Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

Body Accessory Systems Operation 319
All input to the air-conditioning system begins
with the control assembly mounted in the instrument
panel. Temperature control can take the following
forms:
• Manual control
• Semiautomatic control (programmer controlled)
• Fully automatic control (microprocessor or
body computer controlled)
electrically operated. Selecting the mode does not
directly control the actuator; it creates an electrical
input to an independent module or programmer
(Figure 15-7). On Chrysler vehicles, the electronic
servomotor performs the programmer function
(Figure 15-8). Two sensors are added to inform the
programmer of ambient temperature and in-car
temperature (Figure 15-9). The programmer calculates
the resistance values provided by the temperature
dial setting and the two additional sensors
A manual temperature control system does
not provide a method by which the system can
function on its own to maintain a preset temperature.
The user, through the mechanical control
assembly, must make system input. Once the
air-conditioning switch is turned on, the temperature
selection made, and the blower speed
set, the system functions with vacuum-operated
mode door actuators and a cable-actuated airmix
door. Figure 15-6 is a block diagram of
such a system.
Automatic Temperature
Control (ATC)
With a semiautomatic temperature control system,
the user still selects the mode but the actuators are
Figure 15-7. Semiautomatic AC systems use an electronic
programmer to translate mechanical control
movement into actuator signals. (GM Service and Parts
Operations)
Figure 15-8. An electronic servomotor takes the place of a programmer in DaimlerChrysler’s semiautomatic AC
system. (DaimlerChrysler Corporation)