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
Idle Speed Stabilization (Auto. Trans. Engaged)
- Set parking brake. Run engine at idle and warm up to operating temperature. Engine should idle between 570-670 RPM in drive. If idle speed is correct, test is complete. If engine idle is not correct, go to next step.
- Using voltmeter, test voltage input by backprobing CIS-E connector terminal No. 16 for 420SEL and 560 series and terminal No. 28 for 190E and 300 series. With shifter in "P" or "N", reading should be 12 volts. With shifter in "D", reading should drop below battery voltage. If reading is incorrect on either test, repair short in circuit or repair or replace the selector switch. If reading is correct, test is complete.
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.