Stop Lamp Inhibit Relay - Operation

OPERATION

The stop lamp inhibit relay (also known as a brake lamp activation relay or trailer tow damping relay) is used by the Controller Antilock Brake (CAB) to mitigate undesirable brake lamp operation when the CAB activates various brake hydraulic circuits as part of its Traction Control System (TCS) and Trailer Sway Control (TSC) functions. This relay is an electromechanical switch that uses a low current input from the Controller Antilock Brake (CAB) to interrupt a high current output from the brake lamp switch to the brake lamps. Within the relay is 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 stop lamp inhibit relay 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 CAB through a stop lamp inhibit relay control circuit. The CAB controls brake lamp operation by controlling a ground path through this circuit.

- Coil Battery Terminal (86) - The coil battery terminal is connected to a fused ignition switch output (run-start) circuit at all times.

- Normally Open Terminal (87) - The normally open terminal is connected to the brake lamps through a brake lamp switch output circuit and will provide battery voltage 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 not energized.

The stop lamp inhibit relay as well as the hard wired inputs and outputs of the relay may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information.