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Transmission fluid temperature sensor

Transmission fluid temperature sensor

The fluid temperature sensor, which is fitted in the transmission casing, supplies the control module with information on the current fluid temperature.

The sensor is NTC type, i.e. the integral sensor resistance is inversely dependent on the temperature. This means that the resistance decreases as the temperature increases.

Through an integral 1 kohm resistor, the transmission control module measures the temperature of the transmission fluid by applying +5 V to the sensor from pin 6.

The sensor is grounded at grounding point G34S.

In all variants, the control module uses temperature information for the following:

- For calculating when to shift and when to engage the lock-up function.

- At fluid temperatures below 0°C lock-up is not engaged

- At fluid temperatures below 0°C, the system pressure is raised by 3.3 bar for 5 seconds when shifting from N to D or N to R. At fluid temperatures between 0 and 20°C, the same function is implemented but the pressure increase is reduced linearly from 3.3 bar at 0°C to 0 bar at 20°C.

In T7 with bus, the control module uses temperature information for the following:

- At fluid temperatures of about 170°C, the TCM sends “HIGH TRANSMISSION FLUID TEMPERATURE ON” over the bus, and the corresponding message is then shown on the SID. When the fluid temperature has lowered and is below 140°C, “HIGH TRANSMISSION FLUID TEMPERATURE OFF” is sent over the bus, at which the SID message is extinguished and the function of the transmission returns to normal.


Diagnostics

A fault in the temperature sensor can generate the following 3 diagnostic trouble codes:


• If the temperature increases or decreases too slowly during the first 15 minutes of driving, diagnostic trouble code P0711 will be generated.

• If there is a short to B-, the diagnostic trouble code P0712 is generated.

• If there is an open circuit/short to B+ or open circuit, diagnostic trouble code P0713 is generated.



Temperature program 1

If the gearbox fluid temperature rises to 130°, lock-up will be engaged in 3rd and 4th gear. Engaging lock-up prevents slip in the torque converter, which makes the temperature drop. The shifting points are also changed so that upshifting takes place at a higher engine speed and downshifting at a lower engine speed than when using the normal program. Temperature program 1 does not work when:


• WINTER program is selected

• Limp-home is active

Disengaged when:

• temperature falls to 115 degrees


Temperature program 2

If the transmission fluid temperature rises to 145°C, lock-up will be activated in 2nd gear. This may happen, for example, when driving uphill towing a caravan. Activating lock-up prevents slip in the torque converter, which makes the temperature drop. Temperature program 2 will not work when:


• WINTER program is selected

• Limp-home is active

Disengaged when:

• temperature falls to 135 degrees


Temperature program 3

If the temperature rises to 150°C, the engine torque is reduced to 200 Nm. Temperature program 3 is disengaged when:


• temperature has decreased to 140 degrees and stable for 5 seconds

If these programs do not alleviate the problem and the temperature rises to 170 degrees, the "Info display" lamp lights up and the message TRANSMISSION OVERHEATING is displayed on the SID. Diagnostic trouble code P0218 can be read with Tech 2. If this trouble code is displayed, the transmission fluid must be changed.