Tachometer

Tachometer

Fault symptoms
Tachometer inoperative.

Diagnostic help
A reading of the current engine speed can be obtained with the diagnostic tool.
The diagnostic tool can also be used to activate the tachometer.

Check the wiring harness
Jiggle the wiring harness at several points and in different directions to detect intermittent breaks and short-circuits in the leads. Observe the multimeter, test lamp or diagnostic tool during the test.

Diagnostic procedure

1. Choose the system
- Select the engine management system.






TRIONIC
- Go to step 2

MOTRONIC 2.10.3
- Go to step 6

2. Check the reading, Trionic
- Contact Main instrument unit.






- Read value: Engine speed (Main instrument unit).
- Start the engine and vary engine speed.






- Check that the reading obtained is plausible when compared with the actual engine speed.






Is the function correct?
YES
- Go to step 10.
NO
- Go to step 3.

3. Check the control module input, Trionic
- Unplug connector: Main instrument unit (22-pin connector): (540).






- Connect test lamp 86 11 857 to pin 12 of the female connector and a good grounding point.






- Run the starter motor for 10 seconds.






- The test lamp should flash.






Is the function correct?
YES
- Go to step 10.
NO
- Go to step 4.

4. Checking the wiring harness
- Check the lead between pin 12 of the MIU's 22-pin connector and pin 58 of the Trionic.






Is the lead OK?
YES
- Go to step 5.
NO
- Repair or replace the lead between pin 12 of the MIU's 22-pin connector and pin 58 of the Trionic.
- Go to step 10.

5. Final check
- Clear the diagnostic trouble codes in all systems.






- Perform a functional test.






Is the function correct?
YES
- The steps taken to rectify the fault were correct.
NO
- See "Before changing a control module."

6. Check the reading, Motronic
- Contact Main instrument unit.






- Read value: Engine speed (Main instrument unit).
- Start the engine and vary engine speed.






- Check that the reading obtained is plausible when compared with the actual engine speed.






Is the function correct?
YES
- Go to step 10.
NO
- Go to step 7.

7. Check the control module input, Motronic
- Unplug connector: Main instrument unit (22-pin connector): (540).






- Connect test lamp 86 11 857 to pin 12 of the female connector and a good grounding point.






- Run the starter motor for 10 seconds.






- The test lamp should flash.






Is the function correct?
YES
- Go to step 10.
NO
- Go to step 8.

8. Check the wiring
- Check the lead between pin 12 of the MIU's 22-pin connector and pin 6 of the Motronic.






Is the lead OK?
YES
- Go to step 9.
NO
- Repair or replace the lead between pin 12 of the MIU's 22-pin connector and pin 6 of the Motronic.
- Go to step 10.

9. Final check
- Clear the diagnostic trouble codes in all systems.






- Perform a functional test.






Is the function correct?
YES
- The steps taken to rectify the fault were correct.
NO
- See "Before changing a control module."

10. Final check
- Clear the diagnostic trouble codes in all systems.






- Perform a functional test.






Is the function correct?
YES
- The steps taken to rectify the fault were correct.
NO
- See Fault diagnosis, general, "Before changing a control module." Before Changing A Control Module