K 3500 Truck 4WD V8-454 7.4L VIN J SFI (1996)
Control Assembly: Description and Operation
Blower Controls
Front Blower Controls
Battery Voltage is supplied at all times to Front High Blower Relay Pin A2 through the RED (542) wire. When the ignition Switch is OFF or the Front
Blower Switch at the Heater Control Module or Heater and A/C Control Module is OFF, no Voltage is applied to the Relay Coil through the ORN (52)
wire and the Relay remains de-energized. Similarly, no Voltage is applied to the normally closed contact at Pin B2 of the Relay. Battery Voltage is not
applied to the Front Blower Motor and it does not operate at any speed.
When the Ignition Switch is in RUN and the Front Blower Switch is in LO, Voltage is applied through all three Blower Resistors to the normally closed
contact in the Relay. The Relay remains de-energized. Reduced Voltage is applied to the Front Blower Motor through the PPL (65) wire and the Motor
operates at its slowest speed. With the Front Blower Switch in either M1 or M2, current flows through fewer resistors in the Front Blower Resistor and
the Front Blower Motor runs faster.
With the Front Blower Switch in HI, the Blower Resistors are bypassed by the energizing of the Front High Blower Relay. The contacts change state and
Battery Voltage from the Underhood Fuse-Relay Center is applied directly to the Motor. The Front Blower Motor then operates at it's fastest speed.
Rear Auxiliary Blower Controls (Heater W/O A/C)
Battery Voltage is supplied when the Ignition Switch is in RUN to Relay Coil Pin 5 of each of the three Rear Blower Relays through the BRN (341)
wire. Battery Voltage is also supplied when the Ignition Switch is in RUN to Pin 1 of each of the three Rear Blower Relays through the RED (542) wire.
With the Rear Auxiliary Heater Control Switch in OFF, no grounding path is provided for the Relay Coils and the Relays remain de-energized. The
Relay contacts remain open and no Voltage is supplied to the Rear Blower Motor.
When the Ignition Switch is in RUN and the Rear Auxiliary Heater Control Switch is in LO, a ground path is provided for the Rear Low Blower Relay
Coil through the DK BLU (1926) wire to the Control Switch then through BLK (150) wire to Ground G200. The Relay Coil energizes and the Relay
contacts close. This applies Battery Voltage to the Rear Blower Resistor through the YEL (1176) wire. Current flows through two resistors and to the
Rear Blower Motor through the YEL (1172) wire. The Motor operates at its slowest speed.
When the Rear Auxiliary Heater Control Switch is in MED, the Rear Medium Blower Relay is energized. Current flow's to the Motor through one
resistor and the Motor operates at an intermediate speed. When the Rear Auxiliary Heater Control Switch is in HI, the Rear High Blower Relay is
energized. Current bypasses both resistors and the Motor operates at its highest speed.
An illumination lamp is provided at the Rear Auxiliary Heater Control Switch to facilitate Switch operations. The lamp is powered by the Instrument
Panel Dimming system.
Rear Auxiliary Blower Controls (Heater W/ A/C)
The Rear Blower Motor is controlled by either of two Rear Blower Switches when optional rear auxiliary heating and air conditioning is provided. Both
Switches are located in overhead modules, one over the Instrument Panel and the other in the center of the vehicle. The Rear Blower Motor will operate
from the Rear Blower Switch at the Rear Auxiliary Heater and A/C Control Module only when the Rear Blower Switch at the Front Auxiliary Heater and
A/C Control Module is in the REAR position. Refer to HVAC Air Delivery and Temperature Controls for a description of the control priorities for
the ventilation modes.
Battery Voltage is supplied when the Ignition Switch is in RUN to Relay Coil Pin 5 of each of the three Rear Blower Relays through the BRN (341)
wires. Battery Voltage is also supplied when the Ignition Switch is in RUN to contact Pin 1 of each of the three Rear Blower Relays through the RED
(542) wires. With the Rear Blower Switch at the Front Auxiliary Heater and A/C Control Module in OFF and with the Rear Blower Switch at the Rear
Auxiliary Heater and A/C Control Module in OFF, no grounding path is provided for the Relay Coils and the Relays remain de-energized. The Relay
contacts remain pen and no Voltage is supplied to the Rear Blower Motor.
When the Ignition Switch is in RUN and the Blower Switch at the Front Module is in LO a ground path is provided for the Rear Low Blower Relay
through the DK BLU (1925) wire to the Front Blower Switch then through BLK (150) wire to Ground G400. The Relay Coil energizes and the Relay
contacts close. This applies Battery Voltage to the Rear Blower Resistor through the YEL (1176) wire. Current flows through two resistors and to the
Rear Blower Motor through the YEL (1172) wire. The Motor operates at its slowest speed.
When the Rear Blower Switch at the Front Heater and A/C Control Module is in REAR and the Rear Blower Switch at the Rear Heater and A/C Control
Module is in LO, the grounding path is different. The ground path is through the DK BLU (1925) wire to the Rear Auxiliary Heater and A/C Control
Module. The DK BLU (77) wire continues the ground path to the Auxiliary and Heater and A/C Control Logic Module. ORN (77) wire continues the
ground path to the Front Auxiliary Heater and A/C Control Module where is continues to Ground G400.
When the Rear Blower Switch is in MED, the Rear Medium Blower Relay is energized. Current flows to the Motor through one resistor and the Motor
operates at an intermediate speed. When the Rear Blower Switch is in HI, the Rear High Blower Relay is energized. Current bypasses both resistors and
the Motor operates at its highest speed.
An illumination lamp is provided at each Heater and A/C Control Module to facilitate Switch operations. The lamps are powered by the Instrument Panel
Dimming system.
Rear Auxiliary Blower Controls (A/C W/O Heater)
