I have heard that you should switch off the car air-conditioner before turning off the ignition. Is this a motoring myth or is there a sound technical reason for doing so?
During the 1970s, when most automotive air-conditioners were retro-fitted accessories, the average family car suffered 10 to 15 percent 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 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 has reached its 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 in the bad old days.