Grand Cherokee 2WD V6-3.7L (2008)
Trailer Lighting Relay: Description and Operation
Operation
OPERATION
The trailer tow relays are electromechanical switches that use a low current input from the Front Control Module (FCM) (also known 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 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.
-
Coil Battery Terminal (86) - The coil battery terminal is connected to a fused B(+) circuit at all times.
-
Normally Open Terminal (87) - The normally open terminal is connected to the trailer lamps through a right or left trailer tow turn signal 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 as well as the hard wired inputs and outputs of the relays may be diagnosed using conventional diagnostic tools and procedures.
Refer to the appropriate wiring information.
