Allison 1000 & 2000 Gen 5 Fault Code:P0703 Brake Switch Circuit

CIRCUIT DESCRIPTION:
Service brake status is relayed to the TCM by a mechanical switch, or over the J1939 datalink indicating the service brake has been applied. Input wire 162 is used to send a positive signal to the TCM. DTC P0703 is only triggered when input wire 162 is being used for the brake switch and when the number of vehicle accelerations with brake switch “ON” exceeds a calibration value or when the number of vehicle decelerations to a stop with the brake switch “OFF” exceeds a calibration value. This code will not be triggered if the Service Brake Status Input is broadcast on J1939 datalink.

ACTION TAKEN WHEN THE DTC SETS:
1. The TCM forces Variable Modulated Main off.
2. The CHECK TRANS light is not illuminated.
3. No grade braking is allowed.
4. Use the default assumption that the brake is off.
5. Neutral at Stop is disabled (if equipped).

CONDITIONS FOR CLEARING THE DTC:
The diagnostic tool may be used to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
The following procedures are not documented in any OEM procedures or in any OEM Troubleshooting or Service information and should not be used as a method to diagnose any transmission DTCs, function, or shift quality concern:

1. Back-probing any connections used for transmission features or functions may damage and/or unlock terminals from the back-probed connector creating permanent or intermittent shorts and/or open circuits. If possible, use the J-39700 Breakout Box, the appropriate harness adapters, and appropriate magnetic overlays to troubleshoot the vehicle.

2. Load-testing any transmission-related circuits with any other electrical devices such as vehicle lamps or relays, especially with the TCM connected to the harness. Use J-39700 Breakout Box and appropriate harnesses with the diagnostic tool to monitor the circuit performance in question unless otherwise specified in the various Troubleshooting information.

3. Piercing a wire to check for voltages, shorts-to-grounds or other wires anywhere in the circuit but especially at the TCM. This creates a leak path for moisture and damages the wire and insulation.

4. DTC P0703 indicates the TCM did not see the proper positive input signal for service brake status during an acceleration or deceleration cycle. This may indicate an open or short at the brake switch.

Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

Service brake status can be on either a hardwired brake switch or on the datalink. Use the diagnostic tool to determine how the brake status is communicated.

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 fault.

Currently, this DTC is specific to vehicle applications that use grade braking feature only.