ELECTRONIC CLIMATE CONTROL
by Lance Wright

Electronic climate control, also referred to as automatic temperature control, is a system that uses an electronic control unit to monitor and adjust the vehicles interior temperature. The system has the ability to maintain vehicle comfort settings throughout a variety of conditions. System components can consist of some, if not all, of the following: control panel, control module, programmer, in-car temperature sensor, ambient air temperature sensor, evaporator temperature sensor, outside air temperature sensor and sun load sensor. When the driver sets the temperature control setting to his or her comfort level, the control module evaluates the setting and monitors signals from the various sensors to calculate commands to the mode control motors. The control module either directly or through the programmer actuates mode control motors to maintain the desired temperature settings. Sunlight sensors are used on some models to monitor the amount of sunlight shining on the vehicle as an added input for temperature control.

The dashboard control panel contains the blower speed select switch, compressor control switch, and mode settings controls. The compressor control switch can be part of the mode control switch. The control panel also contains a light source that is controlled by the headlamp switch. The light source is used to illuminate the control panel for driver use at night. HVAC mode control is normally accomplished by sending vacuum or electronic signals to the motors that operate the various mode control doors. Cable operated mode control is still in use in some vehicles. Some mode control systems may combine one mode control system with another, using vacuum and electric control or vacuum and cable controls. Vacuum operated mode control motors have been in use for a number of years. Engine vacuum is supplied to the control panel, where it is routed to the mode motor by the mode select switch. Each mode control motor contains a spring loaded vacuum diaphragm that responds to the vacuum signal received from the mode select switch. The mode control motor moves a linkage rod that opens or closes a HVAC mode door according to design intent.

Electric mode control uses electric servo motors to actuate HVAC mode doors. Electrical signals are sent to the mode control motor from the mode select switch, to position the mode control doors inside the HVAC module. Because of their design, the electric mode control motors have the capability to send feedback signals indicating their position to the control panel. Because of this, electric mode control motors are often used for temperature control in HVAC systems that combine vacuum and electric mode control motors. Cable operated mode control is typically used in non air conditioned vehicles and some Asian imports. Cable mode control is often used to control temperature blend doors when combined with vacuum control systems. The cable is operated by moving a lever or knob on the control panel. The moving cable operates a series of inter-connected linkage rods to achieve the desired air flow settings. Diagnosing electronic climate control systems requires nothing less than a offical manufacturer’s auto repair guide which will contain the wiring diagrams necessary for accurate troubleshooting and the steps needed for repair.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)