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.
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HOMESERVICE MANUALSMERCEDES BENZ1989420SELREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETESTING & DIAGNOSISSYSTEM/COMPONENT TESTSFUEL SYSTEMLAMBDA CONTROL (CALIFORNIA MODELS) TEST
1989 Mercedes Benz 420SEL
Lambda Control (CALIFORNIA Models) Test
1989 Mercedes Benz 420SELSECTION Lambda Control (CALIFORNIA Models) Test
- Start and warm engine to normal operating temperature. Connect On-Off Ratio Tester (KDJE-P 600) to diagnostic socket located on left fenderwell. Press IR 100% button on tester. Start engine.
- Press push button in diagnostic test connector for 2-4 seconds. LED should blink once, indicating that no codes are stored in memory. Press push button again for 2-4 seconds, this will program CIS-E control unit to read the on-off ratio. LED should stay on.
- 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.
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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.