Brake Warning Indicator: Description and Operation

BRAKE/PARK BRAKE INDICATOR
A brake indicator lamp is standard equipment on all instrument clusters. The brake indicator lamp is located near the lower edge of the instrument cluster overlay, to the left of the odometer/trip odometer Vacuum-Fluorescent Display (VFD). The brake indicator lamp consists of a red lens behind a rectangular cutout in the instrument cluster overlay. The word "BRAKE" is imprinted in bold black lettering near the center of the red lens. The word "BRAKE" is clearly visible on the face of the instrument cluster when the lamp is not illuminated, and is silhouetted in a bright red glow on the face of the instrument cluster when the brake indicator lamp is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on the instrument cluster electronic circuit board. The brake indicator lamp is serviced as a unit with the instrument cluster.

The brake indicator lamp gives an indication to the vehicle operator when the parking brake is applied, or when there are certain brake hydraulic system malfunctions. This lamp is hard wired on the instrument cluster electronic circuit board, and is controlled by three switch inputs to the cluster. The brake indicator lamp bulb receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (st-run) circuit whenever the ignition switch is in the ON or Start positions; therefore, the lamp will always be OFF when the ignition switch is in any position except ON or Start. The brake indicator lamp will be illuminated under the following conditions:
- Bulb Test - Each time the ignition switch is turned to the Start position the lamp is illuminated as a bulb test.
- Park Brake-ON - If the park brake is applied with the ignition switch in the ON position, the lamp is illuminated.
- Brake Hydraulic System Malfunction - If the pressures in the two halves of the split brake hydraulic system are unequal with the ignition switch in the ON position, the lamp is illuminated.

The hard wired inputs to the brake indicator lamp include:
- Ignition Switch - A contact in the ignition switch provides a ground path to the red brake warning indicator driver circuit whenever the ignition switch is turned to the Start position.
- Park Brake Switch - The park brake switch on the park brake lever mechanism provides a ground path to the red brake warning indicator driver circuit whenever the park brake is applied.
- Brake Warning Pressure Switch - The brake warning pressure switch on the brake combination valve provides a ground path to the red brake warning indicator driver circuit whenever the switch is closed by a pressure differential Spool valve within the combination valve unit.

The ignition switch, the park brake switch, and the brake warning pressure switch are each connected in parallel between ground and the red brake warning indicator input to the instrument cluster so that each of their inputs will illuminate the lamp independently of the others. The brake indicator lamp can be diagnosed using conventional diagnostic tools and methods.