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
After-Start Enrichment
- Connect Multimeter Adapter Cable (102 589 04 63 00) between electrohydraulic regulator and its connector. Set multimeter to mA scale. Disconnect coolant temperature sensor connector. Install 2500-ohm resistor between CTS connector terminals No. 2 and 4 to simulate 68°F (20 °C). Turn ignition on. Multimeter reading should be 20 mA. Start engine.
- After 1-8 seconds, multimeter reading should be 8-12 mA, indicating after-start enrichment is operating. Within 6 more seconds multimeter reading should be 3-7 mA, and will continue at this reading for about 110 seconds or until engine coolant temperature reaches 122°F (50°C), whichever happens first. After this sequence takes place then Lambda control starts.
- If reading is not as described, test coolant temperature sensor as previously described. See Coolant Temperature Sensor under ENGINE SENSORS & SWITCHES. Also see TEST STEP No. 4 and 6 for 6-cylinder engines and TEST STEPS No. 3 and 7 for V8 engines.
- Check starter engagement signal circuit. Connect voltmeter between CIS-E control unit connector terminal No. 11 and ground. Crank starter motor. Cranking voltage signal (circuit No. 50) should be about 10 volts. If cranking voltage signal is not about 10 volts, check and repair wiring. If voltage is correct, go to next step.
- Check wiring between CIS-E control unit connector and electrohydraulic regulator connector. If resistance reading is zero ohms, circuit is good and CIS-E control unit should be replaced. If resistance reading indicates open circuit, repair wiring as required.
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.