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.
Cis-E System: Description
Mercedes-Benz uses a basic CIS-E injection system for fuel delivery and electronic controls for mixture correction functions. CIS-E control units for Federal and California models are not interchangeable.
Electronic controls consist of airflow sensor position indicator (potentiometer), electro-hydraulic actuator, thermo time switch, coolant temperature sensor, Electronic Control Unit (ECU), transistorized ignition system, throttle valve microswitch, altitude sensor, lambda control and oxygen sensor.
If ECU malfunctions, CIS-E system will operate in back-up or "LIMP-HOME" mode until fuel system can be repaired. On 420 and 560 series vehicles, various components of the CIS-E injection system are monitored by a self-diagnosis feature in the control unit. The failure codes are transmitted to the lambda measuring circuit of the diagnostic socket. These codes are displayed on a Bosch "On-Off" Ratio Tester (KDJE-P600 ). The fixed on-off ratio indicates possible malfunctions. California models are equipped with "On-Board Diagnostic System". The on-off ratio readout is only activated after first programming the CIS-E control unit to provide the on-off ratio output.
The EZL electronic breakerless ignition system has computer controlled electronic ignition timing. Characteristics for various engine load conditions are stored in the ignition control module.
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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.