Allison 3000 & 4000 Gen 4 Fault Code: P2743 Retarder Oil Temperature Sensor Circuit High Input

DIAGNOSTIC RESPONSE:
Use default retarder temperature values

CIRCUIT DESCRIPTION:
A retarder Transmission Fluid Temperature (TFT) sensor monitors retarder cavity fluid temperature. The sensor consists of a thermistor that varies its resistance value based on the temperature of the fluid in the retarder housing. The Transmission Control Module (TCM) supplies a 5V reference voltage signal into a voltage-sensing network that is connected to one side of the retarder TFT sensor on W175. The other side of the retarder TFT sensor is connected to the TCM analog ground W158.

The resistance value of the retarder TFT sensor determines the voltage drop in the retarder TFT sensor circuit. As resistance changes, the voltage drop across the thermistor will also change and vary the retarder TFT sensor input voltage on wire 175. When retarder fluid is cold, the retarder TFT sensor resistance is high, and produces a large voltage drop across the retarder TFT sensor circuit. The TCM, therefore, detects a high retarder TFT sensor input voltage during cold conditions. As the retarder fluid temperature warms to normal operating temperature, the resistance decreases and produces a smaller voltage drop across the retarder TFT sensor. As a result, the TCM detects a lower retarder TFT sensor input voltage on W175 during hot oil conditions. The TCM uses retarder temperature information to restrict retarder operation and reduce fluid temperature when a retarder overheat condition is detected.

CONDITIONS FOR RUNNING THE DTC:
1. After TCM initializes normally and engine speed is above 200 rpm for more than 5 seconds, this DTC test runs at 0.25 second intervals.
2. Retarder autodetected or forced present by CMC (Override Retarder Autodetect).
3. This test is disabled for this ignition cycle when a failure is reported for this DTC or DTC P2742.

CONDITONS FOR SETTING THE DTC
The TCM detects retarder oil temperature less than -45°C (-49°F) for more than 10 seconds.

NOTE: The TCM is detecting voltage on W175. An unusually high input voltage implies high thermistor resistance, which corresponds to a low retarder oil temperature measurement.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2743 is active, the following conditions occur:
1. The TCM does not illuminate the CHECK TRANS light.
2. DTC is stored in TCM history.
3. TCM uses default retarder temperature values.

CONDITIONS FOR CLEARING THE DTC/CHECK TRAN 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:
NOTE: If DTC P2743 is accompanied by DTC P0713 or P2185, check for short to battery on W135, W154 and W175. Follow DTC P0713 steps first, if in history with this DTC.

DTC P2743 may be caused by:
1. Incorrect transmission fluid fill.
2. W175 shorted to another wire in chassis harness.
3. W175 open circuit in chassis harness.
4. W158 open circuit in chassis harness.
5. Broken or defective connector assemblies that cause pins to short together.
6. Unlocked connectors, missing or damaged seals, missing seal plugs that allow moisture into connectors.
7. Defective retarder TFT sensor.
8. Defective TCM.

The retarder TFT sensor resistance is available on the diagnostic tool Data Monitor grid. Prior to ‘Connect’ for diagnostic tool, check the ‘Troubleshoot Specific Problem’ box under “Connection Options” and then check the ‘Sensor, Voltage, Ohms, and Counts Data’ box.

Check if retarder TFT sensor resistance is increasing, decreasing, or stationary with fluctuations in fluid temperature. Normal circuit operation is indicated if sensor resistance decreases as fluid temperature increases, and sensor resistance increases as fluid temperature decreases.

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