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Home >> Porsche >> 1987 >> 928 Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Fuel Injection System - BOSCH LH >> Testing >> Test 12 - LH Hot Wire Air Mass Sensor
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Test 12 - LH Hot Wire Air Mass Sensor

  1. It is possible to check positive and ground activation on terminals No. 4 and 2 of a disconnected air sensor plug. Connect voltmeter on terminals No. 4 and 2 and bridge terminals No. 30 and 87 on relay No. XXV.
  2. Display should be battery voltage. An emergency running system is programmed in the LH fuel control unit, which cuts in if the air mass sensor fails. Testing is only possible by exchanging the intake air sensor.
  3. With LH fuel system in emergency running program, injection rate at 2000 RPM should be 2.7 milliseconds. With engine at operating temperature, injection rate should be 5.5 milliseconds at 2000 RPM.
  4. With air mass sensor removed, connect ohmmeter to terminals No. 3 and 6. Reading should be 0-1000 ohms with a basic setting of 382 ohms. With ohmmeter connected to terminals No. 3 and 5, reading should be 3.6-4.1 ohms.
  5. With air mass sensor plug connected, bridge terminals No. 30 and 87 or relay No. XXV on CEB. Connect voltmeter to terminals No. 3 and 5. Reading should be 1-5 volts (hot wire signal blow through the air sensor).
  6. To check the hot wire burn off signal, reinstall relay No. XXV. Burn off cycle is energized only if engine temperature is in excess of 140°F (60°C) and a previous engine speed of greater than 2000 RPM was reached. With the ignition switch in the "OFF" position and after a delay time of approximately 4 seconds, the hot wire must glow bright.
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