Description of operation, function chains

Description of operation, function chains

In the bus system a large number of function chains can be identified. Below are three examples of important ones.

1. Vehicle speed

Vehicle speed is sensed by the ABS through an inductive sensor at each wheel. The sensor supplies the control module with alternating current, the amplitude and frequency of which increase with increasing wheel speed. The frequency is proportional to wheel speed, 29 Hz at a speed of one wheel revolution per second. The ABS control module uses the 4 wheel speeds to prevent wheel lock-up on braking.

Several other systems in the car need to know its road speed. The ABS control module, which is not connected to a bus, therefore has a control module output for each wheel. A transistor grounds the respective output 29 times per wheel revolution.

The main instrument unit is connected by a lead to the speed signal output for the LH rear wheel. The rear wheel is used because the most accurate measurement of the distance travelled is obtained from a non-driven wheel. The main instrument unit converts the frequency modulated signal into km/h and also corrects the value according to the programmed type of tyre.

Since the main instrument unit is connected to a bus, all control modules connected to the bus are supplied with information on the speed of the left-hand rear wheel and the odometer reading.

The table of transmitted bus messages shows that MIU transmits vehicle speed and that it is used by: DICE, SID, ACC, Audio and TWICE. MIU also transmits the odometer reading and this is used by SID for the service indicator.

The various systems use the vehicle speed for the following functions:

•	The DICE uses the value to activate the radiator fans for A/C on, high outdoor temperature and low speed.

•	The SID uses the value for trip computer functions.

•	The ACC uses the value for activating recirculation in connection with high outside temperature and low speed.

•	The Audio system uses the value for increasing the volume at high speeds.

•	The TWICE uses the value for automatic locking of the doors when driving off (certain markets).

EDC 15 on diesel cars has a separate lead from ABS for the front right wheel speed. Bus information is not used because the engine management system (just as with Trionic and Motronic) requires information on the speed of a driving wheel.

The value is used for the cruise control function and to calculate the current gear position.

Diagnostics

The ABS has extremely good diagnostics for its wheel sensors. This means that with a high degree of certainty the wheel speed signals will be present on the control module's four outputs if no diagnostic trouble codes have been generated in the system.

Neither the main instrument unit nor any of the other bus systems have speed signal diagnostics for the LH rear wheel. If the speedometer is working, however, it is certain that speed information is present on the bus and consequently available to all systems connected to the bus.

If the speedometer is not working and the speed reading obtained on any bus system is always 0, and there are no diagnostic trouble codes in the ABS, then the fault is probably in the lead between the ABS and MIU.

2. Immobilizer

When the ignition is turned on, the TWICE checks that the right key is inserted in the ignition switch. A circuit built into the remote control part of the key sends a code by means of inductance to a coil round the ignition switch under the centre console. The code is passed on to the control module, which checks that it is correct. The TWICE also checks that the right MIU is fitted in the car. The MIU continuously sends out an ID number on the bus.

If the key and the MIU identification number are OK, TWICE will transmit the message ”Immobilizer OFF” on the bus.

The information is used by MIU, which sends it on the K lead to Trionic/Motronic. The engine management system then cancels the fuel shut-off, which is always activated on every time the ignition is turned on.

The immobilizer function is used by EDC 15 directly on cars with diesel engines.

Diagnostics

Not OBD II: The immobilizer must be reprogrammed in TWICE after replacing TWICE, MIU or the engine control module. If this is not carried out, the engine will not start and the engine management system will generate diagnostic trouble codes.

TWICE must accept the transponder being used before allowing the immobilizer code to be programmed.

OBD II: The engine can always be started after replacing TWICE, MIU or the engine control module.

TWICE must accept the transponder being used before allowing the starter motor to crank.

3. A/C

A number of conditions must be fulfilled for the A/C relay to be grounded and the A/C compressor's electromagnetic clutch to operate.

In AUTO mode, the ACC unit will request the DICE for A/C engagement if:

•	the outdoor temperature (information via bus from the SID) is above 0°C.

DICE then sends the A/C request if the following conditions are fulfilled:

1.	engine coolant temperature (information via the bus from MIU/EDC 15) is below 125°C.

2.	Refrigerant pressure (own sensor) on the high-pressure side is between 2 and 27 bar.

3.	temperature in the evaporator (own sensor) must be above 4.5°C.

MIU uses the A/C request from DICE and sends on the information on a separate lead to Trionic/Motronic. The engine management system then activates the compressor relay as soon as the engine has started.

EDC 15 on cars with diesel engines uses the A/C request directly and activates the A/C relay as soon as the engine has started. EDC 15 also has a condition that the engine coolant temperature must be below 120°C.

In practice, this means that the condition for the engine coolant temperature in DICE (point 1) is meaningless on cars with diesel engines.

Diagnostics

In the event of a fault in any of the aforementioned sensors in the systems concerned, the A/C compressor will not work. The function chain clearly shows the importance of obtaining diagnostic trouble code readouts from all systems because several systems are involved in any given function.

If there are no diagnostic codes with the symptom description ”A/C not cooling”, all the input values that can block the function must be gone through methodically with the read values command on the diagnostic tool. The compressor relay is activated in the engine management system. Then perform fault diagnosis on the system that ”owns” the sensor or actuator that is faulty.