Speed Signal, Checking

The following procedure requires a test drive. Observe all safety precautions.

Special tools and workshop equipment required 

• multimeter.
• Wiring diagram.

Test requirements

• The Speedometer -G21- OK.
• The Motronic Engine Control Module (ECM) -J623- fuses OK.
  Fig 1: Fuses On E-Box In Engine Compartment, Left Side

  
• Battery voltage at least 12.5 volts.
• All electrical consumers such as, lights and rear window defroster, switched off.
• 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. Refer to PRELIMINARY CHECK 

Start diagnosis

-- Connect the scan tool.

-- Perform a road test with a vehicle speed greater than 5 Km/h.

-- Using the scan tool, Check the vehicle speed:

Diagnostic text		Specified value
Vehicle Speed		Approx. Vehicle Speed

-- Compare the vehicle speed on the scan tool to the Speedometer -G21-.

Specified value: a difference of no greater than 10%.

If the specified value was not obtained or no speed was displayed:

-- Check the wiring from the Motronic Engine Control Module (ECM) -J623- to the Instrument Cluster Control Module -J285- for an open circuit, Short to Battery (+), or to Ground (GND).

-- Check the wiring connection for damage, corrosion, loose or broken terminals.

-- If necessary, repair the faulty wiring connection.

Final procedures

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

1. Check the DTC memory. Refer to DIAGNOSTIC MODE 03 - READ DTC MEMORY .
2. If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
3. If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .