Allison 3000 & 4000 Gen 4 Fault Code: P0703 Brake Switch Circuit

DIAGNOSTIC RESPONSE:
No Neutral to Drive shifts for refuse packer. TCM inhibits retarder operation if a TPS code is also active.

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) can be calibrated to receive a Service Brake Status (SBS) input from either an analog input wire or the digital datalink. An Original Equipment Manufacturer (OEM) switch is attached to the brake pedal and sends a signal to either the TCM directly or another electronic controller in the vehicle. When another controller is used, the TCM typically receives SBS as a datalink message. However, other vehicle controllers could be responsible for switching the input wire to the TCM as well, such as a programmable controller in some bus applications. The SBS is a required or optional input for several transmission features and functions such as the retarder and several I/O functions. These features and functions may cause range inhibits or alter their normal feature characteristics when this code is active.

In OEM 4th Generation Controls, the preferred digital datalink is the SAE J1939 Controller Area Network (CAN). The TCM communicates with the Engine Control Module (ECM) and other controllers by exchanging standardized digital messages over SAE J1939. The physical network consists of a two-wire twisted pair, two 120 Ω termination resistors and, in most cases, a third shielded wire. A 120 Ω termination resistor is located at each end of the network. Vehicle OEMs may choose to install external termination resistors or use internal termination resistors built into many SAE J1939 electronic modules, including the TCM and shift selector. Refer to the OEM Technician’s Library for additional information.

CONDITIONS FOR RUNNING THE DTC:
1. TCM initialized normally and engine speed is above 200 rpm.
2. For vehicles requiring analog SBS input, proper SBS input function must be enabled.
3. For vehicles requiring SBS message, acceptable OEM source address must be autodetected or enabled in calibration.

CONDITIONS FOR SETTING THE DTC:
DTC P0703 sets if the TCM is calibrated to receive a SBS signal and/or SBS datalink message and either of the following conditions is met:
1. The TCM senses three acceleration events with SBS signal/message ON (brake pedal pressed).
2. The TCM senses three deceleration events with SBS signal/message OFF (brake pedal released).

ACTION TAKEN WHEN THE DTC SETS:
1. The TCM does not illuminate the CHECK TRANS light.
2. MY 09 and later illuminates the service indicator in the shift selector.
3. DTC is stored in TCM history.
4. Various features and/or ranges may be inhibited when a valid SBS state is not detected by the TCM. The range inhibit light will illuminate when range is inhibited.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
Use the diagnostic tool 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:
This DTC may be caused by:
1. Malfunctioning brake system control components such as:
– Miswired or defective brake system wiring.
– Defective brake pressure switches.
2. OEM controller hardware or software issue with body, engine, or brake systems controller (e.g., ABS/ATC/ASR, etc.).
3. OEM controller enabled for SBS input, but not wired for SBS input.
4. OEM SAE J1939 datalink integrity issues such as:
– Wiring shorted to ground.
– Wiring open circuit or high resistance.
– Shielding problems.
– Termination resistors missing, defective, or unplugged.
– Electrical noise issues.
– Bus loading issues.
5. Defective TCM.

When reviewing Data Bus Viewer logs, determine which Source Address (SA) brake switch status message is being received by the TCM. SAE J1939 EBC1 and CCVS Brake Switch messages may be received from several source addresses. The TCM locks on the most preferred, even if the message sent by that controller is ‘error’. EBC1 Brake switch messages have preference over CCVS during the SA autodetect process. Reset Autodetect to reset the SA autodetect process.

SBS message status must be monitored using the Data Bus Viewer. The “Brake Pedal Depressed” status is displayed when the service brake is applied. The “Brake Pedal Released” is displayed when the brake is not applied. Verify EBC1 Brake Switch status is shown as “Not Available” before assuming brake switch is being received by the TCM from a CCVS SA.

If the TCM calibration is configured to receive both an analog (hardwire input) vehicle service brake signal (e.g., W162) as well as an SAE J1939 SA SBS message, then the TCM will consider the brake as depressed either when the wire input is in an active ON state or the autodetected brake pedal status message of “Brake Pedal Depressed” is being sent and received. Refer to OEM Datalink Tech Data, the OEM Technician’s Library when troubleshooting brake status related complaints (such as issues with I/O, retarder apply, or DTCs) to determine if either the wired input or the controller message for SBS can be disabled to correct the complaint.

When a controller broadcasts an SAE J 1939-based brake switch parameter, which has been autodetected as the SBS source by the TCM, it is considered an SBS input for the transmission controls and I/O even if the vehicle OEM intended for SBS to only be available to the TCM as an analog only input. Datalink tailoring may be required for some applications to prevent this interaction.

Some controllers might erroneously broadcast a continuous SAE J1939 Brake Switch status message of “Brake Pedal Depressed” if the OEM does not hardwire the brake switch input to that controller. If the OEM brake switch circuit has an issue or the programmable CMC for the controller is not set correctly (e.g., for the brake switch input on the engine TCM) then the SBS will be considered in a stuck active state after several acceleration/deceleration cycles, and the TCM sets DTC P0703.

Not all OEM input functions incorporating Service Brake Status may substitute a SAE J1939-based input for the analog input. The functions that do not allow an SBS datalink message (see information below) must use an analog input for SBS. Refer to the OEM Technician’s Library for additional information. Features and/or ranges may be inhibited when a valid SBS state is not detected by the TCM.

OEM Input Function:
1. E Auxiliary Function Range Inhibit – Single Input (AFRI)
– SBS Datalink Message Allowed

2. AA Service Brake Status
– SBS Datalink Message Allowed

3. CA Brake Based Auto-Neutral For PTO (BBAN)
– SBS Datalink Message Allowed

4. W Direction Change Enable
– SBS Datalink Message Allowed

5. Z Retarder Enable (see Driveline Retarder Control)
– SBS Datalink Message Allowed

6. AK Automatic Neutral – Dual Input with Service Brake Status
– SBS Datalink Message Allowed

7. AL Shift Selector Transition/Secondary Shift Schedule
– SBS Datalink Message NOT Allowed

8. AS Reduced Engine Load at Stop (RELS)
– SBS Datalink Message NOT Allowed

9. BB RELS with Service Brake Status
– SBS Datalink Message NOT Allowed

Vehicle manufacturers may wire the TCM into the CAN 1 backbone in three different ways:
1. The TCM may be on its own stub as in traditional CAN backbones.
2. The TCM may be wired in a pass-through configuration such that the CAN high and low wires are connected to two separate pin pairs in the TCM 80-way connector. Datalink messages pass through but can still be viewed by the TCM.
3. The TCM may represent one end of the backbone. Typically, the internal resistor in the TCM will be used in this setup.

CAN1 and CAN2 devices must not be interconnected because they are incompatible. CAN1 operates at 250 kbps data rate and CAN2 operates at 500 kbps data rate.

For proper datalink communications, it is necessary to have two 120 Ω resistors installed in parallel across the CAN1 High and CAN1 Low terminals. Resistance across CAN1 High and CAN1 Low should measure 60 Ω when wired correctly.

Review applicable information in the OEM Technician’s Library, Control System and Transmission Specifications and other related information.