Part 2

Function

Ignition control





The following components are used for ignition control:
- engine speed sensor (7/25)
- camshaft sensor (7/172-173)
- mass air flow (MAF) sensor (7/17)
- engine coolant temperature (ECT) sensor (7/16)
- throttle position (TP) sensor on the electronic throttle unit (6/120)
- knock sensor (KS) (7/23-24)
- transmission control module (TCM) (4/28)
- spark plugs with ignition coils (20/3-8).
The engine control module (ECM) calculates the optimum ignition advance based on the software and information from the sensors. The engine control module (ECM) cuts the current to the ignition coil mounted on the cylinder to be ignited and produces a spark.
During the starting phase the engine control module (ECM) produces a fixed ignition setting. When the engine has started and the vehicle is being driven, the engine control module (ECM) calculates the optimum ignition setting, taking factors such as the following into account:
- engine speed
- load
- temperature.
The engine control module (ECM) analyses the signal from the knock sensors (KS) when the engine reaches operating temperature. If any of the cylinders knock, the ignition is retarded for that specific cylinder until the knocking ceases.
The ignition then advanced to the normal position or until the knock recurs.
Before the Transmission control module (TCM) is going to shift, sometimes it sends a request for torque limitation to the Engine control module (ECM). Which then lowers the ignition momentarily to reduce the torque and thus give smoother shifting and reduced load on the transmission.
Lowering of ignition can be done in several levels, where the levels depend on the signals from the Transmission control module (TCM). The return signal from the Engine control module (ECM) to the Transmission control module (TCM) confirms that the signal reached the Engine control module (ECM).
For further information, also see: Misfire diagnostics, B6304T4 Misfire Diagnostics
The engine misfires if the fuel does not ignite correctly. For further information, also see: Misfire diagnostics, B6304T4 Misfire Diagnostics

Regulating the air conditioning (A/C) compressor




The air conditioning (A/C) compressor is controlled by the engine control module (ECM) (4/46) on request from the climate control module (CCM) (4/6) via the controller area network (CAN). When the engine control module (ECM) receives a signal from the climate control module (CCM) to activate the air conditioning (A/C) compressor, the engine control module (ECM) grounds the circuit for the relay coil for the A/C compressor. See also: Design, B6304T4 Design
The relay (2/22) closes the circuit between the integrated relay/fuse box in the engine compartment and the clutch for the air conditioning (A/C) compressor (8/3). The air conditioning (A/C) compressor which has a variable cylinder displacement is always running during normal driving. Displacement in the compressor is regulated by a solenoid which is controlled by the engine control module (ECM).
The engine control module (ECM) controls the solenoid (displacement) from the driver's and vehicle's various driving characteristics. At engine start, driving off and at acceleration etc, displacement is controlled so that the AC compressor has the least possible effect on the engine torque. The climate control module (CCM) controls all functions in the climate control system that are related to the vehicle's interface for driver and passenger, i.e., the climate control system buttons on the dashboard environment panel. Also see Design and Function, Climate control module (CCM).
The climate control module (CCM) transmits information to the engine control module (ECM), which determines what must be prioritized. For example, the air conditioning (A/C) compressor in certain extreme cases is switched off completely, regardless of the climate control module (CCM) request. This is to prevent negative engine performance and to protect the air conditioning (A/C) system. As well as the information from the climate control module (CCM), the engine control module (ECM) controls the air conditioning (A/C) compressor based on the information from:
- Air conditioning (A/C) pressure sensor (high pressure side) (7/8)
- the throttle position (TP) sensor (6/120)
- the engine coolant temperature (ECT) sensor (7/16).

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 (AP) position sensor (7/51)
- the throttle position (TP) 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 (ECT)
- mass air flow through the intake manifold
- manifold absolute pressure (MAP) in the intake manifold.
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 AC/DC 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) 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 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.
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 deployed by the integrated power stage in the engine control module (ECM) using a pulse width modulation (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 engine control module (ECM) so that the throttle unit cannot be operated or powered, 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 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 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.

Adaptation of the throttle unit
In ignition position II, 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 engine control module (ECM), 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.

Cold zone Cooling Control
The engine control module (ECM) is located in the cold zone in front of the windshield for optimum cooling, but in extremely hot climates and in hot engine compartments, the internal temperature in the engine control module (ECM) can reach the temperature limit (+105 °C).
By overriding the recirculation shutter and the blower fan, the air flow in the cold zone increases, which cools the engine control module (ECM). This may result in the customer noticing hot air being blown into the passenger compartment without wanting it.