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Chapter Fifteen
HEATING AND AIR-CONDITIONING SYSTEMS Although heating and air-conditioning systems rely heavily on mechanical and vacuum controls, a good deal of electrical circuitry also is involved. Since the late 1970s, air-conditioning systems have become increasingly “smart,” relying on solidstate modules or microprocessors for their operation. This also has complicated the job of servicing such systems.
is turned to its low position, voltage is applied across all of the resistor coils and the motor runs at a low speed. Moving the switch to the next position bypasses one of the resistor coils. This allows more current to the blower motor, increasing its speed. When the switch is set to the highest position, all of the resistors are bypassed and
Heater Fan Heating systems use a heater fan attached to a permanent-magnet, variable-speed blower motor to force warm air into the passenger compartment (Figure 15-1). The higher the voltage applied to the motor, the faster it runs. A switch mounted on the instrument panel controls the blower operation (Figure 15-2). In most heating systems, the switch controls blower speed by directing the motor ground circuit current through or around the coils of a resistor block (Figure 15-3) mounted near the motor. When the switch is off, the ground circuit is open and the blower motor does not run. (Some systems used in the 1970s, however, were wired so that the blower motor operated on low speed whenever the ignition was on). When the switch Figure 15-2. The fan control switch routes current through paths of varying resistance to control motor speed. (DaimlerChrysler Corporation)
Figure 15-1.
An electric motor drives the heater fan.
Figure 15-3. Blower motor resistors are installed on a “block” near the motor. Some resistor blocks have a thermal limiter.
