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Test AQ: Tachometer Inaccurate/Inoperative

  1. If instrument cluster diagnostic system check was performed, go to next step. If instrument cluster diagnostic system check was not performed, go to INSTRUMENT CLUSTER DIAGNOSTIC SYSTEM CHECK  under DIAGNOSTIC SYSTEM CHECKS.
  2. Turn ignition off. Connect scan tool to Data Link Connector (DLC). See Figure. Start engine and test drive vehicle. Using scan tool, select Powertrain Control Module (PCM) data display. Observe ENGINE SPEED parameter. If ENGINE SPEED parameter does not match tachometer reading, go to next step. If ENGINE SPEED matches tachometer reading, problem is intermittent, check for failures in White wire between PCM 80-pin harness connector terminal No. 56 and instrument panel cluster 16-pin harness connector terminal A4. See Figure and Figure .
  3. Check for open, short to ground, short to voltage or high resistance in White or White/Black wire between PCM harness connector C2 (Clear) terminal No. 56 and Instrument Panel Cluster (IPC) connector C2 terminal A4. See Figure and Figure . If no problems are found, go to next step. If any problems are found and corrected, go to step  8.
  4. Check for poor connections at Instrument Panel Cluster (IPC) connector. If no problems are found, go to step  6. If any problems are found and corrected, go to step  8.
  5. Check for short to battery voltage in White wire between PCM 80-pin harness connector terminal No. 56 and instrument panel cluster 16-pin harness connector A4. See Figure and Figure . If no problem was found, go to step  8. If a problem was found and corrected, go to step  10.
  6. Check for poor connections at Instrument Panel Cluster (IPC) connector. If no problems are found, go to next step. If any problems are found and corrected, go to step  10.
  7. Check for poor connections at PCM 80-pin harness connector C1 (Blue). If no problems are found, go to step  9. If any problems are found and corrected, go to step  10.
  8. Replace IPC. See INSTRUMENT PANEL CLUSTER  under REMOVAL & INSTALLATION. After repairs, go to step  10.
  9. Replace PCM. See POWERTRAIN CONTROL MODULE under REMOVAL & INSTALLATION in BODY CONTROL MODULES - DEVILLE article. After repairs, go to next step.
  10. After repairs, recheck operation of tachometer. If tachometer operates properly, repair is complete. If tachometer does not operate properly, return to step  2.
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