Cabin Heater Relay - Operation

OPERATION

The two ISO-standard relays (1) used for the electric positive temperature coefficient (PTC) heater system are electromechanical switches that use a low current ASD power input to control the high current fused battery power output to the PTC heater unit. On each relay, 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 one or more of the heating elements of the PTC heater.

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 terminals for the PTC relays are connected to the vehicle electrical system through receptacles in the diesel accessory fuse/relay block. The inputs and outputs of the PTC relays include:

- Terminals (30) receive battery current through a fusible link at all times.
- Terminals (85) are connected to a ground circuit.
- Terminals (86) are connected to control circuits of the totally integrated power module (TIPM).
- Terminals (87) provide fused battery current to the PTC heating elements through the PTC relays only when the PTC relay coil is energized.
- Terminals (87A) are not connected to any circuit in this application, but provide battery current output only when the PTC relay coil is de-energized.

The two PTC relays cannot be repaired or adjusted and must be replaced if inoperative or damaged. See Wiring Information for diagnosis and testing of the ISO-standard relays and for complete TIPM and HVAC wiring diagrams.