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.
1991 Mercedes Benz 300CEFACTORY OEM SERVICE PROCEDURE
Lambda Control (CALIFORNIA)
- Warm engine to normal operating temperature. Turn engine off. Connect On-Off Ratio Tester (KDJE-P 600) to diagnostic socket located on left fenderwell. Press IR 100% button on tester. Start engine. If any malfunction codes are present, they must be repaired first before testing Lambda control.
- Observe on-off ratio at idle RPM. Record average reading. Observe on-off ratio at 2500 RPM. Record average reading. Compare this value with idle speed reading. The average idle speed value should not vary from the average value at 2500 RPM by more than +/-10. Adjust on-off ratio if it is not within specifications. See TESTING WITH ON-OFF RATIO TESTER in TESTS W/CODES article in this section.
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.