LCT 1000 Allison Transmission

DTC P0703

Circuit Description

A mechanical switch attached to the brake pedal sends a signal to the transmission control module (TCM) indicating the service brake has been applied.


DTC P0703 is a type C DTC.


DTC Descriptor

This diagnostic procedure supports the following DTC:


DTC P0703 Brake Switch Circuit


Conditions for Running the DTC

* DTC P0721 and P0722 are not active.
* The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.



Conditions for Setting the DTC

DTC P0703 sets when the vehicle accelerates 10 times with the brake switch in the ON position, or decelerates 10 times with the brake switch in the OFF position.


Action Taken When the DTC Sets

* The TCM does not illuminate the malfunction indicator lamp (MIL).
* The TCM uses default assumption that the brake is OFF.
* DTC P0703 is stored in TCM history.
* The TCM inhibits torque converter clutch (TCC) engagement.
* The TCM inhibits grade braking.



Conditions for Clearing the DTC

A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.


Diagnostic Aids

DTC P0703 indicates that the TCM did not see the proper input signal for service brake status during an acceleration or deceleration cycle. This may indicate an open or short in the TCC brake switch/cruise control release circuit or a faulty stop lamp switch.

* Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:

- A bent terminal
- A backed-out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- A broken wire inside the insulation

* When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
* You may have to drive the vehicle in order to experience a condition.



Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. This step tests brake switch status.
3. This step tests for voltage at the TCM.
4. This step tests for voltage before the stop lamp switch.
5. This step tests for voltage after the stop lamp switch.