Allison 3000 & 4000 Gen 4 Fault Code: P2742 Retarder Oil Temperature Sensor Circuit Low 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 the TCM analog ground W158.

The resistance value of the retarder temperature sensor determines the voltage drop in the retarder temperature sensor circuit. As resistance changes, the voltage drop across the thermistor will also change which likewise varies the sensor input voltage on wire 175. When retarder fluid is cold the sensor resistance is high, which produces a large voltage drop across the temp sensor circuit. The TCM, therefore, detects a high sensor input voltage during cold conditions. As the retarder fluid temperature warms to normal operating temperature, the resistance decreases producing a smaller voltage drop across the temp sensor. As a result, the TCM detects a lower sensor input voltage on W175 during hot oil conditions. The TCM uses retarder temperature information to restrict retarder operation and thus reduce fluid temperature whenever a retarder over-heat 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, the 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 P2743.

CONDITIONS FOR SETTING THE DTC:
The TCM detects retarder oil temperature greater than 201°C (394°F) for more than 10 seconds.

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

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2742 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 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 warms-up cycles without failure.

DIAGNOSTICS AIDS
DTC P2742 may be caused by:
1. Incorrect transmission fluid fill.
2. W175 shorted to ground, or shorted to another wire in chassis harness.
3. Broken or defective connector assemblies that cause pins to short together.
4. Unlocked connectors, missing or damaged seals, missing seal plugs, that allow moisture into connectors.
5. Defective retarder TFT sensor.
6. Defective TCM.
7. Transmission fluid temperature in retarder reaching over 201°C (394°F) for 10 seconds.
8. Inadequate or obstructed vehicle cooling system.
9. Non-bypassed engine coolant thermostats that are not opening in time may cause overheat condition for transmission fluid temperature. Monitor retarder fluid temperature using diagnostic tool and review transmission overheat and retarder information in General Troubleshooting of Performance Complaints.

The retarder TFT sensor resistance is available on diagnostic tool Data Monitor grid. Prior to ‘Connect’ for diagnostic tool, check the ‘Troubleshooting Specific Problem’ box under the “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.

Refer to DTC P2740 and DTC P0218 or General Troubleshooting of Performance Complaints as guidelines for troubleshooting vehicle cooling issues.

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