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Tachometer (rev counter) (MIU2)

Tachometer (rev counter) (MIU 2)


Fault symptoms

Tachometer needle does not move.

Diagnostic procedure



1. Start the engine.

2. Connect the Scan Tool and select "READ VALUES" followed by "ENGINE RPM".

3. Increase and decrease engine speed. If the rpm signal is OK, a corresponding increase and decrease (in rpm) should be shown on the Scan Tool's display. If this is the case, continue with point 7.

4. If no rpm signal is present, check the signal sent to the main instrument display panel by connecting a BOB to the 56-pin connector behind the main instrument display panel (which should not be connected).


Connect the Scan Tool's test leads to pin 6 and ground. Then select "MEASURE" followed by "PULSE". Start the engine and check whether any pulses are generated.

About 40 Hz (4-cyl.) at idling speed. About 30 Hz (6-cyl.) at idling speed. If pulses are generated, continue with point 8.

5. If no pulses are generated, check the wiring by connecting another BOB to the connector of the engine management system's control module.


The control module should not be connected.

6. Check for continuity/shorting to ground by taking a reading across the engine management system and pin 6 on the BOB connected to the main instrument display panel.


If the lead is OK, continue fault diagnosis in the engine management system, see Trionic , Motronic 2.10.1 or Motronic 2.8.1 .

7. Select "TEST FUNCTIONS" followed by "TACHOMETER TEST" on the Scan Tool.


If in proper working order, the rev counter needle should rise to 7000 rpm and then return to 0.

If the tachometer does not work, it could be due to an open circuit in the connection between the processor card and the intermediate part of the main instrument display panel.

Remove the main instrument display panel's rear section complete with processor card and check that all leads connected to the connector marked CN3 are securely located. If any lead has come loose or is broken, continue on .

If the leads connected to the connector are all OK, the fault is probably in the intermediate part with the indicating instruments. Continue on .

8. Select "TEST FUNCTIONS" followed by "TACHOMETER TEST" on the Scan Tool. If in proper working order, the rev counter needle should rise to 7000 rpm and then return to 0.


If the tachometer works, there is probably an open circuit between the connector and the processor on the processor card in the main instrument display panel. Continue on .