Headlamp Relay: Description and Operation

Fig.14 Power Distribution Center:

The headlamp (or security) relay is located in the Power Distribution Center (PDC) near the battery in the engine compartment. See the fuse and relay layout label affixed to the inside surface of the PDC cover for headlamp relay identification and location. The headlamp relay is a conventional International Standards Organization (ISO) micro relay. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions. The relay is contained within a small, rectangular, molded plastic housing. The relay is connected to all of the required inputs and outputs through its PDC receptacle by five male spade-type terminals that extend from the bottom of the relay base. The ISO designation for each terminal is molded into the base adjacent to the terminal. The ISO terminal designations are as follows:
- 30 (Common Feed) - This terminal is connected to the movable contact point of the relay.
- 85 (Coil Ground) - This terminal is connected to the ground feed side of the relay control coil.
- 86 (Coil Battery) - This terminal is connected to the battery feed side of the relay control coil.
- 87 (Normally Open) - This terminal is connected to the normally open fixed contact point of the relay.
- 87A (Normally Closed) - This terminal is connected to the normally closed fixed contact point of the relay.

The headlamp relay cannot be adjusted or repaired. If the relay is damaged or faulty, it must be replaced.

The headlamp (or security) relay is an electromechanical switch that uses a low current input from the high-line or premium Central Timer Module (CTM) to control a high current output to the headlamps. The movable common feed 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 electromagnetic field draws the movable relay contact point away from the fixed normally closed contact point, and holds it against the fixed normally open contact point. When the relay coil is de-energized, spring pressure returns the movable contact point back against the fixed normally closed contact point. A resistor or diode is connected in parallel with the relay coil in the relay, and helps to dissipate voltage spikes and electromagnetic interference that can be generated as the electromagnetic field of the relay coil collapses.

The headlamp relay terminals are connected to the vehicle electrical system through a connector receptacle in the Power Distribution Center (PDC). The inputs and outputs of the headlamp relay include:
- The common feed terminal (30) is connected to ground at all times through a take out and eyelet terminal connector of the right headlamp and dash wire harness that is secured by a ground screw to the left fender inner shield near the PDC in the engine compartment.
- The coil ground terminal (85) is connected to the Central Timer Module (CTM) through the security relay control circuit. The CTM energizes the headlamp relay control coil by internally pulling this circuit to ground.
- The coil battery terminal (86) is connected to battery current at all times through a fused B(+) circuit that is internal to the PDC.
- The normally open terminal (87) is connected to the headlamps at all times through the beam select switch low beam output circuit. This circuit provides a path to ground for the headlamps through the common feed terminal when the headlamp relay control coil is energized by the CTM.
- The normally closed terminal (87A) is not connected to any circuit in this application, but is grounded through the common feed terminal when the headlamp relay control coil is de-energized.

The headlamp relay can be diagnosed using conventional diagnostic tools and methods.