Air Conditioning, '88 S4 need infomation!

12 Jul 2016 08:32 #19129 by Balz
I recently acquired a low mileage '88 S4, which is good. AC not blowing cold, which is not so good. A slurry of bubble in the sight glass. Compressor turning, evaporator fan blowing, Using a set of recently acquired R-12 gages, measuring: Low side pressure is high. High side pressure is low. No change in pressure readings from compressor-on to compressor-off. Do I have a bad compressor? It is still an R-12 system. Holds pressure in the system. Any recommendations? Anyone done the full changeover to R-134a? Any good resources/technical documents? Do I need to get an R134a-specific compressor, or will a direct replacement work with 134a? It does not have the rear ac unit. How much R-134a will it take and how much oil. I am assuming new compressor and new dryer. How challenging is it to get to all the seals and do this swap-over? (scale of 1-10? 1=tire pressure check, 10=engine rebuild) Photos attached. the one with the really high low-side pressure is when I tried adding R-12 (not so smart).

Sure could use some advice. I am in Northeast Florida. It's about a zillion degrees and 100% humidity. Black car, black interior. If anyone needs the AC, I do! I am a long time 911 owner and added the 928 to the collection. It's a beauty, but is is hot as .....

This message has attachments images.
Please log in or register to see it.

Please Log in or Create an account to join the conversation.

12 Jul 2016 15:30 - 12 Jul 2016 15:40 #19132 by davek9
Humm lots of question here, don't know where to start.
Good news is the system is still holding some pressure, however not enough and most likely has moisture is in it.

Yes you can get an AC shop to vacuum down the system and recharge w/ the correct version of 134a that's compatible w/ old R12 systems, I've done it and it works well, this is the easiest way to get your AC going and to source any leaks too.

If you real really want to do it all correctly, you will need to replace the dryer, the two main rubber hoses at the compressor, all the seals and hopefully the compressor doesn't have a leak or is not worn out, you also should replace the valve as it will be disconnected anyway while doing the seals.
Compressors, (rebuilt units) run about $225, so if its in your budget just do it along w/ the other new parts and be done w/ it.

As diagnosing your Pressure issues, the reading all depend on the outside Temp at the time they are taken, the systems quantities are all available to any AC shop and or searchable on-line and will vary slightly if you have rear air. On a 1 to 10, 10 being the hardest to replace the seals I'd give this one a 7 and I've done several 928 engines ;)

Edit: FYI... the Low pressure fitting is located just below the upper Radiator hose along the Right Fender just ahead of the 14 Pin connector.

Good luck,
Dave K
Last edit: 12 Jul 2016 15:40 by davek9.
The following user(s) said Thank You: Balz

Please Log in or Create an account to join the conversation.

