1989 Mercedes Benz 300CEFACTORY OEM SERVICE PROCEDURE

Fuel Pump Relay

  1. Remove fuel pump relay from board. Connect positive lead of multimeter to socket No. 8 (terminal No. 30) and negative lead to socket No. 11 (terminal No. 31) of connector. Measure voltage. If reading is 12 volts, go to step 3). If reading is zero volts, go to next step.
  2. Remove negative lead of multimeter from socket No. 11 (terminal No. 31) and connect to vehicle ground. If reading is 12 volts, check Brown wire from terminal No. 31 for short. If voltage is zero, check Red wire from terminal No. 30 for short. Repair wiring if necessary.
  3. Turn ignition on and check voltage at socket No. 9 (terminal No. 15) of connector. If reading is zero volts, check Black/Red wire from terminal No. 15 to fuse box for short. Repair wiring if necessary. If reading is 12 volts, go to next step.
  4. Using dwell meter, connect positive lead to socket No. 10 (terminal TD). Dwell angle should be 7-34 degrees. If dwell is incorrect, inspect Green/Yellow wire from jack 10 (terminal TD) to TZL (transistor ignition) switching unit for short. If wiring is acceptable, replace switching unit. If dwell reading is within specifications go to next step.
  5. Bridge socket No. 7 (terminal No. 87) and socket No. 8 (terminal No. 30) with jumper wire. If fuel pump is running, test is completed. If fuel pump is not running, go to next step.
  6. Verify Black/Red/White wire from terminal No. 87 to fuel pump is not shorted. If no short, replace fuel pump relay. If fuel pump still does not run, replace fuel pump.
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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.