1983 BMW 633CSi AutomaticFACTORY OEM SERVICE PROCEDURE

Test 3: MOTRONIC Control Unit

  1. Check power supply to Motronic Control Unit (MCU): Connect BMW Service Test unit and perform L-Jetronic Test step 1). Refer to BMW TEST KIT, L-JETRONIC PROCEDURES BROCHURE. If values are correct, replace MCU.
  2. If voltage value between wire 18/35 and 5, or 4 and 5 is insufficient or incorrect, connect voltmeter between wire 4 on disconnected MCU plug and ground. Start engine. If voltmeter does not show battery voltage, trace circuit and repair wiring. See Figure .
  3. Check wire 18 and 35 for breaks or poor connections. Connect ohmmeter between wire 5 on MCU plug and ground. Resistance should be zero. Connect ohmmeter between wire 18 or 35 on MCU plug, and wire 87 on plug of Relay 2. Resistance should be zero. If either test shows resistance, repair wiring.
  4. If resistance values are correct, check power supply to Relay 2: Disconnect Relay 1 and connect voltmeter between terminal 86 on relay socket and ground. Turn ignition on. Voltmeter should read battery voltage. If not, trace circuit and repair wiring. See Figure .
  5. If voltage is correct, check ground of Relay 2: Pull off relay and check resistance between connector 85 and ground. Resistance should be zero. If not, trace circuit and repair wiring. See Figure .
  6. If resistance is correct, check power supply of Relay 2: Pull off relay and connect voltmeter between connector 30 of relay socket and ground. Voltmeter should read battery voltage. If not, trace circuit and repair wiring. If voltage reading is correct, replace Relay 2.
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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.

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