Service Information: Coolant thermostat

S20.10-4566-06

Please remember that with regard to the above-mentioned vehicles there have been several instances recently of coolant thermostat replacements because of the following customer complaints:

 Coolant temperature too low, engine fails to reach operating temperature

 Poor heater output, as a result vehicle interior is not sufficiently heated,

 etc.

However, no fault could be found on the removed coolant thermostats (function, material or manufacturer fault).

Therefore, to support the investigation of causes - until further notice - a TIPS case should be created up to headquarters for all repair operations conducted for cases with the above-mentioned customer complaints (where available). The document SI01.10-S-0001A must be referred to here. The analysis sheet OF20.10-S-0001-01A must be filled out and attached to the TIPS case.

 The title of the TIPS case must be "Coolant thermostat for engine 651".

The expenditures for filling out the analysis sheet OF20.10-S-0001-01A and for creating the TIPS case can be invoiced within the implied warranty/warranty and goodwill period under operation number 02-3952 (model 447) or 02-8522 (model 906). To do so, use damage code 20102 52 (Use of coolant thermostat – does not operate correctly).

 The coolant thermostat heating element (R48/2) must not be removed from the thermostat housing (1).
Otherwise, correct operation of the coolant thermostat function can no longer be guaranteed.

 The coolant temperature sensor (B11/17) can (if necessary) be removed from the coolant thermostat housing (1). In this case, the sealing ring must always be removed.
Otherwise, the leak tightness of the coolant circuit cannot be guaranteed.

 When removing / installing the coolant thermostat the corresponding WIS documentation (for model 906, document AR20.10-D-2460WE or for model 447, document AR20.10-S-2461E) must be referred to.

Coolant thermostat task

The coolant thermostat conducts map-controlled regulation of the coolant temperature in a range from approx. 55 °C to 106 °C. As from approx. 106 °C coolant temperature, the coolant thermostat is fully opened irrespective of the closed-loop control.

Coolant thermostat function

The function of the coolant thermostat is subdivided into the following partial functions:

 Short-circuit operation position (A)

 Mixed fuel operation position (C)

 Radiator operation position (C)

 The orifice area of the coolant thermostat is determined by the duty cycle of the electrical current. The limp-home function ensures that the coolant thermostat is fully opened as from approx. 106 °C, irrespective of actuation.

Partial functions of coolant thermostat

Short-circuit operation position (A)

Temperature up to 55 °C: Coolant flow in engine only, the passenger compartment heating can be flowed through.

Mixed fuel operation position (C)

Temperature as from 55 °C to 106 °C: Coolant thermostat opens, coolant flow through cooler starts.

Radiator operation position (C)

Temperature as from 106 °C: Coolant thermostat open, full cooler flow through.

Advantages of closed-loop control

By regulating the coolant temperature the following benefits result:

 Engine operating temperature is reached quicker

 Emissions reduced

 Improved heating comfort