1987 Mercedes Benz 300EFACTORY OEM SERVICE PROCEDURE

Acceleration Enrichment

  1. Disconnect airflow sensor potentiometer (position indicator). Ensure fuel pump is NOT running. Connect ohmmeter between terminals No. 14 and 18 of potentiometer. Reading should be 3200-4800 ohms with sensor plate in rest position.
  2. Connect ohmmeter between terminals No. 14 and 17. Resistance should be 550-1050 ohms with sensor plate in rest position. With sensor plate at fully open position, resistance should be 3760-5640 ohms. If readings are not correct, adjust or replace potentiometer. If readings are correct, go to next step.
  3. Connect meter adapter cable to differential pressure regulator. Set multimeter to mA scale. Connect 2500-ohm resistor wire between coolant sensor wire and ground to simulate coolant temperature of 60°F (20°C). Disconnect microswitch and turn ignition on. Current reading should be 9-14 mA. Deflect sensor plate to full open position and check that current reading increases. If readings are incorrect, go to step 5). If readings are correct, go to next step.
  4. Disconnect differential pressure regulator. Measure resistance of regulator. If resistance is in range of 18-21 ohms, regulator is good and test is complete. If reading is out of specified range, replace differential pressure regulator.
  5. Measure acceleration enrichment voltage between terminal No. 18 of sensor plate potentiometer and ground. Voltage reading should be 7.4-8.6 volts. If not, repair wiring circuit. If reading is correct, go to next step.
  6. Measure voltage between terminals No. 18 and 14 of connector to airflow sensor potentiometer. Reading should be 8 volts. Also measure between terminals No. 17 and 14 of connector, which should give reading of 7.5 volts. If readings are not correct, repair wiring circuit. If readings are correct, go to next step.
  7. Check wiring between ECU connector and differential pressure regulator connector. See MERCEDES-BENZ 190E CIS-E WIRING DIAGRAM 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 (4), repair wiring.
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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.