If it was NECESSARY, it would have been designed in by the engineers to do that automatically. What I would rather have you ask is "do I have to turn the heater / ac fan off first?" The fan draws a real lot of current which is hard on the already cheap ignition switch contacts. It will also overheat the switch terminals and wiring. These are very common problems and usually associated with frequent high speed fan use.
Besides the fan, the ac compressor uses electrical current to engage the clutch, but the ignition switch isn't involved in that circuit. The clutch is commanded on by the engine computer, and powered through a separate relay. A relay is a switch that turns a high current on and off through use of a very small current. Therefore, if your goal is to reduce component damage and wear, turn the fan down before turning the engine off, and use it on the lowest setting possible that is still comfortable. Turning the ac off won't accomplish much. In fact, you'll notice it doesn't kick in right away after starting the engine. The computer delays engagement so the engine doesn't have to struggle to get up to speed when first started.
Friday, August 28th, 2009 AT 5:24 PM