Allison 3000 & 4000 Gen 4 Fault Code: P2185 Engine Coolant Temperature Sensor Circuit High Input

DIAGNOSTIC RESPONSE:
Use default coolant engine values

CIRCUIT DESCRIPTION:
The Transmission Control Module (TCM) receives an input from an Engine Coolant Temperature (ECT) sensor. The TCM supplies a 5V reference voltage signal into a voltage-sensing network that is connected to one side of the ECT sensor on W135. The other side of the ECT sensor is connected to TCM analog ground on W158.

The resistance value of the ECT sensor determines the voltage drop in the ECT sensor circuit. As resistance changes, the voltage drop across the ECT sensor circuit also changes and varies the sensor input voltage on W135. The TCM uses engine coolant temperature information to restrict retarder operation when an engine coolant 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 and ECT sensor autodetected or forced present by CMC (Override Retarder Autodetect//Override Engine Coolant Source).
3. This test is disabled for this ignition cycle when a failure is reported for this DTC or DTC P2184.
4. The retarder reduction and preselect, based on engine coolant temperature feature, is enabled in the calibration.

CONDITIONS FOR SETTING THE DTC:
The TCM detects engine coolant temperature less than -45°C (-49°F) for more than 10 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2185 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 engine coolant 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 warm-up cycles without failure.

DIAGNOSTIC AIDS:
NOTE: If DTC P2185 is accompanied by DTC P0713 or P2743, check for short to battery on W135, W154 and W175. Follow DTC P0713 steps first, if in history with this DTC.

DTC P2185 may be caused by:
1. Incorrect transmission fluid fill.
2. W135 shorted to another wire in chassis harness.
3. W135 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 ECT sensor.
8. Defective TCM.

The ECT 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 “Connection Options” and then check the ‘Sensor, Voltage, Ohms, and Counts Data’ box.

Check if ECT 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.