Engine Speed Sensor G28, Checking

Engine Speed Sensor G28, Checking

NOTE: Use only gold-plated terminals when servicing terminals in the electrical harness connector of the Engine Speed (RPM) Sensor G28.

Special tools, testers and auxiliary items required
- Multimeter.
- Wiring diagram.

Test requirements
- The Motronic Engine Control Module (ECM) J623 fuses OK.
- Battery voltage at least 12.5 volts.
- All electrical consumers such as, lights and rear window defroster, switched off.
- Vehicles with automatic transmission, shift selector lever into position "P" or "N".
- A/C switched off.
- Ground (GND) connections between engine/transmission/chassis OK.
- Ignition switched off.

Test procedure
- Perform a preliminary check to verify the customers complaint. Preliminary Check

Start diagnosis
The Engine Speed (RPM) Sensor G28 detects RPM and reference marks. Without an engine speed signal, the engine will not start. If the engine speed signal fails while the engine is running, the engine will stop immediately.

Function test
- Connect the scan tool.
- Switch the ignition on.
- Using the scan tool, check the engine speed:

Diagnostic text Specified value
Engine rotations per minute (RPM) Idle speed

- End diagnosis and switch ignition off.

If the specified value was not obtained:

Checking internal resistance







- Disconnect the Engine Speed (RPM) Sensor G28 electrical harness connector - 5 -.







- Using a multimeter, check the Engine Speed (RPM) Sensor G28 terminals 2 to 3 for resistance.

Specified value: 0.7 to 1000 k Ohms

- Check Engine Speed (RPM) Sensor G28 terminals 1 to 2 and 1 to 3 for short circuits.

Specified value: (infinity)

If any of the specified values are not obtained:
- Replace the Engine Speed (RPM) Sensor G28.

If the specified values are obtained:

Checking wiring
If the manufacturers test box is being used. Perform the following step.
- Install the test box.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) J623.







- Using a Multimeter, check the Engine Speed (RPM) Sensor G28 electrical harness connector terminals to the Motronic Engine Control Module (ECM) J623 electrical harness connector T121 terminals for resistance.

Engine Speed (RPM) Sensor Motronic Engine Control
G28 electrical harness Module (ECM) J623 electrical
connector terminals harness connector T121 terminals or test box sockets
1 98
1 108
2 82
3 90

Specified value: 1.5 Ohms Max.

If the specification was not obtained:
- Check the wiring for an open circuit, short circuit to each other, Battery (+), and Ground (GND).
- Check the electrical harness connector for damage, corrosion, loose or broken terminals.
- If necessary, repair the faulty wiring connection.

If no malfunction is detected in the wiring:
- Remove the Engine Speed (RPM) Sensor G28 and check the sensor wheel for proper seating, damage and run-out.

If the sensor wheel is OK:
- Erase the DTC memory. Diagnostic Modes 01 - 09
- Perform a road test to verify repair.

If the DTC does not return:
Repair complete, Generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes
- End diagnosis.

If the DTC does return and no malfunction is detected in the wiring and the voltage supply was OK:
- Replace the Motronic Engine Control Module (ECM) J623.
- Assembly is performed in the reverse of the removal.

Final procedures
After repair work, the following work steps must be performed in the mentioned sequence:

1. Check the DTC memory. Diagnostic Modes 01 - 09
2. If necessary, erase the DTC memory. Diagnostic Modes 01 - 09
3. If the DTC memory was erased, generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes

End of Diagnosis