Prizm L4-108 1.8L DOHC VIN 8 MFI (1998)
Control Module HVAC: Description and Operation
Blower Controls
Battery voltage is applied at all times through the HTR fuse to the switch contact of the heater relay. Ignition voltage is applied to the coil of the heater
relay when the ignition switch is in the ON and START positions. When the blower speed selector switch is moved to any position other than OFF, the
coil of the heater relay is provided with a ground through the blower speed selector switch to G2o4. With power and ground provided, the relay
energizes; its contacts close and voltage is applied to the blower motor. The blower motor speed is maintained by controlling the voltage potential to the
blower motor. This is achieved by increasing or decreasing the resistance within the blower motor's ground path.
With the blower speed selector switch in LO, current must travel through the entire resistive element in the blower motor resistor before the blower
motor's ground path is completed through the blower motor resistor at G203. This reduces the voltage potential to the blower motor greatly and in turn
causes the blower motor to run at low speed.
When the blower speed selector switch is in the M1 position, the blower motor ground path travels only partially through the resistive element in the
blower motor resistor before completing the ground path at G204 through the blower speed selector switch.
Since less of the blower motor resistor is in the circuit, the resistance in the circuit is lower, thus making the voltage potential of the circuit higher.
Therefore, the blower motor operates at a higher speed than it does with the blower speed selector switch in the LO position.
M2 operation is identical to M1 operation with the exception that even less of the blower motor resistor is in the circuit. Therefore, the blower motor
operation in M2 is faster than it is in both Mi and LO.
With the blower speed selector switch in HI, the ground path of the blower motor bypasses the blower motor resistor entirely and passes directly through
the blower speed selector switch to G204. Therefore, the resistance in the ground path is nearly zero and the blower motor operates on full system
voltage. Under these conditions, the blower motor operates at full speed.
