Allison 1000 & 2000 Gen 4 Fault Codes: P0712 Transmission Fluid Temperature Sensor Circuit Low Input (High Temperature)

CIRCUIT DESCRIPTION:
The Transmission Fluid Temperature (TFT) sensor is part of the Pressure Switch Manifold (PSM) which is located in the transmission oil pan. The TFT sensor is a thermistor that changes its resistance value based on the temperature of the transmission fluid. The Transmission Control Module (TCM) supplies a 5V reference to the thermistor. The TCM’s internal impedance provides resistance at the TCM so that voltage drop can be measured between the TCM and the thermistor. When the transmission is cold, the sensor resistance is high and the TCM detects high signal voltage. As the transmission fluid temperature warms to a normal operating temperature, the resistance becomes less and the signal voltage decreases. The TCM uses this information to control shift quality and to determine torque converter clutch applies.

If the TCM detects a continuous short to ground in the TFT sensor or signal circuit, DTC P0712 is set.

CONDITIONS FOR RUNNING THE DTC:
1. No TFT sensor DTC P0711 or P0713.
2. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
3. The engine is greater than 200 rpm for more than 5 seconds.
4. The engine has been running for 20 seconds with a coolant temperature above 20°C (68°F).

CONDITIONS FOR SETTING THE DTC:
DTC P0712 sets when the TFT sensor indicates a voltage of less than 313mV for 2.5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
1. The TCM does not illuminate the CHECK TRANS light.
2. The TCM stores DTC P0712 in TCM history.
3. TCM uses calibration default for temperature values.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
1. A Scan Tool may be used to clear the code from the TCM history.
2. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.
3. The TCM cancels the DTC default actions when the fault no longer exists and the ignition has been cycled.

DIAGNOSTIC AIDS:
Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

A short to ground on wire 210 may allow a code P0712 to set. A default value of 160°C (320°F) TFT on the Scan Tool would be a good indication this has occurred.