Commander 4WD V8-4.7L (2008)
Blower Motor Relay: Description and Operation
Controls - Front
Description
DESCRIPTION
The blower motor relay (1) for the front heating-A/C system is an International Standards Organization (ISO)-type relay. Relays conforming to the ISO
specifications have common physical dimensions, current capacities, terminal functions and patterns (2). The front blower motor relay is a
electromechanical device that switches battery current through a fuse in the power distribution center (PDC) directly to the front blower motor when
equipped with the manual temperature control (MTC) system, or to the front blower motor power module when equipped with the automatic temperature
control (ATC) system. The front blower motor relay is energized when the relay coil is provided battery current by the ignition switch.
The front blower motor relay is located in the PDC in the engine compartment.
Operation
OPERATION
The ISO-standard front blower motor relay is an electromechanical switch that uses a low current input from the ignition switch to control the high
current output to the front blower motor. The movable, common feed relay contact is held against the fixed, normally closed relay contact by spring
pressure. When the electromagnetic relay coil is energized, it draws the movable common feed relay contact away from the fixed, normally closed relay
contact and, holds it against the fixed, normally open relay contact. This action allows high current to flow to the blower motor.
When the relay coil is de-energized, spring pressure returns the movable relay contact back against the fixed, normally closed contact point. The resistor
or diode is connected in parallel with the relay coil, and helps to dissipate voltage spikes and electromagnetic interference that can be generated as the
electromagnetic field of the relay coil collapses.
The front blower motor relay terminals are connected to the vehicle electrical system through a receptacle in the power distribution center (PDC). The
inputs and outputs of the front blower motor relay include:
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Terminal (30) receives fused battery current at all times.
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Terminal (85) is connected to a ground circuit at all times.
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Terminal (86) receives fused battery current through the run relay only when the run relay coil is energized.
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Terminal (87) provides fused battery current to the front blower motor through the front blower motor relay only when the front blower motor
relay coil is energized.
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Terminal (87A) is not connected to any circuit in this application, but provides a battery current output only when the front blower motor relay coil
is de-energized.
The front blower motor relay cannot be repaired and must be replaced if found inoperative or damaged. Refer to the appropriate wiring information for
diagnosis and testing of the ISO-standard relay and for complete HVAC wiring diagrams.
