First Speeds Repairs

There was a time when the heating and ventilation system on most cars was simple. A three-position switch,

a couple of levers and maybe some separate vents that would allow outside air to enter the passenger compartment. Times have changed and today’s car buying public wants to dial in a desired temperature and turn over regulation to a processor (See Photo 1). Many cars have multiple-zone HVAC systems where different temperatures can be dialed in for certain areas of the car interior.

BMW has used the same basic system for the last 25 years or more for regulation of interior temperature, window defrosting and ventilation. The focus of this article will be the 3 and 5 Series cars of the last 15 years, but the information on repair and diagnosis is applicable to all models. Like a jigsaw puzzle, there are a lot of pieces to this system, but when they are all put together, the picture on how it all works is pretty clear.

COMPONENT PARTS
Beyond the obvious parts of the HVAC system, the radiator and heater core, blower motor, A/C evaporator and condenser, there are other components that help regulate the interior temperature regardless of the outside conditions. On all of these models, there is an auxiliary coolant pump, a switching valve assembly and various temperature sensors, flap actuators and a control head and processor (See Photo 2). Later models will have at ...