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Home >> BMW >> 1990 >> 325is Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Testing >> Fuel System >> MOTRONIC Control Unit (MCU) >> Power Circuits
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Power Circuits

  1. Ensure ignition is off. Use DVOM to check for constant battery voltage at MCU pin No. 18 (Red wire). See MCU LOCATION table. Battery voltage should also be present at pin No. 36 (Brown wire). Pin No. 36 is power feed from main relay. Computer will ground pin No. 36 to activate main relay during cranking and running operation. If no voltage exists on pin No. 36, check main relay. See RELAYS & SOLENOIDS in SYSTEMS & COMPONENT TESTING article.
  2. Turn ignition on. DO NOT start engine. Battery voltage should be present at pin No. 37 (Red/Blue wire).
  3. During cranking, battery voltage should be present at pin No. 27 (Green wire). See WIRING DIAGRAMS article for more information.
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