24 Jul 2016 16:29 #19179 by Frank Mullozzi
1st, make sure you have the gauges on correctly. Red hose on high side by the drier. Blue on the low side by the battery charge (jump start?) terminal on passenger side fenderwall. Normal high side running pressures w/R12 vary from 150-300 psi depending on outside temp. 300 psi is the compressor cut off pressure by means of the high/low pressure switch located next to the drier (that round black thing about 3"x6" located near the passenger headlight front of the condenser radiator. Low side gauge should read 32-40 degrees. This is assuming the compressor is running. Not running, pressure should be equal on each gauge. At an outside temp of 90 degrees, a properly working R12 system should discharge air at the center dash vent somewhere around 40 degrees or cooler. Varies with air temps, but this is a thumbnail guide to operation.
When you turn on the A/C, temp set to coldest, fan high, discharge on up/down. Motor running, press A/C button in. You should hear a loud click, the electric fans come on and the engine rpm should dip than return to normal idle. Yes some of this may sound simple, but never assume anyone's knowledge level.
The compressor center of the pulley should be turning. If not, the pulley clutch may be the problem. There may be an electrical problem between the dash A/C switch and the compressor pulley clutch. The electrical flow is from the dash switch thru the freeze switch thru the high/low pressure switch to the compressor clutch to earth ground. Use a meter and check the freeze switch located under the removable panel where the wiper linkages are. Freeze switch can be temporarily bypassed for test purposes if you don't read 12-14 volts DC on each side measured to earth ground (where all those brown wires are attached to the body).Freeze switch has one wire that's purple in color. If you have power going thru this switch, move on to the high/low switch. High/low switch as a rubber connector that pulls off (pull on connector, not wires). You should have power on one of the wires. There should by continuity across the switch ( Ohm reading on the meter should be 0). It may be necessary to make 2 wire leads at least 4" long with male and female spade connectors on each end to be able to attach the meter. If you see an open or infinite Ohm reading on either switch, the switch may be bad or in the case of the high/ low switch. no refrigerant in the system. Freeze switch protects the evaporator (under dash/fan part) from ice buildup and damage. High/low switch protect system from bursting and compressor from under lubricating. Check the wire going to the compress for a connector possibly located in front of the passenger side engine timing belt cover. This should be where the compressor power lead should run.
There should be power at the compressor lead. WATCH THE FANS !!!!!!!!!!! They run constantly while the A/C is on. If the fans don't come on and you don't have power to the compressor, there may be a problem in the dash control unit. Bad relay (Porsche undersized the electrical relay component) or a printed circuit board trace failure. Mine had both and a trashed compressor. There are posts on how to repair these problems. If you have a leak, oil will leak out with the usual indication of dirt accumulation on the leak area. Continuing to add gas without oil will starve the compressor and it will become a nice compact boat anchor.
R12 and R134 use different oils that aren't too compatible. Switching refrigerants can be done, but without the proper methods, I wouldn't recommend it. Refrigerant carries the oil thru the system to lubricate the compressor. No gas, no oil= locked up trashed compressor. If you switch to R134, the system should be flushed with R11 to clean out the old oil. Some oil tends to sit at the bottom of the condenser coil (radiator thingy out front of the regular radiator). Yes R12 is ridiculously $$$$$. But not impossible to obtain. Also check online about certification requirements for handling refrigerant.
Given the grief involved in compressor replacement, I would suggest a new unit. O rings are green in color. Drier must be replaced, system evacuated, pressurized (usually with Nitrogen) and checked for leaks. Evacuated and properly charged with refrigerant (measured in grams/ounces) . Any time the system is opened, the possibility for water moisture entering exists. Water will freeze in the expansion vale and block the operation. Also cause corrosion, and the compressor is only designed to pump gas, liquid refrigerant in the compressor will brake the valves. Air is also not a good thing to have in the system. Drier absorbs moisture and filter any debris that may be floating around the system.
Oh, if the compressor pulley clutch has given up the ghost, this would indicate an electrical open or infinite Ohm reading, you will need to replace the entire compressor. The few suppliers I've found charge as much as a new compressor and clutch package. Plus they are a huge huge pain to replace.
Most problems are not impossible to repair, just require the right tools. Multimeter (even cheap will work), gauges, thermometer, scale to weigh refrigerant, vacuum pump or charging equipment, money is helpful and an abundant supply in keeping with 928's needs is important.
It is illegal to discharge refrigerants into the atmosphere. Don't touch the car body while measuring electricity (12 VDC can really hurt in the correct circumstances). Goes with the watch your hands, scarves and shirtsleeves around rotating things disclaimer/ warning. Refridge Oil is nasty in your eyes. Not too great on skin either.Use protection. Remember- HIGH PRESSURE SYSTEM, PROPERLY remove old refrigerant before opening. Enough with the warnings.
Good luck, check electrical first then refrigerant side.

1990 GT
30+ years Commercial/ Industrial Heating A/C Electronic controls, safety devices and systems
R11, R12, R22, Lithium Bromide, Largest system 22,000 pounds R22 (yes thousands), 350 and 700 HP compressors. 1,000,000+ sq ft of conditioned space.
Lots more, but this is not a resume'.

Please Log in or Create an account to join the conversation.

25 Jul 2016 14:07 - 25 Jul 2016 14:10 #19191 by billvvOC
I'd stick with the R-12 until you can replace all of the O-rings and flush the system.

Hi and Low pressures should be equal with the compressor not running.

Low High-side pressure and high suction pressure can indicate a poorly functioning compressor.

Here's info from the Climate Control folder in the CD Set (Roger at 928Rus has them for sale)

1 bar = 14.5 psi

This message has attachments images.
Please log in or register to see it.

Last edit: 25 Jul 2016 14:10 by billvvOC.

Please Log in or Create an account to join the conversation.

18 Aug 2016 14:09 #19256 by Balz
Frank, great detailed information. think i will start diagnosis from scratch using your protocol. i will make no assumptions until verified with data. am prepared to replace hoses, drier, compressor. hope not to need switches, expansion valve, condenser or evaporator.

22 k lb system sounds interesting. thanks a "ton" as they say in HVAC class. very helpful.

Please Log in or Create an account to join the conversation.

18 Aug 2016 15:07 - 18 Aug 2016 15:10 #19257 by billvvOC
I replaced all the O-rings, plus dryer, expansion valves and rear evaporator. I'd say this is a 3-4 in difficulty. Much easier than timing belt, torque tube bearings. Probably on a par with changing the HVAC vacuum system diaphragms. Hardest part was finding the correct O-rings.

Your post-fill pressures indicate a bad compressor. It can't compress enough gas to make liquid, so it pumps gas, with the difference between hi and lo side being the pressure drops in the system. At least that's my take.

The hard part with the 132a changeover is making sure all the old oil is out. 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. You'll have to flush the evaporator and condenser.

Refrigerating oil qty
6E171: 280cc (9.46 oz)
10PA20C: 120cc (4.36) oz

Refrigerant R12
950g or 2.094 lbs
1150g or 2.54 lbs with addit. evaporator in rear compartment

Here's an O-ring diagram I made....

This message has an attachment file.
Please log in or register to see it.

Last edit: 18 Aug 2016 15:10 by billvvOC.

Please Log in or Create an account to join the conversation.

Moderators: Rich928mm928billvvOC

Don't have an account yet? Register Now!