1989 Mercedes Benz 300CEFACTORY OEM SERVICE PROCEDURE

After-Start Enrichment Test

  1. Connect meter adapter cable to differential pressure regulator. Set multimeter to mA scale. Unplug O2 sensor under right front floor mat. Current reading at operating temperature should be 7-9 mA. If reading is not correct, test coolant temperature sensor as previously described. If reading is correct, go to next step.
  2. Disconnect Green wire of ignition switching unit (located on firewall) or install Protective Plug (102 589 02 21 00) into diagnostic socket. Simulate coolant temperature of 68°F (20°C) by connecting 2500-ohm resistor wire between coolant temperature sensor lead and ground. Crank engine for about 3 seconds and let key return to ON position. DO NOT turn ignition off.
  3. After 4 seconds (including 3 seconds of cranking), current reading should increase to 24 mA. After 20 seconds, current reading should drop to 9-14 mA, which is basic warm-up current value. If readings are correct, test is complete. If readings are not correct, go to next step.
  4. Connect voltmeter between ECU connector terminal No. 24 and ground. Crank starter motor. Cranking voltage signal (terminal No. 50) should be 10 volts. If canking voltage signal is not 10 volts, check and repair wiring. If voltage is correct, go to next step.
  5. Check wiring between ECU connector and differential pressure regulator connector. See WIRING DIAGRAM article for wiring color and terminal numbers. If resistance reading is zero ohms, circuit is good and ECU should be replaced. If reading indicates open circuit, repair wiring.
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.