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Tachometer Control Circuit Diagnosis (4.3L)

NOTE: Before clearing DTCs, use scan tool to capture and save freeze frame records for reference. Information will be deleted when DTCs are cleared.
  1. Perform powertrain diagnostic system check. See POWERTRAIN DIAGNOSTIC SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing powertrain diagnostic system check, go to next step.
  2. Install scan tool. Using scan tool, select BODY (IPC) diagnosis tachometer sweep test function. Sweep tachometer needle on and off. If tachometer needle sweeps when commanded, go to next step. If tachometer needle does not sweep when commanded, diagnose instrument cluster. See appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT.
  3. Using scan tool, select POWERTRAIN tachometer control. Start engine and let idle. Using scan tool, command tachometer output to 600 RPM, and then to 6000 RPM. Compare scan tool display to actual tachometer value displayed on instrument cluster. If scan tool display matches instrument cluster display, system is okay. If scan tool display does not match instrument cluster display, go to next step.
  4. Disconnect instrument cluster connector that contains tachometer output circuit. Using scan tool, select POWERTRAIN tachometer control. Connect DVOM between ground and tachometer circuit of instrument cluster connector (harness side) and measure AC frequency (press Hz button) of tachometer circuit. Start engine and let idle. Using scan tool, command tachometer output to 600 RPM, and then to 6000 RPM. Compare DVOM display to specifications in table. See FREQUENCY VS. RPM  table. If DVOM display matches table specifications, diagnose instrument cluster. See appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. If DVOM display does not match table specifications, go to next step.
  5. Check for open, short to ground or short to voltage in tachometer output circuit. See WIRING DIAGRAMS article. If problem exists, go to next step. If problem does not exist, go to step  7.
  6. Repair circuit as necessary. After repairs, go to step  3.
  7. Check for poor connection at VCM. If problem exists, repair as necessary and go to step  3. If problem does not exist, go to next step.
  8. Replace VCM and reprogram. Perform VTD password relearn procedure and perform crankshaft variation learn procedure. See COMPUTER RELEARN PROCEDURES in GENERAL INFORMATION article. After repairs, go to step  3.
FREQUENCY VS. RPM

Commanded Output (RPM) VCM PWM Signal (Hz)
500 16.4
1000 32.9
1500 49.4
2000 66.7
2500 83.1
3000 99.7
3500 116.1
4000 133.6
4500 150.1
5000 166.6
5500 183.1
6000 200.4
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