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Home >> Mercedes Benz >> 1992 >> 300SD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System/Component Tests - 3.5L Diesel >> Idle Speed Control
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Idle Speed Control

  1. Connect a tachometer to diagnostic connector terminal No. 2 (engine RPM signal) and terminal No. 1 (ground). Diagnostic connector is located in right rear corner of engine compartment. With engine idling and engine coolant temperature approximately 176°F (80°C), engine speed should be 590-630 RPM. If engine speed is 590-630 RPM, go to next step. If engine speed is not 590-630 RPM, proceed to step 3).
  2. Disconnect idle speed control actuator electrical connector. Engine speed should drop to 520-590 RPM. If engine speed drops within specification, idle speed control system is functioning properly. If engine speed does not drop within specification, adjust engine speed at injection pump. See ON-VEHICLE ADJUSTMENTS article.
  3. Repair or replace idle speed control actuator, injection pump, and engine speed sensor. Check Electronic Diesel System (EDS) control unit connector. Repair as necessary.
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