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AUGUST TECH NOTES ALLAN CALDWELL Technical Editor 28 Spiel – August 2021 EARLY PORSCHE AIR CONDITIONERS A fter three record-setting, over- 100°F temperature days this year in Seattle along with a lot of mid-to-high 80° days, Porsche air conditioners suddenly became of interest. The PCA's past experience with 911 factory air conditioning was spotty until the mid-1970s, and we really didn't get a fully integrated heating and cooling system until the 964 type 911s were introduced in the 1990s along with the introduction of the later refrigerants that took the place of the original Freon refrigerants. Considering the long life of many early 911s, it is suspected that there still may be a number of PNWR 911s, 911SCs, and 911 Carreras that were never upgraded to later systems. In our past 911 air conditioner tech articles (References 1 and 2), we reviewed some of the interesting design features and modes of operation that might still be important for later use and/ or upgrades to the earlier systems. All of our past experience with the early fuel injection and air conditioners indicated that the electrical power supplies to the units were often more important failure points than the basic air conditioning equipment. Figure 1 shows a schematic drawing of the early 911 system. One of the early common complaints was a lack of cold air coming out of the vents coming from the primary AC blower which is located in the luggage compartment condenser box. Reference 1 points out that simply removing the decorative firewall vent trim above the foot pedals around the outlet can provide a surprisingly increased flow of cold air into the passenger compartment. SUDDEN AC BLOWER STOPPAGES In my 911SC, we also encountered some strange sudden stoppages of the evaporator-blower fan and cold air coming from the vents that was not a blower failure or anything amiss in the refrigerant system. Changing the position of the thermostat control or the blower speed control had no effect. After reaching the end of a day's trip, a check of the main fuses and electrical connections revealed everything was okay. Initially, it was thought that perhaps the wire to the compressor clutch might have been inadvertently disconnected in the engine compartment, although, it didn't seem like that would have caused the evaporator-blower to stop. Anyway, after a couple of hours of cool-down, the air conditioner would work fine again. On following trips, the air conditioner worked for the first half hour or so on the highway, but then suddenly stopped again as it had previously. After reaching home, a check of the wiring diagram revealed some interesting features. Figure 2 shows a simplified version of the wiring diagram for a 911 without a front condenser. All of the electrical power going to the air conditioner goes through fuse S-20 which is the far aft fuse on the luggage compartment fuse panel. The main power wire from the fuse panel is a red and white wire that goes to the air conditioner relay located in the air conditioner compartment under the front luggage compartment floor. The red output wire from this relay runs everything on the air conditioner , including the electro-magnetic clutch and evaporator-blower fan. This relay was immediately suspect because if it doesn't close, no power can get to the system. So why is the relay there? Note that the two wires that control operation of the relay consist of a black wire that comes from the input side of fuse S-11 (hot with ignition or starter engaged) and a yellow wire connected