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.
1990 BMW 535iFACTORY OEM SERVICE PROCEDURE
Fuel Pump Relay
- If fuel pump fails to operate, ensure battery voltage exists on Red and Red/White wires of fuel pump relay and that the fuel pump fuse is okay. If no voltage exists, check power circuits and ground circuits for Motronic Control Unit (MCU). MCU controls fuel pump relay ground circuit. See MOTRONIC CONTROL UNIT (MCU) in this article.
- If battery voltage exists at fuel pump relay, note wire color and location on fuel pump relay terminals. Apply battery voltage to Red and Red/White wire terminals of relay. Ground the Brown/Green wire terminal. Voltage should now exist at Green/Violet wire terminal of the relay. Replace relay if voltage does not exist.
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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