Re: Air Condition hot up front, cold in the rear? (archive)

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Dual A/C system:
I just repaired my system with the same symptom. I personally replaced the front expansion valve($125) and the problem was resolved. My front expansion valve would completely shut down Freon flow to the evaporator. This could be diagnosed when a gauge set was connected and the low side pressure would drop to 15inHg vacuum with the rear unit also turned/cycled off. The other caveat to my system symptoms, although it is not necessarily indicative of a failed expansion valve, was the front evaporator would cool for a minute or two when I first turned the A/C on after it had been off for more than an hour. Also, the correct parts for the dual A/C unit are only found in the ECE portion of the Mobile Tradition parts catalog. For some unknown reason the US parts catalogs do not show the correct parts for the front A/C unit and no rear A/C unit.

However, I also noticed that when I was charging(R-12) the system and both controls were set to maximum. The rear evaporator will start to function long before the front unit. If I then shut down the rear evaporator control the front unit would then cool. Therefore, it is possible for these systems to have lost some Freon and only one of the two evaporators will function and the remedy would be to re-charge the system. These dual system have a pressure regulator. I have not been unable to find anything specific on this low side pressure regulator. However, I think the pressure regulator will shut down the front expansion valve by way of the capillary tube when the suction side Freon line pressure become low, signifying a low Freon charge. This will allow the rear unit to function when the system would have insufficient Freon charge to operate both front and rear evaporators. So an insufficient Freon charge could be diagnosed by the effect of the rear evaporator shut/cycles off and then the front evaporator unit will then cool.

The down side to the Freon pressure regulator is the low pressure side of the compressor can go to a vacuum when the front expansion valve fails, which means the compressor is running without coolant and lubrication. The Freon cools and carries the lubricant for the compressor. I would caution anyone that has a dual system with only the rear unit working, DO NOT RUN YOUR A/C. This could lead to a compressor failure!! Then you'd be into some repair bucks.

Knowledge:
You will need a basic understanding of mobile Freon A/C systems. Some automotive repair manuals do a good job with this topic others avoid the topic. A significant factor in the success of A/C repair/charging is the techniques of line purging to prevent contaminating and mixing air into the Freon lines as the Freon gas is transferred. This is technique must be mastered or your efforts will all be in vane.
This URL is a manual on the EPA training requirements for 609 certification and handling of R-12 Freon. It also includes a lot of Freon properties information. From this you will find it is possible for the DIY'er to discharge a mobile A/C system without violating the EPA laws.
http://www.qwik.com/manual609.html

Tools:
I used an empty 30 lb. Freon container as a recovery tank, 5 gal. pail and party ice. You will also need a charges gauge set, vacuum pump, 20 pound scale (the more accurate the better) and hand tools (flare nut wrenches etc.).

Instructions:
1) Measure the weight of the recovery tank. Evacuate the Freon recovery tank if it is contaminated with air. Connect the gauge set and purge the lines as you connect. Connect the recovery tank to the gauge set charge line, purge the line. Place the recovery tank in the 5 gal. pail with party ice and add a couple if quarts of water to make an ice slurry. Open the recovery tank valve, then slowly open the low side gauge set valve until the pressure drop stabilizes (this will prevent an excess amount of oil from being transferred with the Freon gas). This will condense the Freon into a liquid in the recovery tank as it is discharged from your system. Running your engine and rear A/C before you start will help put some heat in the vehicle A/C system and this will help the pressure differential and system discharge process. Monitor the gauge set and once the low side pressure drops to approximately 30 psig. and stable the gas Freon in your A/C system will now be discharged onto the recovery tank and condense to a liquid Freon because Freon will condense to liquid at approximately 30 psig @ 32 degrees.
2) Disassemble your center console starting at the console between the rear seats, then the E-brake console then finally the shifter console. You will need to remove the driver side knee bolster and passenger side glove box to remove the shifter console and blower/evaporator assembly .
3) Remove the insulation tape(usually white) over A/C fittings (glove box side) and disconnect the three A/C lines (2 large and 1 small capillary line). Use rubber stoppers to plug the large system lines as soon as they are open. A small rubber cap can be installed over the end of the capillary line. This will prevent air contamination of you're A/C system and the receiver/dryer should not require replacement if your system remains sealed and free from air humidity(it is your choice to replace the receiver/dryer).
4) Remove the blower/evaporator assembly.
5) Replace the expansion valve. BMW p/n 64-50-1-380-658 and install new "O" rings at every fitting.
6) Test the "O" ring seal of the re-assembled blower/evaporator assembly, using the vacuum pump or 50 psig. air and necessary miscellaneous fittings (this is a what is necessary and available to you parts for the job).
7) Re-assemble A/C system and evacuate the system from the low side charge gauge set valve as soon as the last fitting is connected (this will prevent the introduced air from saturating into the receiver/dryer). It is also a good idea to use a heat lamp or heat tape on the receiver/dryer to help drive out any residual moisture.
8) Re-charge system with 3.97 pounds of Freon using the recovered Freon with some makeup or all new Freon. Purge the lines. This decision should be based on your confidence that your recovered Freon is pure and uncontaminated. The charge label on your car will indicate 3.69 pounds however the Freon charge references in the BMW OFS manual and TIS CD-ROM indicate a charge level of 3.97. You can also use the receiver/dryer sight glass. From what I can tell the system will perform well when the system is charged to a point that the sight glass is bubble free (this will occur at approximately 3.6 pounds of Freon).

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