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 525i AutomaticFACTORY OEM SERVICE PROCEDURE
Coolant Temperature Sensor
- Disconnect coolant temperature sensor. Turn ignition switch to RUN position. Check for 5 volts between Brown/Red wire of harness side connector and ground. If no voltage is present, check continuity of wiring to MCU (in control box at right shock tower).
- Check for 5 volts between Brown/Red and Brown/Orange wires of harness side connector. If no voltage is present, check for open or short in Brown/Orange wire to ground.
- If voltage was okay, reconnect coolant temperature sensor and erase trouble code from MCU memory. Test drive vehicle and recheck for trouble code. If no code present, problem may be intermittent. See INTERMITTENTS in TESTS W/O CODES article.
- If code is still present, replace coolant temperature sensor. Ensure resistance of coolant temperature sensor is 2300-2700 ohms at 72°F (20°C) and 300-360 ohms at 186°F (80°C). Erase trouble code from MCU memory. Test drive vehicle and recheck for trouble code. If code still present, replace MCU with a known good unit.
Factory Service Tools
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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.