I have heard that you should switch off the car air-conditioner before turning off the ignition. Is this a myth or is there a sound technical reason for doing so?
During the 1970s, when most car air-conditioners were retro-fitted accessories, the average family car suffered 10 to 15 per cent loss in power.
With these units, it was helpful to have the air-conditioner turned off when the engine was shut down, so that the starter motor had less of a strain the next time the car was started.
So, if you run a classic car, do remember to at least ensure that the air-conditioner is switched off before starting up the engine. Otherwise, the air-conditioner compressor load puts a strain on the starter motor and battery.
With all modern cars that are fuel injected and fitted with electronic engine management, this routine is not necessary. This is because the compressor is not engaged - and thus does not impose a load - until the engine is at normal idling speed.
Also, the higher efficiency of the air-conditioning system and the engine of modern cars means that performance and fuel economy losses are not as drastic as they used to be.