Throttle Control

Throttle Control







To ensure that the correct throttle angle is reached, the engine control module (ECM) controls the throttle shutter in the throttle unit (6/120), mainly using the signal from:
- accelerator pedal (AP) position sensor (7/51)
- clutch pedal sensor (7/123)
- stop lamp switch (3/9)
- the throttle position (TP) sensor on the electronic throttle unit (6/120)
- brake control module (BCM) (4/16), signal for the brake pedal sensor.

Additional signals and parameters are used to ensure optimum throttle control. For example there is compensation for the load from the air conditioning (A/C) compressor, load from the transmission depending on the gear selected (automatic), engine temperature etc. In cars with Four-C (Continuously Controlled Chassis Concept), the throttle characteristic changes depending on the driving mode selected.

The position of the throttle is measured by two potentiometers, in the throttle position (TP) 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, the difference between the minimum and maximum airflow is considerable. The smaller air flows need more thorough regulation, so the potentiometer signal from potentiometer 1 is amplified approximately 4 times in the engine control module (ECM) before it reaches the Analog/Digital converter in the engine control module (ECM). This means that there are three, two real and one fictitious, input signals available to the engine control module (ECM). 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 engine control module (ECM) first uses the signal from potentiometer 1 to measure the throttle opening. The signal from potentiometer 2 is mainly used to check that potentiometer 1 is functioning correctly. The engine control module (ECM) 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 engine control module (ECM) which depend on this information so that the throttle can be correctly regulated.

There is an adaptation (learning) in the engine control module (ECM) so that the control module can calculate how the damper motor needs to be controlled. See "Adaptation of the throttle unit" below. This adaptation is carried out during manufacture of the car, when the engine control module (ECM) deploys the throttle disc to the different positions and reads off and registers the actual values from the potentiometers.

The throttle angle is regulated so that the actual angle (actual value) is the same as the angle calculated by the engine control module (ECM) (desired value). The engine control module (ECM) also uses the values that were stored during adaptation of the throttle angle, and the actual signals from the potentiometers.

The damper motor is controlled by a power stage integrated in the engine control module (ECM) using a pulse width modulation (PWM) signal. The torsion from the opening and return springs in the throttle unit is also used. If there is a fault in the engine control module (ECM) so that the throttle unit cannot be operated or supplied with power, the springs in the throttle unit will turn the throttle disc to the limp home position (return position). This return position gives a throttle angle large enough to allow the car to be driven to a workshop, although with considerably reduced drivability.

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 stored. 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 values, the engine control module (ECM) 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 limp home mode (return position).

Adaptation of the throttle unit
Normally the engine control module (ECM) adapts the throttle unit during manufacture of the car. There are a number of other occasions when the throttle unit must be adapted. A new adaptation is obviously required if the throttle unit or the engine control module (ECM) is replaced, but also if the software in the engine control module (ECM) is replaced. This is because the adaptation values are stored in the EEPROM memory in the control module, at a predetermined location in the memory for each software. These storage locations may vary between different software, in which case a new adaptation will be required. The final reason for adaptation is that the values for the throttle unit may have changed during its service life. Therefore there is a routine which begins adaptation of the throttle unit if the car is left with the engine off and the throttle on for 30 seconds (and other basic conditions for adaptation of the throttle unit are met). A new adaptation is then carried out. If the "new" values are different from the "old" values, the new values are used. Adaptation usually takes place automatically when the engine control module (ECM) detects that the adaptation has not been run or that the values from a previous adaptation are outside certain tolerances. However a number of basic conditions must be met to ensure that the adaptation is carried out correctly:

Basic conditions for adaptation of the throttle unit:
- ignition on/engine off
- battery voltage above 10 V
- accelerator pedal (AP) released (unaffected)
- the car must be stationary (no speed signal)
- engine coolant temperature (ECT) between 5 and 100 degree C
- intake air temperature (IAT) above 5 degree C.

During adaptation, the engine control module (ECM) checks that the values obtained are within reasonable limits. This helps to determine whether the throttle unit is functioning correctly or not. When the adaptation has been carried out, all relevant values are stored in the EEPROM memory in the control module so that they can be retrieved if the power supply is broken. This means that if the battery is replaced for example, a new adaptation is not necessary.