Dakota 2WD V8-4.7L VIN J (2005)
Trailer Towing Relay: Description and Operation
Vehicles equipped with an optional factory-installed trailer towing package have two trailer tow relays, one for the trailer right turn and brake lamps and
the other for the trailer left turn and brake lamps. The trailer tow relays are conventional International Standards Organization (ISO) micro relays. Relays
conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions. Each relay is
contained within a small, rectangular, molded plastic housing and is connected to all of the required inputs and outputs through five integral male
spade-type terminals that extend from the relay base plate.
The trailer tow relays are located in the Power Distribution Center (PDC) in the engine compartment near the battery. Refer to the layout label on the
underside of the PDC cover for specific relay cavity assignment information. A trailer tow relay cannot be adjusted or repaired and, if faulty or damaged,
the unit must be replaced.
The trailer tow relays are electromechanical switches that use a low current input from the Front Control Module (FCM) (also referred to as the
Integrated Power Module/IPM) to control a high current output to trailer brake and turn signal lamps. Within each relay are an electromagnetic coil, a
movable contact and two fixed contact points. A resistor is connected in parallel with the coil, and helps to dissipate voltage spikes and electromagnetic
interference that can be generated as the field of the relay coil collapses.
The movable common supply 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 field draws the movable contact away from the normally closed contact, and holds it
against the normally open contact. When the relay coil is de-energized, spring pressure returns the movable contact back against the normally closed
contact.
The inputs and outputs of the trailer tow relays include:
-
Common Supply Terminal (30) - The common feed terminal is connected to a fused B(+) circuit at all times.
-
Coil Ground Terminal (85) - The coil ground terminal is connected to a fused B(+) circuit at all times.
-
Coil Battery Terminal (86) - The coil battery terminal is connected to a control output of the FCM through a right or left trailer tow turn relay
control circuit. The FCM controls trailer brake and turn lamp operation by controlling a ground path through this circuit.
-
Normally Open Terminal (87) - The normally open terminal is connected to the trailer lamps through a right or left trailer tow turn relay output
circuit and provides battery voltage to the right or left trailer brake and turn lamps whenever the relay is energized.
-
Normally Closed Terminal (87A) - The normally closed terminal is not connected to any circuit in this application, but will have battery voltage
present whenever the relay is de-energized.
The trailer tow relays can be diagnosed using conventional diagnostic tools and methods.
