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Home >> GMC >> 1995 >> Forward Control 4.3 Z, Standard >> Repair and Diagnosis (Single Page) >> Transmission >> Automatic Trans >> Automatic Transmission Diagnosis - 4L80-E >> Technical Service Bulletins (TSB) >> Lack Of Power, Early, Late Or Erratic Shifts >> Early, Late Or Erratic Shifts (All Models)

Early, Late Or Erratic Shifts (All Models)

  1. Check for trouble codes which may relate to condition. If no codes are present, go to next step. If codes are present, correct affected systems, then check for correct shift timing. If condition still exists, go to next step.
  2. Operate vehicle through 2/4 high position shifts. If shifts are early and/or erratic, go to next step. If shifts are late and/or erratic, go to step 5). If shifts are correct, problem is intermittent. Operate vehicle until condition occurs, then repeat step 1). If condition does not occur, check for stuck 4WD switch, loose connections at PCM and 4WD switch, or for circuits which may be shorted to ground. Repair as necessary.
  3. Using a scan tool, monitor 4WD-low input. If 4WD-low input indicates low position even though vehicle is in high position, go to next step. If 4WD-low input indicates truck is not in low position, scan tool value for 4WD-low reading will be NO at all times. If condition exists, check TP sensor, MAP sensor or VSS sensor for proper operation. Repair or replace as necessary.
  4. Scan tool value for 4WD-low position will be YES at all times if 4WD-low input indicates low position even though vehicle is in high position. A short to ground in 4WD-low position circuit, 4WD switch or related circuit may exist. Perform 4WD-LOW DIAGNOSTIC CHECK.
  5. Check TP sensor, MAP sensor or VSS sensor for proper operation. Repair or replace as necessary. If sensors are okay, check tire size and axle ratio. If tire size and axle ratio has been modified from stock applications, buffer (DRAC) must be recalibrated or replaced with correct calibration.
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