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 PERFORMANCESYSTEMSYSTEM/COMPONENT TESTSIGNITION SYSTEM (EZL IGNITION)ENGINE RUNNING
1989 Mercedes Benz 420SEL
Engine Running
1989 Mercedes Benz 420SELSECTION Engine Running
- Run engine at idle (operating temperature). Check ignition timing for 7-11 degrees. If timing is not 7-11 degrees, go to next step. If timing is 7-11 degrees, go to step 4).
- Pull plug from throttle valve switch connector. Test resistance of throttle valve switch on connector. Reading between terminal Nos. 1 and 2, (idle speed position) about zero ohms; between terminal Nos. 2 and 3, (full load position) about zero ohms; between terminal Nos. 1 and 2 and Nos. 2 and 3, (partial load) infinity. If readings are not as specified, adjust or replace throttle valve switch. If okay, replace ignition control module.
- Run engine at 3200 RPM. Check ignition timing (vacuum line connected) for 40-44 degrees ATDC at 3200 RPM. If timing is not as specified, go to next step. If okay, go to step 6).
- Check vacuum line from intake manifold to ignition control module for leaks. If no leaks are present, check resistance of reference resistor (EZL). Pull sensor signal input plug on ignition control module (Brown/Yellow Green/ Black wire). Check resistance with ohmmeter between connector terminal No. 3 and ground.
- Resistance should be 750 ohms. If okay, replace ignition control module. If resistance is not as specified, check wire for open or replace connector. Go to next step.
- Pull vacuum line from ignition control module and run engine at 3200 RPM. Check ignition timing for 27-31 degrees at 3200 RPM. If okay, go to step 8). If timing is incorrect, go to next step.
- Unplug coolant temperature sensor. Test resistance from sensor terminal to ground. Test resistance at 2 different temperatures. See Figure . If not within specifications, replace sensor. If values are okay, go to next step.
- Run engine at 3200 RPM (vacuum line connected). Check dwell angle for 24-53 degrees. If dwell angle is not correct, replace ignition control module. If okay, test is complete.
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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.