1989 Mercedes Benz 300CEFACTORY OEM SERVICE PROCEDURE

Control Unit Relay Test

  1. Unplug CIS-E control unit (located under dash on passenger's side). Turn ignition on and check voltage between terminal Nos. 1 (+) and terminal 2 (-). See Figure and Figure . Battery voltage should be present. If voltage is correct, control unit relay is OK. If battery voltage is not present, go to next step.
  2. Check voltage between terminal No. 1 (+) and ground (battery). Voltage should be approximately 12 volts. If voltage is incorrect, inspect fuse on overvoltage protection relay (on fuse panel in engine compartment). Inspect wire connections and ensure wire routing is OK. If voltage is approximately 12 volts, go to next step.
  3. Check wire from terminal No. 2 to ground for continuity. Resistance should be zero ohms. If resistance is not about zero ohms, repair circuit. If okay, go to next step.
  4. Check wire between diagnostic socket terminal No. 3 (located on left side of engine compartment) and CIS-E control unit plug (terminal No. 23) for continuity. Resistance should be zero ohms. If okay, go to next step. If resistance is not zero ohms, repair open.
  5. Disconnect oxygen sensor connector and ground male terminal. Start engine. Measure voltage between female terminal of O2 sensor and ground. Voltage should be greater than 450 mV. If resistance is not greater than 450 mV, replace oxygen sensor. 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.