The fuel injection system used on all Mercedes-Benz gasoline engine vehicles uses a basic CIS injection system for fuel delivery and electronic controls for mixture correction functions.

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.

The EZL electronic breakerless ignition system has computer controlled electronic ignition timing (firing point) adjustment. Data about ignition characteristics for various engine loads is stored in the ignition control module and can be recalled according to actual engine operating conditions.

Fig 1: 420 & 560 CIS-E Fuel Injection Layout

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.

Safety Warning

Cis-E System: Description

Use the Manual With the Right Hardware

When to See a Mechanic