Allison 3000 & 4000 Gen 4 Fault Code: P0990 Retarder Pressure Sensor Failed High

DIAGNOSTIC RESPONSE:
None

CIRCUIT DESCRIPTION:
Some transmission retarder applications may use an optional retarder pressure sensor (also called Electronic Braking System (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 EBS. Electronic Braking Systems typically use service brakes, engine brakes, and driveline retarders (like the retarder) for an 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 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 P0990 not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0990 sets when signal voltage on W144 is more than 4.5V for longer than 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0990 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 P0990 indicates the TCM has detected a voltage signal from the retarder pressure sensor in the high error zone and may be caused by the following:
1. Retarder pressure sensor set in calibration for a non-retarder pressure sensor transmission application.
2. Defective retarder pressure sensor.
3. Defective retarder pressure sensor sharing 5V supply.
4. W144 shorted to battery.

NOTE: Short to battery in W144 (retarder pressure sensor signal) only causes DTC P0990. Short to battery in W112 or open in W158, may cause other sensor DTCs like P070D, and/or C1313.

5. Broken or defective connector assemblies allowing pushed back pins to short together.
6. Unlocked connectors, missing seal plugs allowing moisture in connectors.
7. W158 open circuit.
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.