RAM 2500 Truck 2WD V10-8.0L VIN W (2003)
Brake Light Switch: Description and Operation
BRAKE LAMP SWITCH
The plunger type brake lamp switch is mounted on a bracket attached to the base of the steering column, under the instrument panel.
CAUTION: The switch can only be adjusted during initial installation. If the switch is not adjusted properly a new switch must be installed.
The brake lamp switch is hard wired to the Center High Mount Stop Lamp (CHMSL) and also monitored by the Instrument Cluster for use by the
brake lamp, speed control brake sensor circuits and electronic brake distribution (EBD). The brake lamp circuit is open until the plunger is depressed.
The speed control and brake sensor circuits are closed until the plunger is depressed. When the brake lamp switch transitions, the CHMSL transitions
and instrument cluster transmits a brake applied/released message on the bus. The Integrated Power Module (IPM) will then transition the brake
lamps.
When the brake light switch is activated, the Powertrain Control Module (PCM) receives an input indicating that the brakes are being applied. After
receiving this input, the PCM maintains idle speed to a scheduled rpm through control of the Idle Air Control (IAC) motor. The brake switch input is
also used to disable vent and vacuum solenoid output signals to the speed control servo.
Vehicles equipped with the speed control option use a dual function brake lamp switch The PCM monitors the state of the dual function brake lamp
switch
The brake switch is equipped with three sets of contacts, one normally open and the other two normally closed (brakes disengaged) The PCM sends a
12 volt signal to one of the normally closed contacts in the brake switch, which is returned to the PCM as a brake switch state signal With the contacts
closed, the 12 volt signal is pulled to ground causing the signal to go low The low voltage signal, monitored by the PCM, indicates that the brakes are
not applied When the brakes are applied, the contacts open, causing the PCM's output brake signal to go high, disengaging the speed control, cutting
off PCM power to the speed control solenoids
The second set of normally closed contacts supplies 12 volts from the PCM any time speed control is turned on Through the brake switch, voltage is
routed to the speed control servo solenoids The speed control solenoids (vacuum, vent and dump) are provided this voltage any time the speed control
is ON and the brakes are disengaged
When the driver applies the brakes, the contacts open and voltage is interrupted to the solenoids The normally open contacts are fed battery voltage
When the brakes are applied, battery voltage is supplied to the brake lamps
The brake lamp switch can only be adjusted once That is at the initial installation of the switch If the switch is not adjusted properly or has been
removed, a new switch must be installed and adjusted.
BRAKE LAMP SWITCH CIRCUIT
This switch prepares the CAB for a possible antilock stop.
The antilock system uses an input signal from the brake pedal switch when activated. A released brake pedal will close the switch circuit and read 0
volts. When the driver applies the brake pedal, the circuit voltage is 12 volts. This signal tells the CAB that the pedal is depressed. The brake lamp
switch is located on the brake pedal under the driver dash.
