Transmission Oil Cooling With Transmission Oil Heat Exchanger, Function - GF27.55-P-2008ANI

Transmission 725.0 in model 253 

Shown on cooling circuit transmission oil in engine 651 

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Circuit 87 ON
• Engine running
• Threshold for transmission oil temperature exceeded

Oil cooling 

To keep the operating temperature of the automatic transmission constant in a temperature range adapted to the respective operating state, the oil cooling is regulated as required.

The transmission oil heat exchanger is designed as a plate-type oil cooler. Its operating principle means it can be kept small and provide high efficiency when cooling the transmission oil.

The transmission oil is pumped by the oil pump (primary pump) from the automatic transmission to the transmission oil heat exchanger. The transmission oil heat exchanger forms the interface between the transmission oil and the coolant circuit. The waste heat of the transmission oil is absorbed by the coolant within the transmission oil heat exchanger (plate-type oil cooler).

The transmission oil flows from the transmission oil heat exchanger back to the automatic transmission.

Regulation of transmission oil circuit 

The transmission oil feed from the transmission to the transmission oil heat exchanger is regulated by the oil thermostat. The oil thermostat is bolted to the transmission oil heat exchanger and opens or closes the supply to the transmission oil heat exchanger depending on the current transmission oil temperature (stroke start at approx. 80°C, maximum stroke as of approx. 90°C). When closed (cold-start phase), the transmission oil in the small circuit is quickly brought to operating temperature inside the automatic transmission.

When open, the flow of transmission oil is routed via the transmission oil heat exchanger (large circuit) in order to cool the transmission oil.

Control of coolant inflow 

The low-temperature circuit circulation pump 1 (M43/6) (except engine 177.9) or low-temperature circuit circulation pump 3 (M43/8) (engine 177.9) is actuated as and when required to match the transmission oil temperature. The transmission oil temperature is recorded by the transmission oil temperature sensor (Y3/8s2) and passed on by the fully integrated transmission control unit (Y3/8n4) via the drive train CAN (CAN C1) to the powertrain control unit (N127). This compares the input signals against a threshold value filed in the characteristics map and actuates the low-temperature circuit circulation pump 1 (except engine 177.9) or the low-temperature circuit circulation pump 3 (Motor 177.9) and the low-temperature circuit switchover valve (Y73/1) (engine 177.9) to match the evaluated input signal over the drivetrain LIN (LIN C3).