Expedition 4WD V8-4.6L SOHC VIN W (1999)
Control Module HVAC: Description and Operation
Blower Motor Speed Control
The A/C blower motor speed control is a Pulse Width Modulated (PWM) type which provides variable blower speed control of the blower motor. The
A/C blower motor speed control is controlled by switching the control signal voltage of 5 volts OFF and ON 2,000 times per second. By varying the
ratio of time on (+5 volts) to time off (0 volts), the Electronic Automatic Temperature Control (EATC) module tells the A/C bower motor speed control
how fast the blower motor should run. The A/C blower motor speed control has the following features:
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Has circuitry to protect the blower motor from burning out in the event of a locked rotor condition.
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Can compensate for changes in battery positive voltage (B+) which prevents the blower speed from slowing down when the engine is idling and
will provide up to a maximum of 30 amperes for blower motor operation with battery voltages between 10 and 16 volts.
