Trailer Tow Relay: Operation

WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.

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 SYSTEM WIRING DIAGRAMS .