Trailer Tow Relay - Operation

OPERATION

The trailer tow relays are electromechanical switches that use a low current input from the Body Control Module (BCM) (also known as the Common Body Controller/CBC) to the tail lamps to control a high current output to either the trailer park lamps or the right or left 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 of both the left and right trailer tow stop/turn relays is connected to their respective left or right trailer tow stop/turn signal pins of the 4-way trailer tow connector at all times. The common feed terminal of the trailer tow park lamps relay is connected to the trailer tow park lamps pin of the 4-way trailer tow connector at all times.
- Coil Ground Terminal (85) - The coil ground terminal of both the left and right trailer tow stop/turn relays is connected to a fused B(+) circuit at all times. The coil ground terminal of the trailer tow park lamps relay is also connected to a fused B(+) circuit at all times.
- Coil Battery Terminal (86) - The coil battery terminal of both the left and right trailer tow stop/turn relays is connected to their respective left or right trailer tow stop/turn relay control circuits from the BCM at all times. The coil battery terminal of the trailer tow park lamps relay is connected to the trailer tow park lamps relay control from the BCM at all times.
- Normally Open Terminal (87) - The normally open terminal of both the left and right trailer tow stop/turn relays is connected to a fused B(+) circuit at all times. The normally open terminal of the trailer tow park lamps relay is also connected to a fused B(+) circuit at all times.
- Normally Closed Terminal (87A) - The normally closed terminal of all three trailer tow relays are connected to their respective common supply (30) terminals whenever the relay is de-energized. The normally closed terminals of the trailer tow relays are not connected to any circuit in this application.

The trailer tow relays and the hard wired circuits for the relays may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the electronic controls and communication between other modules and devices that provide some features of the trailer tow stop/turn and park lamps lighting system. The most reliable, efficient, and accurate means to diagnose the electronic controls and communication related to trailer tow stop/turn and park lamps lighting operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.