1.Use the coolant temperature sensor wrench (MB992041) to remove the coolant temperature sensor.
2.Connect the removed sensor to the vehicle wiring harness wire, and then immerse the sensor into hot water.
3.Disconnect the cooling fan motor connector.
4.Turn off the air conditioner to make the READY (ready to drive) indicator illuminate.
5.Heat or cool the water where the coolant temperature sensor has been immersed while the water temperature does not reach 80°C.
6.When the voltage is measured between the connector terminals at the cooling fan controller side, the measured valued should alternate among (1), (2) and (3) as below: (Check both of the controllers 1 and 2)
(1) 0V
(2) 6.8 ± 2.6 V
(3) Battery voltage: ±2.6 V

7.Replace a defective cooling fan controller. (For how to install, refer to )