Headlamp Low Beam 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 headlamp low beam relay is an electromechanical switch that uses a low current input from the Body Control Module (BCM) to control a high current output to the headlamp low beam filaments. Within the 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 point 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 headlamp low beam 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 common feed terminal is connected to a fused B(+) circuit at all times.
Coil Battery Terminal (86) - The coil battery terminal is connected to a control output of the Body Control Module (BCM) through a low beam relay control circuit. The BCM controls headlamp low beam operation by controlling a ground path through this circuit.
Normally Open Terminal (87) - The normally open terminal is connected to the headlamp low beam filaments through the low beam relay output circuit and provides battery voltage to the headlamp low beams whenever the relay is energized.
Normally Closed Terminal (87A) - The normally closed terminal is not connected in this application, but will have battery voltage present whenever the relay is de-energized.

The headlamp low beam relay can be diagnosed using conventional diagnostic tools and methods. Refer to SYSTEM WIRING DIAGRAMS .