Allison 3000 & 4000 Gen 4 Fault Code: P0712 Transmission Fluid Temperature Sensor Circuit Low Input

DIAGNOSTIC RESPONSE:
Use default sump temperature

CIRCUIT DESCRIPTION:
The Transmission Fluid Temperature (TFT) sensor is incorporated into the internal wiring harness. The TFT sensor is thermistor (thermally sensitive resistor) that changes its resistance value based on the temperature of the transmission fluid. The Transmission Control Module (TCM) supplies a 5V reference voltage signal to a reference resistor inside the TCM that is in series with the TCT sensor. The voltage drop across the TCT sensor is constantly changing as the TFT sensor resistance varies due to the change in transmission fluid. When the transmission is cold, the sensor resistance is high and the TCM detects high signal voltage on W154. As the transmission fluid temperature warms, the resistance of the TCT sensor decreases and the signal voltage on W154 also decreases. The TCM uses this voltage on W154 to calculate transmission fluid temperature and optimize shift quality.

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 will run at 0.25 second intervals.
2. If engine coolant temperature sensor is present and engine has not been above 200 rpm for more than 10 minutes, engine coolant temperature must be above 20°C (68°F) for more than 20 seconds.
3. This test will not run in current ignition cycle with active DTCs P0712 or P0713.

CONDITIONS FOR SETTING THE DTC:
The TCM detects transmission fluid temperature greater than 160°C (320°F) for more than 2.5 seconds.

NOTE: The TCM is detecting voltage present on W154. An extremely low input voltage implies low thermistor resistance, which corresponds to an unusually high transmission fluid temperature measurement.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0712 is active, the following conditions occur:
1. The CHECK TRANS light is illuminated.
2. DTC is stored in TCM history.
3. TCM uses engine coolant temperature or default temperature calibration value for transmission fluid temperature.

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 forty warm-up cycles without failures.

DIAGNOSTIC AIDS:
DTC P0712 can be caused by:
1. Incorrect transmission fluid.
2. W15 shorted to ground or to another wire in chassis harness.
3. W154 shorted to ground or to another wire in the internal transmission harness.
4. Broken or defective connector assemblies allowing pushed back pins to short together.
5. Unlocked connectors, missing seal plugs allowing moisture in connectors.
6. Defective TFT sensor.
7. Defective TCM.
8. Transmission fluid temperature in control valve module reaching over 160°C (320°F) for 2.5 seconds. Monitor fluid temperature using the diagnostic tool and review transmission overheat information in General Troubleshooting of Performance Complaints.

TFT sensor resistance is available on the diagnostic tool data monitor grid. Prior to ‘Connect’ for the diagnostic tool, check the ‘Troubleshoot Specific Problem’ box under the “connection options” and then check the ‘Sensor, Voltage, Ohms, and Counts Data’ box.

Check if 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.