Thermostat: Description and Operation

Characteristic Map Thermostat

Characteristic map thermostat
The characteristic map thermostat is attached to the housing of the coolant pump.
The coolant temperature influences:
- Fuel consumption
- Power output
- Quality of the mixture formation
- Pollutant emissions
- Mechanical load on components

Optimisation of these variables does not permit a fixed temperature with the different engine speed and load states.
Optimisation requires a temperature range that corresponds to each operating point.
The characteristic map thermostat enables convergence towards the optimal temperature.

Functional description
Input variables for calculation by the engine management system are:
- Engine speed
- Load
- Road speed
- Intake air temperature
- Coolant temperature

The engine management system uses the above input variables to calculate the optimal coolant temperature for each operating point. The optimal coolant temperature is influenced by specific heating of the wax element in the characteristic map thermostat as well as activation of the electric fan according to requirements. At full load, lower coolant temperatures improve the cylinder filling. Moreover, lower engine temperatures reduce the risk of knocking. The power output and torque can thus be positively influenced.







A heater resistor is located in the wax element of the characteristic map thermostat. The engine management system supplies the heater resistor with current. This expands the wax element and closes the inlet of the cylinder head using spring pressure.
The spring has the task of pushing the characteristic map thermostat back into the rest position when the wax element cools down.
The cooling circuit with a cold engine passes via the inlet of the cylinder head via the characteristic map thermostat to the return line to the coolant pump.

Structure and inner circuit
The characteristic map thermostat is connected by means of a 2-pin plug-in connection.
The characteristic map thermostat is supplied with 12 Volts vehicle voltage by the engine management system. Activation by the engine management system is on the earth side.







Temperature characteristics and target values
The engine management system activates the heating element via a characteristic map depending on the current driving situation.







Observe the following target values for the coolant-temperature sensor:

Variable Value
Voltage range 9 to 16 Volts
Opening temperature, unheated 97 °C
Minimum stroke 8 mm
Minimum stroke in the control range 4.5 mm
Maximum output current 20 mA
Temperature range -40 °C to 150 °C

Diagnosis instructions

Failure of the component
If the characteristic map thermostat fails, the following Behavior is to be expected:
- Fault code memory entry in the engine control unit
- Conventional operation via wax element
- US version: The emissions warning lamp lights up in the instrument cluster

No liability can be accepted for printing or other errors. Subject to changes of a technical nature