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most owners would want for safe, reliable operation during highway driving trips. Figure 1. 996 Carrera Airflow Paths If the Porsche was driven during the winter, spring driving preparations should include a visual inspection of the underside of the car and, depending on the specific model, the condition of any airflow spoilers, deflectors, and ducts that help guide underside airflow through front coolers, around front brake calipers, and underneath mid and rear engine models. On the liquid cooled Boxsters, Caymans and Carreras, the front radiator areas can become amazingly cluttered with stray road trash, gravel, leaves, and cigarette butts, which are worth cleaning out periodically. And there are often non obvious spoilers. For example, on the 2005-2008 997 Carrera S models there is a small downward spoiler that is mounted across the width of the bottom under tray just ahead of the front axle area. This will occasionally drag on driveways or pavement irregularities. It is worthwhile to inspect the condition of such equipment to make sure it is not damaged and firmly in place. Underside CheCk It is also a good check on the air-cooled engine cars to inspect the bottom side of the engine cylinders where the blower cooling air is dis- charged (just above the heat exchangers) for any obstructions. We have seen and/or had reported everything from paper cleaning towels, shop towels and road debris that get lodged in the lower blower channels and prevent the full flow of blower cooling air out of the bottom of the engine compartment. And while you are down there enjoying the view, it is a good time to make sure that the heater control valves coming out of the heat exchanger on each side are fully closed when the heater control is set to the off position so the hot air will all go out the bottom of the engine compartment. Incidentally, for 964 Carrera C4 and C2 owners (1989-1994), there is a cover under the engine with strategically placed openings for the hot cooling air to get out. Addition of this cover helped reduce the overall aerodynamic drag of the 964 by reducing engine compartment underside turbulence. In warm climates, many owners found that removal of the lower engine shield helped reduce their oil temperature when operating under warm conditions. Another accessory that usually doesn���t get any attention in fall and winter driving is the air conditioning system. Nothing is worse on the first warm day than switching on the air conditioner system and getting no response. Like a number of other Porsche maintenance areas, the Porsche air conditioners fall into two categories: later models where the air conditioner is integrated with the rest of the cooling system and early models where the air conditioner is a stand-alone add-on option. In the case of the air-cooled engine cars, the stand alone systems ended after 1989, while the liquid cooled engine cars had fully integrated systems starting with the 944 in 1983 and 924 in 1980. interior eqUipment Figure 2. Pre 1989 911 Air Conditioner Electrical Wiring Showing the Relays for the Main blower and Front Condenser. The last of the 911 standalone factory air conditioners were on the 1978-1989 models, including the 911 SC models and first 911 Carreras. These systems had some interesting features that resulted in unusual failure modes (References 3 and 4). Foremost of these was the addition of an electrical relay in the main power supply line from the fuse panel to the air conditioner. This relay is shown in the Figure 2 wiring diagram as the ���A/C Main Power Supply Relay.��� The purpose of this relay is to cut off the power to the air April 2011 37