Liberty 4WD L4-2.4L DOHC VIN 1 (2005)
Trailer Towing Relay: Description and Operation
Trailer Tow Relays
Vehicles equipped with an optional factory-installed trailer towing package have four trailer tow relays. 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 include one each for brake lamps, right turn lamps, left turn lamps and a fused ignition switch output (run) circuit which are
provided for the trailer through the rear body wiring and connectors. The relays are located in a connector bank above the right rear wheelhouse and
behind the quarter trim panel. The connector bank and relays are wrapped within a foam rubber isolator envelope for sound deadening and anti-rattle
protection.
Refer to Trailer Tow in the wiring information 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 each use a low current input from the circuit that they isolate to control a high current output to
the trailer brake and turn signal lamps, and a trailer accessory circuit. 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 the circuit connected to the normally open terminal (87) when the
relay is energized, or to the circuit connected to the normally closed terminal (87A) when the relay is de-energized.
-
Coil Ground Terminal (85) - The coil ground terminal is connected to a ground circuit at all times.
-
Coil Battery Terminal (86) - The coil battery terminal is connected to the circuit that controls the relay output. When battery voltage is applied to
this terminal, the relay is energized.
-
Normally Open Terminal (87) - The normally open terminal is connected to the circuit connected to the common supply terminal (30) whenever
the relay is energized.
-
Normally Closed Terminal (87A) - The normally closed terminal is connected to the circuit connected to the common supply terminal (30)
whenever the relay is de-energized.
The trailer tow relays can be diagnosed using conventional diagnostic tools and methods. Refer to the appropriate wiring information.
