Allison 3000 & 4000 Gen 4 Fault Code: P2740 Retarder Oil Temperature Hot

DIAGNOSTIC RESPONSE:
None

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 temperature sensor on W175. The other side of the retarder temperature sensor 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 and vary the retarded temperature sensor input voltage on wire 175. When retarder fluid is cold, the retarder temperature sensor resistance is high, and produces a large voltage drop across the retarder temperature sensor circuit. The TCM, therefore, detects a high retarder temperature 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 temperature 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 reduce fluid temperature when 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, this DTC test runs at 0.25 second intervals.
2. Retarder autodetected or forced present by CMC (Override Retarder Autodetected).
3. This test is disabled for this ignition cycle when a failure is reported for this DTC, DTC P2742 or DTC P2743.
4. The TCM detects retarder oil temperature less than 201°C (394°F) for more than 10 seconds.

CONDITIONS FOR SETTING THE DTC:
Retarder TCT sensor signal for fluid temperature higher than 166°C (331°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 P2740 is active, the following conditions occur:
1. The TCM does not illuminate the CHECK TRANS light.
2. DTC is stored in TCM history.
3. Retarder command reduced.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
The DTC goes inactive and the TCM cancels the DTC active actions when the retarder TFT sensor signals that the fluid temperature is below 166°C (331°F) for 10 seconds or more.

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: DTC may occur in vehicles with history or DTC P2742. Follow the diagnostics steps in DTC P2742 before proceeding to the diagnostic steps for this DTC. Repairing the condition that sets DTC P2742 will likely eliminate this DTC.

DTC P2740 may be caused by:
1. Incorrect transmission fluid fill.
2. Defective retarder TFT sensor.
3. Defective TCM.
4. Transmission fluid temperature in retarder reaching over 166°C (331 °F) for 10 seconds.
5. Inadequate or obstructed vehicle cooling system.
6. 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 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.

Using the diagnostic tool, monitor the transmission fluid temperature to verify that it rises in cycles and then stabilizes.

Monitor and record snapshot fluid temperature.

Follow transmission overheat troubleshooting in General Troubleshooting of Performance Complaints for problems: OVERHEATING INDICATED BY DTCs P0218, P2740 OR FLUID CONDITION, and if equipped with a retarder follow problem RETARDER FEELS WEAK.

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