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 Mercedes Benz 300CEFACTORY OEM SERVICE PROCEDURE
Full-Load Enrichment Test
- Disconnect wiring between throttle valve switch and ECU. Check resistance of throttle valve switch at male plug. In idle speed position, resistance should be infinity. In full load position, resistance should be zero ohms. If readings are not correct, replace throttle valve switch. If readings are correct, go to next step.
- Connect meter adapter cable to differential pressure regulator. Set multimeter to mA scale. Bridge connector of throttle valve switch that connects to ECU. Turn ignition on. Current reading should have constant nominal value of 7-9 mA. If current reading is correct, test is complete. If current reading is not correct, go to next step.
- Check wiring between ECU and differential pressure regulator for continuity. Resistance reading should be zero ohms. If reading is correct, replace ECU. If reading is not correct, repair short in wiring.
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.