DESCRIPTION AND OPERATION
The following provides a brief description of each of
the sensors and actuators used by the PCM for proper
Powertrain Control Module (PCM)
Brake Pedal Position (BPP) Switch
Transmission Control Indicator Lamp (TCIL)
Engine Coolant Temperature (ECT) Sensor
Intake Air Temperature (IAT) Sensor
4 X 4 Low Switch
Accelerator Pedal Position (APP) Sensor
The operation of the transmission is controlled by the
Powertrain Control Module (PCM). Many input
sensors provide information to the PCM. The PCM
then uses this information to control actuators which
determine transmission operation. Refer to Figure 14
for PCM location and connector terminal information
The brake pedal position (BPP) switch supplies
battery voltage to the PCM, that the brake pedal is
applied. The PCM uses this information to release the
torque converter clutch, speed control, and auxiliary
idle (if equipped).
The Tow/Haul Switch is located on the end of the
manual shift lever and is a momentary contact switch.
The Tow/Haul Switch provides a signal to the PCM
when pressed by the operator, resulting in a change in
shift and TCC scheduling. When the Tow/Haul
Switch has been pressed, the indicator lamp that is
located at the end of the manual shift lever will
illuminate "Tow/Haul - ON". When the switch is
pressed again, Tow/Haul will be cancelled and the
TCIL will turn off (See Figure 4).
The TCIL is used along with the Tow/Haul Switch.
The TCIL is located near the end of the manual shift
lever and will illuminate "Tow/Haul - ON" when the
Tow/Haul switch has been pressed. The PCM
controls the operation of the TCIL. The PCM may
also flash the TCIL on and off, to alert the driver that a
transmission operational error has occured, when
certain faults in monitored sensors, solenoids or other
transmission components are detected (See Figure
The engine coolant temperature (ECT) sensor is a
thermistor in which resistance changes when the
temperature changes. The resistance of the sensor
increases as engine temperature decreases and the
voltage sent to the PCM increases. The PCM uses this
information to help determine TCC operation.
The intake air temparature (IAT) sensor is a
thermistor in which the resistance changes with
temperature. The resistance decreases as the intake
air temperature increases. The IAT provides air
temperature information to the PCM, which is used to
help determine transmission line pressure and shift
The 4X4 Low Switch, located on the dash on the right
hand side of the driver, sends a ground signal to the
instrument cluster when the vehicle is in 4X4 Low.
The PCM then receives 4X4 Low status from the
instrument cluster and adjusts the transmission shift
schedule accordingly. Four wheel "High" can be
selected while moving at any speed up to 55 MPH.
The accelerator pedal position (APP) sensor is
mounted on the accelerator pedal on 6.0L diesel
applications. The APP sensor detects the position of
the accelerator pedal and inputs this information, as a
voltage to the PCM. The PCM uses APP sensor
information to help in determining line pressure, shift
scheduling and TCC operation.
Failure of the APP sensor will cause transmission to
operate at a higher than normal line pressure to help
avoid damage to the transmission. This will result in
harsh upshifts and harsh engagements.
AUTOMATIC TRANSMISSION SERVICE GROUP
Technical Service Information
Copyright © 2004 ATSG