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.

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

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

•	The Trionic has a separate lead from the ABS for RH front wheel speed. The bus value is used as a backup for the Cruise Control system, both values must be above 40 km/h for this to work. The value is also used as a substitute value if no RH front wheel speed is obtainable.

•	The DICE uses the value to activate the radiator fans for A/C on, high outside 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).

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.

The Trionic has diagnostics for the RH front wheel speed signal (lead from the ABS) but a lengthier period of driving without a speed signal may be needed before the diagnostic trouble code will be generated.

A readout of the two speed signals used in the car can be obtained from the Trionic using the diagnostic tool.

2. Immobilization

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's ID number are OK, the TWICE sends out the message ”Immobilization OFF” on the bus. This information is used by the Trionic which cancels fuel shut-off, which is always activated every time the ignition is switched on.

Diagnostics

On replacement of the TWICE, MIU or Trionic control module, reprogramming of the immobilization function must be carried out in the TWICE. If this is not done, the engine will not start and a diagnostic trouble code will be generated in the Trionic.

If the diagnostic trouble code is generated in spite of programming having been carried out, fault diagnosis will have to be carried out on the immobilization function in the TWICE.

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:

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

The DICE then requests the TRIONIC control module to ground the A/C relay if the following conditions are fulfilled:

1.	Coolant temperature (information via bus from the Trionic) is below 125°C.

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

3.	The temperature in the evaporator (own sensor) must fulfil the following conditions:

3.a.

If the temperature in the evaporator is above 3°C, the DICE sends via the bus a request to the Trionic control module for A/C engagement.

3.b.

If the temperature in the evaporator is below 0°C, the A/C compressor can only be engaged if the conditions in points 4 and 5 are satisfied.

4.	The outside temperature (information via bus from the SID) must be above 25°C.

5.	One of the following conditions must also be met:

5.a.

the engine must not have been running for more than 30 minutes (information via bus from the MIU).

5.b.

Vehicle speed is below 5 km/h (information via bus from the MIU).

The Trionic will then activate the compressor relay.

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 trouble codes with the ”A/C not working” symptom description, all input values which can block the function must be checked methodically by means of the diagnostic tool's read value commands. The compressor relay is activated in the Trionic system. Then carry out fault diagnosis in the system which ”owns” the defective sensor or actuator.