? It is the part fitted right in the front of the car, under the bumpers and has a square mesh grille protector over it. Thanks!
March 27, 2015
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Glenn Comments: I have replaced everything in the system but the compressor, recharge it every year, but still cooling is operable for only a week. Such a great driver but not up to Oklahoma summers. Such
July 12, 2014
  Followup from the Pelican Staff: Sounds like you have a leak. I would monitor system pressure, then evacuate and perform a leak test. - Nick at Pelican Parts  
wwest Comments: Good enough for a Tech Note...?

1988 Carrera Special Edition.

I have long been of the firm belief, and remain so, that refrigerant
leakage in our Porsche's is the result of a design flaw on the part of

Many naysayers will chime in and attempt to convince you that
the problem lies solely with the use of non-barrier hoses. But many
other marques of this era made use of these very same non-barrier
hoses and yet hold their refrigerant charge inatct for decade after

So what, then...?

Inadvertent over-pressurization, high side pressurization well beyond the 350-450 PSI that appears to be an acceptable high end pressure for A/C systems of this era.

Why, how....?

Many systems of this era used a HPRV BOV, a vent to atmosphere
pressure relief valve ***1 should the high side pressure exceed the normal pressure range. In the alternative some systems of this era used a high side pressure sensing switch to disable the compressor clutch once system pressures reached 350 PSI.

Some even used both...

***1. Yes, R-12 VENT-TO-ATMOSPHERE....!!! Even in my 1992 Ford AeroStar...

Why make use of both provided the pressure switch prevents the
compressor from forcing the high side pressure from being

Because there is MORE to the equation.

Suppose the compressor has just driven the pressure to the limit, 350 PSI, but now you switch off the ignition...?? Now you have just
disabled the airflow that was cooling the rear lid and front lip
condensers, keeping the high side pressure "in check", against the
CONSTANT engine/exhaust radiant heating, as it were. Loss of cooling, now NOTHING to compensate, overcome engine/exhaust RADIANT heating of the rear lid condenser, the high side pressure will undoubtedly begin to rise.***2

More to the equation.....engine and exhaust manifold HEAT.

Even without the rea lid condenser being problematic Porsche engineers recognized the problem of engine compartment HEAT in the 964 and 993 series, and yes, even in our 996/997 cars.

With the 964 came a new design aspect that helped to prevent the
engine compartment HEAT from rising too high in certain conditions.
With extended idling, say iwith consistent, prolonged, rush hour stop and go driving, if the engine compartment temperature, convection + radiant, exceeded 70C the cabin heater blower would be powered on to help "wick" the exhaust manifold HEAT away from rising up through the engine cooling fins/vanes.

The new design aspect also included an "after-run" engine compartment cooling capability. If, as you switched off the ignition, the engine compartment temperature was above 30C then a 15 minute time delay circuit was "armed". Now, for the next 15 minutes, if the engine compartment temperature continued to rise and reached 70C the cabin heater blower/fan would be powered. Again, to wick away the exhaust manifold heat and discharge it into the rear wheelwells.

***2. For purposes of this dissertation lets make the following assumption, which is not exactlly a rare situation, circumstance.

Assume that as we switch off the ignition the A/C evaporator core is already at ~33F, thus the TXV is mostly closed, the R/D is filled with seevral ounces of liquid refrigerant.

And now with no air being circulated through the evaporator cooling fins/vanes it might take many minutes for the R/D's
liquid refrigerant to be "exhausted" and then for the high side and
low side to begin equalize. More than 20 minutes according to Charlie at Griffiths.

Sidebar: Google..

Bob Tindel cooling

Solution for leaking non-barrier hoses.

Tape and insulate a thermistor to the metal pipe as it enters the rear lid condenser. Use the thermistor value to discern if/when the high side pressure exceeds 300 PSI. Once 300 PSI is sensed then power the cabin heater blower/fan. Should the sensor indicate 350 PSI disable the compressor clutch circuit. Ignition on or off, POWER the cabin heater blower/fan as long as, whenever, the sensor value indicates 300 PSI.

This may not only prevent the loss of refrigerant on a 1 or 2 year
cycle, it might well raise the overall A/C system performance to a more adequate level.

With the engine idling at ~800 RPM I meaured ~100 Ft/m airflow at the center/top of the rear grille, with the cabin heater blower powered it went to ~200 Ft/m. Single point measurement, no attempt to calculate overall inlet airflow.

Thermistor alternative: The EPA requires the use of a high side pressure switch when converting an R-12 refrigerant A/C system to the use of R134a. This is because R-134a, absent some sort of control, wil operate with pressures well above your system's design specifications. I recommend using the Red Dot trinary pressure switch. First, because it will limit the operating high side pressure within the limits of R12***3.

Second, the 3rd function of the Red Dot trinary pressure switch can be used to control the cabin heater fan. And the front lip condenser fan if you wish.

***3. An R-134a trinary or binary pressure switch will allow the high side pressure to rise ABOVE the system design specifications whereas the Red Dot switch will not.
August 6, 2013
  Followup from the Pelican Staff: Thanks for the additional Info and the time it took to put this together. We appreciate it. - Nick at Pelican Parts  
Kurt Comments: I am attempting to install an A/C system in a '78 gray market SC. Can anyone tell me the layout of the wiring harness for this endeavor and would anyone have access to or know of a source for the original dealer installation instructions? Any light shed will be greatly appreciated.
July 16, 2013
  Followup from the Pelican Staff: I opened a post in our forums. A Pelican community member may be able to answer your question.

- Nick at Pelican Parts
pit Comments: AC Blower runs well but only when directly connected to battery...same happens with cool air blower motor...fuses are ok...how can I see the wiring diagram for my 911 1987 Carrera...what might be the problem?
September 7, 2012
  Followup from the Pelican Staff: You either have a bad blower motor switch or more likely a bad blower motor resistor. This resistor drops the current to the motor so you can change the motors speed. Try replacing that first - Nick at Pelican Parts  

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