1990 Mercedes Benz 560SELFACTORY OEM SERVICE PROCEDURE

Lower Chamber (Differential) Pressure

  1. Connect Meter Adapter Cable (102 589 04 63 00) between differential pressure regulator and ammeter. Disconnect O2 sensor lead at connector located under right front floor mat. Turn ignition on. Meter reading for lower chamber pressure should be constant 7-9 mA. If not, verify correct operation of lambda control. See TESTS W/CODES article.
    NOTE: During warm-up and at operating temperature, current to differential pressure regulator is determined by coolant temperature sensor, O2 sensor and ECU.
  2. Bridge fuel pump relay board terminals No. 7 and 8. Lower chamber pressure should be about 5.8 psi (.4 kg/cm2 ) less than system pressure reading determined earlier. Unplug coolant temperature sensor.
  3. Note lower chamber pressure and differential pressure regulator current. If coolant temperature sensor plug is disconnected, temperature sensor resistance should be infinity. Differential pressure regulator current should be about 114-132 mA and lower chamber pressure should be about 15.95-18.85 psi (1.1-1.3 kg/cm2 ) less than system pressure. Reconnect coolant temperature sensor.
  4. With coolant temperature at 68°F (20°C), verify the following: coolant sensor resistance is 2200-2800 ohms, differential pressure regulator current is 9-14 mA and lower chamber pressure is about 5.8 psi (.4 kg/cm2 ) less than system pressure.
  5. With coolant temperature at 176°F (80°C), verify the following: sensor resistance is 290-370 ohms, differential pressure regulator current 7-9 mA and lower chamber pressure is about 5.8 psi (.4 kg/cm2 ) less than system pressure.
  6. With coolant temperature at 212°F (100°C), verify the following: sensor resistance is 140-220 ohms, differential pressure regulator current is 7-9 mA and lower chamber pressure is about 5.8 psi (.4 kg/cm2 ) less than system pressure.
  7. If test results do not conform to specifications, check coolant temperature sensor for malfunction. Coolant temperature sensor should also be checked if warm-up problems exist. Verify correct operation of lambda control. If lower chamber pressure is excessive, verify that orifice in fuel distributor is not restricted. Install fuel pump relay. Reconnect O2 sensor.
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