LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> GMC >> 1999 >> Forward Control 4.3 W, Automatic >> Repair and Diagnosis >> Engine Performance >> Engine Adjustments >> Injection Pump Timing (Diesel) >> TDC Offset Learn Procedure
April 5, 2026: LEMON Manuals is launched! Read the announcement.

TDC Offset Learn Procedure

This procedure is only used when one of the following conditions exist:

  1. Start engine and allow it to reach normal operating temperature. Connect scan tool to Data Link Connector (DLC). Clear all DTCs. Turn ignition on, with engine off. Using scan tool, select OUTPUT TEST/INJ PUMP mode. Activate TDC OFFSET LEARN.
  2. If learned TDC OFFSET reading is between -.25 and -.75 degree, procedure is complete. If reading is not as specified, hold accelerator pedal in wide open throttle position for at least 45 seconds. Turn ignition off for 30 seconds. Start engine. TDC OFFSET reading should be zero. If reading is not zero, repeat step  1.
  3. With engine running, PCM will learn new TDC OFFSET when normal operating temperature is obtained and engine speed is less than 1500 RPM. New TDC OFFSET will overwrite previous reading. If new learned TDC OFFSET reading is not between -.25 and -.75 degree, turn ignition off.
  4. Slightly rotate injection pump and repeat step  1. A .039" (1.0 mm) pump movement equals about a 2 degree timing change. To achieve a negative reading, rotate pump toward driver's side. To achieve a positive reading, rotate pump toward passenger's side.