Allison 3000 & 4000 Gen 4 Fault Code: P0989 Retarder Pressure Sensor Failed Low

DIAGNOSTIC RESPONSE:
None

CIRCUIT DESCRIPTION:
Some transmission retarder applications may use an optional retarder pressure sensor (also called EBS pressure sensor, and/or retarder pressure transducer). The retarder pressure sensor provides an analog voltage input to the TCM proportional to the retarder charge pressure. The retarder pressure sensor signal is monitored by TCM software and is used to calculate the delivered retarder torque. The calculated torque message is broadcast by the TCM over the Controller Area Network (CAN) for use by other vehicle controllers located on the CAN. For example, this feature is used when the vehicle has an Electronic Braking System (EBS). EBSs typically use service brakes, engine brakes, and driveline retarders (like the OEM retarder) for the optimal brake combination to slow or stop a vehicle.

NOTE: Refer to Technical Document (TD) 182 on the OEM Extranet for the description and requirements of an EBS.

NOTE: The retarder pressure sensor interface uses the same three TCM pin locations and wire numbers as those assigned to an analog TPS. Therefore, a retarder pressure sensor cannot be integrated on a vehicle with an analog or PWM TPS source.

NOTE: The retarder pressure sensor can potentially be damaged by continuity testers or Digital Multimeters (DMM) set to check continuity. Check voltages only if troubleshooting retarder pressure sensor performance.

The TCM is connected to the retarder pressure sensor by:
1. a 5V reference voltage (W112),
2. a retarder pressure sensor signal (W144), and
3. an analog ground (W158).

CONDITIONS FOR RUNNING THE DTC:
1. The retarder pressure sensor is enabled by calibration.
2. After TCM initializes normally and engine speed is above 200 rpm for more than 5 seconds, this DTC test will run at 0.10 second intervals.
3. DTC P0989 not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0989 sets when W144 signal voltage is less than 0.125V for more than 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0989 is active, the following conditions occur:
1. The TCM does not illuminate the CHECK TRANS light.
2. DTC is stored in TCM history.
3. The CAN message containing retarder torque data is broadcast as “Not Available” (FE hex or 254 decimal).

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 failure.

DIAGNOSTIC AIDS:
DTC P0989 indicates the TCM has detected a voltage signal from the retarder pressure sensor in the low error zone and may be caused by the following:
1. Open or short to ground in W112 or W144 circuits.

NOTE: Shorts or opens in W112 will typically log other sensor failed low DTCs that are using W112 (5V), such as the Oil Level Sensor (OLS) or Retarder Modulation Request (RMR) Sensor. Shorts or opens in W144 (retarder pressure sensor signal) only causes DTC P0989.

2. Retarder pressure sensor set in calibration for a non-retarder pressure sensor transmission application.
3. Missing, defective, or unplugged retarder pressure sensor.
4. Defective retarder pressure sensor sharing 5V supply.
5. 5V wire open or grounded in transmission internal harness.
6. Broken or defective connector assemblies allowing pushed back pins to short together.
7. Unlocked connectors, missing seal plugs allowing moisture in connectors.
8. TCM issue.

Checking the ‘Autodetected and SEM Information’ Data Grid on the diagnostic tool for the ‘Retarder Pressure Transducer Status’ will display whether the retarder pressure sensor is enabled or not enabled in the calibration. ‘Present’ means enabled, ‘Not Present’ means not enabled.

Review applicable information in Control System and Transmission Specifications to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.