my mercedes ml320 y reg 2001 air con doesnt work when i press the air con on the fans on the front grill dont come on nor can i hear the pump come on , i had a tracker fitted other day and he tested the relay in fuse box and it works ok when he attched earth to the relay pin and the fan came on, the light comes on the switch in car ,he also said some good cars have safety feature if gas low or empty which stops all pump and fan from coming on