Blower Motor Relay: Description and Operation

The blower motor relay for the rear heating-A/C system is a International Standards Organization (ISO)-type relay. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions. The rear blower motor relay is a electromechanical device that switches battery current from a fuse in the power distribution center (PDC) directly to the rear blower motor. The rear blower motor relay is energized when the relay coil is provided a voltage signal by the ignition switch.

The rear blower motor relay is located in the power distribution center (PDC) in the engine compartment. Refer to the PDC label for rear blower motor relay identification and location.

The black, molded plastic case is the most visible component of the rear blower motor relay. Five male spade-type terminals extend from the bottom of the base to connect the relay to the vehicle electrical system, and the ISO designation for each terminal is molded into the base adjacent to each terminal. The ISO terminal designations are as follows:
- 30 (Common Feed) - This terminal is connected to the movable contact point of the relay.
- 85 (Coil Ground) - This terminal is connected to the ground feed side of the relay control coil.
- 86 (Coil Battery) - This terminal is connected to the battery feed side of the relay control coil.
- 87 (Normally Open) - This terminal is connected to the normally open fixed contact point of the relay.
- 87A (Normally Closed) - This terminal is connected to the normally closed fixed contact point of the relay.

The rear 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 rear blower motor. The movable common feed contact point is held against the fixed normally closed contact point by spring pressure. When the relay coil is energized, an electromagnetic field is produced by the coil windings. This electromagnetic field draws the movable relay contact point away from the fixed normally closed contact point, and holds it against the fixed normally open contact point. When the relay coil is de-energized, spring pressure returns the movable contact point back against the fixed normally closed contact point. The resistor or diode is connected in parallel with the relay coil in the relay, and helps to dissipate voltage spikes and electromagnetic interference that can be generated as the electromagnetic field of the relay coil collapses.

The rear 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 rear blower motor relay include:
- The common feed terminal (30) receives a battery current input from a fuse in the PDC through a fused B(+) circuit at all times.
- The coil ground terminal (85) is connected to a ground at all times.
- The coil battery terminal (86) receives a battery current input from a fuse in the junction block (JB) through a fused ignition switch output (run) circuit only when the ignition switch is in the On position.
- The normally open terminal (87) provides a battery current output to the rear blower motor through the rear blower motor relay output circuit only when the rear blower motor relay coil is energized.
- The normally closed terminal (87A) is not connected to any circuit in this application, but provides a battery current output only when the rear blower motor relay coil is de-energized.

The rear blower motor relay cannot be repaired and, if faulty or damaged, it must be replaced. Refer to the appropriate wiring information for diagnosis and testing of the ISO relay and for complete HVAC wiring diagrams.