Brake Warning Indicator: Description and Operation

The BRAKE Indicator in the Instrument Cluster is provided to alert the operator to problems and conditions in the vehicle's braking system. Voltage is applied to the Instrument Cluster through the PNK (39) wire from GAUGES Fuse 4 when the Ignition Switch is in either the RUN or the START position. The indicator lamp is illuminated whenever the TAN/WHT (33) wire from the Instrument Cluster is grounded.

The BRAKE Indicator lamp is tested whenever the vehicle's engine is started. Placing the Ignition Switch in the START position connects a ground path from the Ignition Switch to Ground G201 through the BLK (150) wire. With this ground connection, In-Line Diode D201 is forward biased, current flow is allowed through the TAN/WHT (33) wire and the indicator bulb is illuminated. After the engine is started, the Ignition Switch is in the RUN position and the lamp is off.

The BRAKE indicator is also illuminated when the parking brake remains applied and the Ignition Switch is in RUN or START. This alerts the operator to release the parking brake. The Park Brake Warning Switch is mechanically grounded. The switch contacts close when the parking brake is applied. This grounds the TAN/WHT (33) wire.

The Daytime Running Lamps module also senses the grounded condition from the Park Brake Warning Switch. This condition prevents the operation of the Daytime Running Lamps while the parking brake is applied. In-Line Diode D201 prevents this Daytime Running Lamp interlock from operation due to lamp operation by the Brake Pressure Differential Switch or the Electronic Brake Control Module. For school bus models, the Daytime Running Lamps will also not operate if a low power brake booster fluid flow and a low power brake booster pressure alarm condition is present.

The Brake Pressure Differential Switch closes it's contacts when conditions in the hydraulic brake system are abnormal. This grounds the TAN/WHT (33) wire through the BLK (150) wire to Ground G103.

The Electronic Brake Control Module also grounds the TAN/WHT (33) circuit if there are abnormal conditions in the Antilock Brake System. The BRAKE indicator lamp and the ABS indicator lamp are both lighted for ABS failures. See detailed information about the operation and grounding of the Antilock Brake System.

School Buses are equipped with additional brake warning equipment. An audible alarm will sound and the BRAKE indicator lamp will light if both the power brake booster fluid flow and fluid pressure is low. Voltage is available to the P/B Booster Fluid Flow Alarm Delay Module through the
The YEL (43) wire In-Line Resistor R208 and the P/B Booster Fluid Flow Alarm The YEL/BLK (965) wire.

This voltage is only available when the Ignition Switch is in either the ACC or the RUN position. The P/B Booster Fluid Flow Alarm does not operate unless there is sufficient current flow through In-Line Resistor R208. Without a ground path, there is no current flow. Without current flow, there is no voltage drop across the resistor. With current flow, the voltage drop across the resistor provides the operating voltage to drive the audible alarm.

The delay module is only grounded through the following ground path: The LT ORN (966) wire to the P/B Booster Fluid Pressure Alarm Switch The closed contacts of the P/B Booster Fluid Pressure Alarm Switch when fluid pressure is low The DK GRN (860) wire to the P/B Booster Fluid Flow Switch The closed contacts P/B Booster Fluid Flow Switch when fluid flow is low The BLK (150) wire to Ground G112

In-Line Diode D202 prevents the Park Brake Warning Switch from providing a ground path which would cause the P/B Booster Fluid Flow Alarm to operate. However, if the low fluid pressure and flow condition is present, the diode allows the ground path to light the BRAKE indicating lamp on the Instrument Cluster.

When both the low fluid pressure and low fluid flow conditions are present, the normally open contacts in the P/B Booster Fluid Pressure Alarm Switch and the P/B Booster Fluid Flow Alarm Switch are both closed. This provides a ground path for the P/B Fluid Flow Alarm Delay Module. The module has an internal time delay function that prevents the alarm from sounding unless the condition is present longer that the normal transients seen in the system during operation. The normal operation of the brakes causes momentary low pressure conditions. The Power Steering Pump, which supplies the hydraulic pressure to the brakes, soon returns the pressure to normal. The delay module prevents these short-term conditions from driving the alarm.

The alarm may be present for a short period of time after the vehicle is first started. When the Power Steering Pump starts operating and increases pressure and flow to normal, the alarm is silenced.