Instrument Cluster (Basic) - Part 1 of 2
Instrument Cluster (Basic)
General
At series launch, the instrument cluster (basic) features two bus interfaces which enable the information link (data link/serial) to the connected control modules.
These busses are:
- K-bus (body bus)
- Diagnosis bus (link to diagnostic socket).
A further bus interface provides the link to the automatic transmission unless the instrument cluster has a CAN interface. On manual transmission vehicles, the reverse gear selector status is read in via this connection.
Coded data determining the scope of functions of the instrument cluster are stored in a non-volatile data memory (data is retained when supply voltage is disconnected).
The various scope of functions includes, for example, the vehicle model and the country language variant.
Replacing Instrument Cluster
The four cases described in the following show under which circumstances which steps must be taken to ensure correct replacement of the instrument cluster electronics.
Case 1: The instrument cluster control module is defective and the light check module control module (LCM) is OK.
Case 1:
Case 2: The instrument cluster control module is OK. and the light check module control module (LCM) is defective.
Case 2:
Case 3: The control module for the instrument cluster and the control module for the light check module (LCM) must be replaced.
Only replace both control modules at the same time if this is unavoidable (stored total odometer reading is irretrievably lost).
NOTE: Disconnect battery.
Case 3:
Case 4: As a check, replace the control module for the instrument cluster or the light check module control module (LCM).
NOTE: Although exchanging one of the two control modules is possible in principle, it should be avoided wherever possible.
Case 4:
Scope of function
The following are displayed or used for a function:
Scope Of Function:
Test functions
The test functions which could be called up on the on-board computer in previous series can now be called up in the instrument cluster.
System test (test No. 2): The system test is used to test the display unit together with its elements which can be controlled from the instrument cluster.
These elements are:
- All pointer instruments
- All LC displays with a segment test and their background lighting
Indicator and warning lamps for:
- Belt warning
- Fuel reserve
- General brake warning lamp
- Parking brake
- Left and right turn signal indicators
- Front and rear fog light
- High beam
- Gear indicator (automatic transmission)
- Service interval display SIA
- Check control displays (if coded)
- Air suspension
- Fault automatic transmission
- Engine oil level
Pointer instruments: During the system test, operation of the pointer instruments is diagnosed and a corresponding fault code is set in the fault code memory if an electrical fault occurs. In addition, particular care must be taken to observe whether the pointers move smoothly and continuously over their entire indication range during the first test pass. During the second pass within a system test sequence, the pointer instruments are not moved continuously and smoothly over their entire indication range. In this case, the electrical drive data is measured at various indication angles of the pointers.
Selecting test functions:
All test functions, apart from test numbers one and two, are interlocked and must be released by means of test function number nineteen.
Procedure: Press and hold (approx. 5 s) the trip recorder reset button (left-hand button in the instrument cluster) with terminal R "ON" until "__tESt__1._" appears in the instrument cluster display. The corresponding test (system test corresponds to test 2 "__tESt__2._") can be selected by pressing/releasing the button again within 1 second. The first subfunction of the test selected then appears after approx. 1 second. The subfunction (the system test has no further subfunction) of the corresponding test can be selected by pressing the button again. Since there are no further subfunctions associated with test 2 (system test), the system test is triggered after the "__tESt__2.0" display appears and the trip recorder button is pressed. If the test is a locked test (test 3 to 21) the lock is released by pressing the trip recorder button when "_L_oFF" appears in the display. The display then jumps to test 0 so that the corresponding test can be selected by briefly pressing the button.
Test Function
No.
1 Identification Instrument Cluster
2 System test
3 SIA data
4 Current consumption values in l/100 km and l/h
5 Range consumption and current range
6 Fuel tank content values
7 Coolant temperature, outside temperature, current engine speed, current road speed
8 ADC values (system voltage, voltage at left and right lever sensor, voltage at photo transistor and BVA, voltage at coolant and outside temperature sensor)
9 System voltage terminal 30 in volts
10 Read out country-specific code
11 Read out units (AM/PM or mm.dd/dd.mm) etc.
12 not used
13 Triggering acoustic signals
14 Read out of error bytes (self-diagnosis)
15 Display of I/O port statuses
16 free
17 free
18 free
19 Locking and Unlocking Test Functions
20 Entry of a correction factor for average consumption
21 Reset instrument cluster (software reset)
0 End of test, the test mode can be quit with this function.
Test function 20 Correction factor for the "average fuel consumption function".
Test function 20 has 4 subfunctions 20.0 (indicates the stored correction factor), 20.1 (enables setting of the units decimal position), 20.2 (enables setting of the tens decimal position) and 20.3 (enables setting of the hundreds and thousands decimal position where here only the number pairs 07, 08, 09, 10, 11 and 12 are possible. The adjustment range of the correction factor is defined from 0750 to 1250. The factor must be set to 1000 (factory setting) in order to carry out a correction otherwise it will not be possible to calculate a new factor with the formula. When the factor is set to 1000, a new correction factor is derived from the actual consumption in liters per 100 km divided by the displayed consumption in liters per 100 km multiplied by 1000. If other units of measure are displayed, i.e. miles, liters or miles per gallon and kilometers per liter these units of measure must be converted into liters per 100 km for the purpose of calculating the correction factor. This new factor can now be set with the subfunctions 20.1 to 20.3.
