A DIY Air Conditioning Rehab on My 911

[dkirk]

When I purchased my 1988 Carrera coupe, it had been converted to R-134 refrigerant by the previous owner. The system never worked very well and needed a charge boost every spring. The tell-tale streak of oil on the underside of the engine lid showed evidence that the compressor seal was leaking. All the hoses were original, as well as the front and rear condensers. After owning the car for several years and slowly getting it back into 100% mechanical condition, I finally tackled what I considered the worst job in the whole restoration – the A/C system.

Last winter I removed the original equipment Denso compressor and replaced the shaft seal.



My compressor was fitted with a lip-type seal versus the spring-loaded face seal. A new seal and shaft protector were procured and the installation was quite easy.



I also received a Kuehl center air vent from my son as a birthday present, which was easily installed.



This spring, a complete set of barrier hoses was purchased from Rennaire. All the original factory A/C hoses were removed and replaced with the new equipment – not a pleasant job, taking about 10 – 12 hours for the entire ordeal. It was made somewhat easier using my friends assistance and car lift. The evaporator motor assembly was removed from the smugglers box, disassembled, motor oiled and evaporator fins cleaned. There’s no wonder why previous airflow and cooling were not up to par, as the evaporator was about 1/3 blocked by debris, hair, fuzz, and other ugly stuff. Rinsing in the sink with hot soapy water and a brush had it looking like new in about 15 minutes. The old expansion valve was discarded and a new one fitted. All components were then reassembled, reinstalled, and refrigerant lines hooked up. The last component was the compressor, which was filled with the proper amount of ester-based lubricating oil (4.7 fl oz) before mounting.

We then attached gauges and vacuum pump and let the pump run for 1.5 hours, pulling a 28.0 in Hg vacuum on the system.





During this time a heat gun was applied to the dryer for a few minutes at a time, just to assist in expelling any entrained water (seemed like the logical thing to do).

The bottle of R-134 was placed on a digital scale to measure mass of refrigerant introduced.



Blankets were placed around the engine bay such that a respectable seal would be attained when the lid is closed, and with the engine running, airflow through the rear condenser would be established.



An electric blower was positioned in front of the car to move air through the forward-located condenser.



With all gauges hooked up, the appropriate valves were opened and refrigerant began to flow into the evacuated system. The engine was then started and run at a steady 2000 rpm and the A/C system turned to maximum cool and high blower. A digital thermometer and thermocouple wire were positioned in the center instrument panel vent to monitor air temperature. Here are the results of the test:

Ambient air temp – 80 deg F
Mass of R-134 added – 47 fl oz (per the Bentley manual)
Low side pressure – 29 psi (@ 2000 rpm)
High side pressure – 225 psi (@ 2000 rpm)
Center duct air temp – 32.7 deg F



The recommended (chart) values for pressures at 80 deg F ambient are:

Low side – 40-50 psi
High side – 175-210 psi

I believe these would have been seen had I checked with the engine idling at 850 rpm, but I forgot to do this.

The real test was the drive home – about a 30 mile jaunt at varying highway speeds. Immediately apparent was the colder air at a much improved duct velocity on high blower speed. With the digital thermometer still reading center duct temperatures, air out remained in a 32 – 35 deg F range, depending on engine speed. I then turned the blower to medium speed and the temperature quite rapidly dropped to around 27 deg F! And at one point, I even saw 25.5 degrees at 2800 engine rpm. I continued running in this mode just to see if an evaporator ice-up would occur, but it never did. It actually got cold enough that I dialed back the temperature controller to about mid position, and the air temp followed quite rapidly, settling out at about 40 deg F.

What a nice surprise! I’m totally pleased with the outcome of this work. This stock system is certainly working better than it ever has before and actually cooling the cabin perfectly well, albeit on an 80 degree day. Now being stuck in traffic on a 100+ day would be another story, but getting a 50 degree temperature differential while underway is really quite respectable. I think the performance of this system is perfectly adequate for my upper Midwest location, but the real test will be the wife’s comments. I plan to surprise her this coming weekend.

The car still looks the same on the outside.

[wwest]

You need to check and be sure your compressor is being cycled OFF in the max cooling position and in the appropriate conditions. Those vent temps seem too low for a properly operating thermostatic switch.

Since the Porsche A/C system is always in recirculate mode the atmospheric relative humidity is not as important with regards freezing up. Human metabolism will do the trick it just takes longer.

[GH85Carrera]

Good job. Replacing the hoses is a dirty time consuming job. I did not find it that hard, just a pain to do. A lift would make thing a lot easier.

