Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1989 BMW 635CSi AutomaticFACTORY OEM SERVICE PROCEDURE
MCU Power & Ground Circuits
- Check power and ground circuits. SEE MOTRONIC CONTROL UNIT (MCU) in this article.
- Check that battery voltage exists from pin No. 1 of MCU to the ignition coil. If voltage does not exist, check for open or short in wiring. On models equipped with anti-theft systems, check that no voltage exists at pin No. 38 of MCU. If voltage exists, anti-theft system will prevent vehicle from starting.
NOTE:
If all components are okay, engine speed/reference sensor should be checked. Sensor provides signal to MCU to provide ignition operation. See ENGINE SPEED/REFERENCE SENSOR & CIRCUIT in this article.
Factory Service Tools
Use the Manual With the Right Hardware
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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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