To select test function 20: Press and hold (approx. 5 s) the trip recorder reset button (left-hand button in the instrument cluster) with terminal R "ON" until "__tESt__1._" appears in the instrument cluster display. Test 20 can be selected by repeatedly pressing/releasing the button within 1 second. The first subfunction of the test selected then appears after approx. 1 second. The subfunction of the corresponding test can be selected by pressing the button again. The set correction factor is indicated in test function 20, subfunction 0 (20.0). The units decimal position is counted down (0-9) by switching to subfunction 20.1. The units decimal position continues changing automatically until the next subfunction is selected. The position last displayed is adopted in the correction factor. The same procedure also applies to subfunction 20.2 which counts down the tens decimal position (0-9). The hundreds and thousands decimal position are counted down in subfunction 20.3 where only the digit pairs 07, 08, 09, 10, 11 and 12 are shown so as to maintain the valid range for the correction factor from 750 to 1250.
Speed display
The speed display is equipped with various types of speedometer depending on the vehicle model.
The instrument cluster receives the position signal tw from the antilock brake system (ABS)/automatic stability control (ASC)/dynamic stability control (DSC) control module. The instrument cluster uses this to control the speedometer, total odometer reading and trip recorder reading displays.
The instrument cluster derives the speed from the position signal tw and from the position pulse number (K-number) stored as coding data in the instrument cluster. In addition, the instrument cluster provides a speed signal (Speed-A ) for the connected control modules. The speed information is provided in the form of a telegram on the body bus (K-bus ).
Engine speed display
The instrument cluster uses the engine speed signal from the engine control module (td for gasoline engines) to determine the engine speed. Adaptation to the various types of engines uses the stored coding data.
From terminal 15 "ON", the engine speed instrument (rev counter) is controlled by the instrument cluster with the corresponding engine speed signal. The engine speed information is also provided on the body bus (K-bus) for other control modules.
Phased-in as from 9/97, the instrument cluster is equipped with a CAN connection. Coding data defines whether the engine speed signal is routed via the CAN bus or via a separate line to the instrument cluster.
Fuel consumption display
The fuel consumption signal t(KVA) is derived from the injection signal supplied by the engine control module. In combination with the position signal, it yields to the consumption per unit of distance (e.g. l/100 km).
The fuel consumption signal t(KVA) is not only used for calculating the fuel consumption but also for controlling the service interval display and therefore displaying the service interval.
Phased-in as from 9/97, the instrument cluster is equipped with a CAN connection. Coding data defines whether the fuel consumption signal is routed via the CAN bus or via a separate line to the instrument cluster.
Fuel gauge
The tank content is measured by two lever sensors which are separately linked to the instrument cluster. Each lever sensor is connected by a separate ground (analog ground) and a sensor line (analog positive) to the instrument cluster. There is no reserve contact in the level sensor to activate the fuel reserve warning lamp. The fuel reserve warning lamp is switched as a function of the tank contents by comparison with a reserve threshold value.
The "fuel reserve" signal is output as information for engine management. It is coupled to the fuel reserve warning lamp and is switched on for 2 seconds as a function check when terminal 15 is switched on (predrive check).
The different methods of determining the fuel level for each tank half is determined by means of the coding data.
Coolant temperature display
The instrument cluster determines the current coolant temperature from the coolant temperature sensor (NTC resistor). The coolant temperature sensor is connected by a separate ground (analog ground) and a sensor line (analog positive) to the instrument cluster.
The instrument cluster places the "coolant temperature" information on the body bus (K-bus) for other systems.
Phased-in as from 9/97, the instrument cluster is equipped with a CAN connection. Coding data defines whether the coolant temperature signal is routed via the CAN bus or via a separate line with separate coolant temperature sensor to the instrument cluster.
Service interval display
The service interval display (SIA) is an indication to the driver and the workshop that an engine oil service, distance-dependent services or time-dependent service. The service intervals of the vehicle are related to the consumption. The current service interval status is displayed from terminal 15 "ON" for 10 seconds after a minimum engine speed of 400 rpm is reached.
The service interval display (SIA) data is also stored in the light check module (LCM ).
Reset service interval display
The service interval display (SIA) is reset by pulses of defined length at the service interval reset input. Each reset procedure can be carried out individually (service interval display [SIA] reset). The oil service, time-dependent service and/or distance-dependent service can be reset.
Total odometer reading
Display of the total distance covered is a component part of the LC display. The current mileage is indicated on the display from terminal R "ON". With terminal R "OFF", pressing and releasing the trip recorder reset button displays the mileage for approx. 25 seconds. The unit of distance (km/miles) dependent on the coded country-specific version is displayed between the total distance recorder and the trip recorder. The total distance is placed on the body bus as a telegram (K-bus).
The total distance is stored in the instrument cluster and in the light check module (LCM). "999999" is displayed as the total distance if the instrument cluster cannot read the total distance and it cannot be determined by way of the light check module (LCM).
Trip recorder reading
Display of the distance covered on a trip is a component part of the LC display. The current trip recorder reading is shown on the display from terminal R "ON". At terminal R "OFF", pressing and releasing the instrument cluster button displays the mileage for approx. 25 seconds. The unit of distance (km/miles) dependent on the coded country-specific version is displayed between the total distance recorder and the trip recorder.