Throttle Control
To ensure that the correct throttle angle is reached, the Engine Control Module (ECM) (4/46) controls the throttle shutter in the throttle unit (6/120), mainly using the signal from:
- accelerator pedal position sensor (7/51)
- the throttle position sensor on the electronic throttle unit.
Additional signals and parameters are used to ensure optimum throttle control. By example by compensating for:
- the load from the air conditioning (A/C) compressor
- the load from the transmission depending on the selected gear mode
- engine coolant temperature
- Mass Air Flow through the intake manifold
- Manifold Absolute Pressure in the intake manifold
The position of the throttle is measured by two potentiometers, in the throttle position sensor, which are on the throttle unit. These are connected, so that potentiometer 1 produces a higher voltage as the throttle angle increases, while potentiometer 2 does the opposite.
In a combustion engine there is a large difference between the minimum and maximum airflow and the smaller air flows need more thorough regulation. Therefore the signal from potentiometer 1 is amplified approximately 4 times in the control module before it reaches the AC/DC converter in the control module. This means that there are three, two real and one fictitious, input signals available to the control module. These signals are used to determine the position of the throttle and to deploy the damper motor to the correct position. In general the amplified signal is primarily used for small throttle angles (small air flows), which are desirable when a high degree of accuracy is required, for idle air trim for example.
Because the signal is amplified, it reaches its maximum value as early as approximately a quarter of maximum deployment.
The control module primarily uses the signal from potentiometer 1 as a measurement of throttle opening. The signal from potentiometer 2 is mainly to check that potentiometer 1 is working. The control module then uses the signal to calculate a throttle angle (actual value). This is the actual throttle angle. The value for the actual throttle angle is used by those functions in the control module which depend on this information so that the throttle can be correctly regulated.
There is an adaptation (learning) in the control module so that the control module can calculate how the damper motor needs to be controlled. See "Adaptation of the throttle unit" below.
The throttle angle is regulated so that the actual angle (actual value) is the same as the angle calculated by the control module (desired value). The control module also uses the values that were stored during adaptation of the throttle angle, and the actual signals from the potentiometers.
The damper motor is deployed by the integrated power stage in the control module using a Pulse Width Modulated (PWM) signal. The torsion from the opening and return springs in the electronic throttle unit is also used. If there is a fault in the control module so that the throttle unit cannot be operated or powered, the springs in the throttle unit will turn the throttle disc to the return position (limp home). This return position gives a throttle angle large enough to allow the car to be driven to a workshop, although with considerably reduced driveability.
Throttle angle
The throttle angle is usually gauged by potentiometer 1. For small angles the amplified signal is used to obtain a clearer signal. The Engine Control Module (ECM) also monitors the throttle unit signals from the potentiometers to check that they are plausible, that they are within the minimum and maximum limits and that the signals correspond to the same throttle angle. If there is a difference in the signals, a fictitious throttle signal is calculated from the load signal, the engine speed (RPM) and the prevailing conditions, particularly pressure and temperature.
The potentiometer whose signal is closest to the calculated throttle angle will then be assumed to be correct. The other potentiometer is then classified as not functioning and a Diagnostic Trouble Code (DTC) is generated. The system then constantly monitors the throttle angle of the remaining potentiometer in comparison to the calculated throttle angle. If there is a difference between these, the control module will not rely on any of the throttle unit potentiometers. The power stage in the throttle unit is then disengaged, and the throttle switches to the return sprung mode (limp-home).
Adaptation of the throttle unit
During run-on, i.e. just before the control module switches off, the Engine Control Module (ECM) carries out adaptation of the electronic throttle unit. Adaptation is carried out by the throttle disc being mechanically controlled to the closed position and the current throttle position being read off. If previous adaptation values are missing in the control module, for example, if the control module has not been powered, the current throttle angle is stored as the adaptation value. If, in addition, there is a previously stored value, the average value of the previous one and the current throttle angle is stored as the new adaptation value.
HINT: When replacing the electronic throttle unit, the Engine Control Module (ECM) must therefore always be switched off.