You will need a long road trip to really test the system. After an hour on the highway on a HOT day with the system at MAX you will know if you have the sensor bulb placed properly. It took me a couple of tries to get it just right.

Good write up. You must have some experience with AC systems.

[dkirk]

Gentlemen,

Thank you for the good feedback and compliments. I was a bit concerned when the compressor didn't seem to cycle when the outlet air temperature was below 32 F. I've been considering installation of a pilot light, located between the A/C panel control knobs, that would be lit when the clutch is engaged.

I'm not an A/C expert but my friend is and he provided the equipment and knowledge. We relied on the Bentley manual which specifies the quantities of refrigerant and lubricant to use, even though this is based on R-12. But I don't think there is any significant difference in the refrigerant properties to alter the quantity of the refrigerant in the system, nor the high / low side pressures when in operation. Is this a correct assumption?

Forgot to mention that the sensor capillary tube went right back into the same spot in the evaporator which it was originally located. The new expansion valve was wrapped in that same black tar-based tape that was used originally. I should have taken more pictures but was getting less than enthused with the job at that point.

[wwest]

If you use a LED for a pilot light don't make Kuehl/Griffith's mistake. Be sure and add a hefty damping diode across the compressor clutch coil.

[sacoffee]

Originally Posted by wwest

If you use a LED for a pilot light don't make Kuehl/Griffith's mistake. Be sure and add a hefty damping diode across the compressor clutch coil.

My Kuehl switch came with diode

[wwest]

Originally Posted by sacoffee

My Kuehl switch came with diode

Yes, he learns by doing, screwing up..., but slowly....

I hope it is a HUGE diode.

Anyone with any real lmowledge would have included a low resistance value POWER resister to put in series with the diode.

[sacoffee]

Originally Posted by wwest

Yes, he learns by doing, screwing up..., but slowly....

I hope it is a HUGE diode.

Anyone with any real lmowledge would have included a low resistance value POWER resister to put in series with the diode.

Site need an AC forum for you to play with yourself

[tmsautoart]

Originally Posted by wwest

Yes, he learns by doing, screwing up..., but slowly....

I hope it is a HUGE diode.

Anyone with any real lmowledge would have included a low resistance value POWER resister to put in series with the diode.

and I'm out....already....

[dicklague]

When do we put in the tri-urinal switch??

[Ronnie's.930]

Originally Posted by sacoffee

Site need an AC forum for you to play with yourself

Originally Posted by dicklague

When do we put in the tri-urinal switch??

Oh WOW - too funny!

I've never know anyone to have such a hard*&@ for someone else, in an auto enthusiast forum, as wwest does for Kuhel!

[Discseven]

Nice work Dave!

[wwest]

Originally Posted by Ronnie's.930

Oh WOW - too funny!

I've never know anyone to have such a hard*&@ for someone else, in an auto enthusiast forum, as wwest does for Kuhel!

Moreso that I DISLIKE marketing of SNAKE OIL ahead of real functional products out of the very same quiver.

[dkirk]

Yesterday we had a good A/C test day here in Wisconsin – an ambient of 88 deg F and 53% relative humidity. I took the 911 out for a 45 minute drive just to see how the revitalized system would work on a day when A/C is a welcomed accessory.

Started by relocating the thermocouple wire in the center dash vent:



Blower speed was put to high and temperature control was set to full cold. The digital thermometer started dropping right away and settled out at about 42 F at a steady 2600 rpm. After full warmup, I noticed that oil temperature was running slightly higher that usual at a steady 60 mph (2400 rpm):



The cabin temperature became comfortable and I turned the fan down to medium speed, as the fan noise is annoying on high. After a few minutes, I recorded the lowest temperature on the thermometer that I saw at this fan speed:



The temperature would drift between this low value up to about 35 F. The cabin remained at an ideal temperature and was comfortable at the medium fan speed. I could not detect the compressor cycling at all. At this vent temperature, I would not be concerned with an evaporator freeze-up. One interesting observation – the faster the engine turns, the lower the temperature drops. The above temperature was recorded going through a speed zone in third gear at 3000 rpm. Turning the engine faster than 3000 rpm produced no lower reading.

Question – is this due to the faster engine fan speed (more airflow through the rear condenser) or due to the higher compressor rpm?

A longer drive in hotter weather would be a better and more realistic test, but I’m quite pleased with the results thus far. To conclude, it seems that the stock A/C system isn’t really all that bad as long as all the details are addressed, such as proper charge mass, clean heat exchangers, and new expansion valve.

Here is what I saw at 60 mph with cold air blowing in my ugly face.