It is not an unknown phenomenon that car owners with DSG transmissions (Direct Shift Gear) periodically experience strongly fluctuating cooling water temperatures and lack of cabin heat. However, it is very rare that the condition / error can be recreated in the workshop. Here is a tip, what could be wrong.
Vehicles with this type of transmission have a separate transmission oil thermostat. The problem arises in situations where, for example, you drive downhill for a long time with the gear and transmission engaged, but without engaging the accelerator pedal. In such situations, both the cooling water and transmission oil temperatures may fall below the normal operating temperature.
The thermostat for regulating the oil temperature in the gearbox is controlled by the cooling water circuit in the vehicle and the heat of the DSG gearbox oil is transferred to the cooling water via a heat exchanger. If the thermostat does not work properly - and remains in the open position, for example - the transmission oil - but also the engine - will reach its normal operating temperature much more slowly.
Illustration of system design. The thermostat (8620 48775 or 8620 48780) is located on the inlet hose for the heat exchanger
8620 48775/8620 48780
Important! Observe the installation direction (arrow) when replacing the thermostat
IMPORTANT! Modern cooling circuits are very complex systems and must therefore be repaired carefully. Assuring good ventilation of the system is of particular importance.