RELAY-HEATER UNIT
DESCRIPTION
Three relays (1) are used for the electric positive temperature coefficient (PTC) heater system when equipped with the 3.0L diesel engine. The relays are International Standards Organization (ISO)-type relays. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal functions and patterns (2). The PTC relays are electromechanical devices that switch fused battery current directly to the heating elements of the PTC heater unit as determined by the engine control module (ECM). The PTC relays are energized when provided a ground path by the ECM and the front control (FCM).
The three PTC relays are located in the power distribution center (PDC) in the engine compartment.
OPERATION
The three 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 power distribution center (PDC). The inputs and outputs of the PTC relays include:
- Terminals (30) receive fused battery current at all times.
- Terminals (85) are connected to a control circuit (ground circuit) of the engine control module (ECM) or front control module (FCM) (depending on the relay).
- Terminals (86) receive fused battery current through the ASD relay only when the ASD relay coil is energized.
- 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 three PTC relays 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 relays and for complete ECM, FCM and HVAC wiring diagrams.
REMOVAL
NOTE: LHD model shown. RHD model similar.
1. Disconnect and isolate the negative battery cable.
2. Open the cover of the power distribution center (PDC) (1) located in the engine compartment.
NOTE: Refer to the fuse and relay layout map on the inside of the PDC cover for PTC relay locations.
3. Remove the three positive temperature coefficient (PTC) relays (2) as necessary from the PDC.
INSTALLATION
NOTE: LHD model shown. RHD model similar.
1. Position the three positive temperature coefficient (PTC) relays (2) as necessary into the proper receptacles of the power distribution center (PDC) (1).
NOTE: Refer to the fuse and relay layout map on the inside of the PDC cover for PTC relay locations.
2. Align the PTC relay terminals with the terminal cavities in the PDC receptacles and push down firmly on each relay until the terminals are fully seated.
3. Close the cover of the PDC.
4. Reconnect the negative battery cable.