This report presents the development of a dynamic temperature model for an automatic transmission in a Volvo Cars passenger vehicle. The model should simulate the oil to cooler temperature and flow from the transmission. A mathematical approach to use lumped masses for different parts of the transmission was used. To tune the response of the lumped masses and heat transfer coefficients; temperature measurements were done on a vehicle in a chassis dyno. To verify the model, simple drive cycles were performed with temperature measurement in the same chassis dyno and on the same vehicle. The verification on the model shows that the model can simulate the behavior of a transmission with an error of 2.5 °C during normal behavior and 6.5 °C for a few minutes when a sudden change in the temperature from the cooler have a large transient increase. Because of this, the model is considered to be fairly accurate. However, in order to make the model compatible with Volvo Cars existing simulation software, Vsim, a "cooler model" has to be created.