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Test E: Tachometer Inaccurate: Coupe

  1. Using scan tool, observe RPM output Parameter Identification (PID) over range of engine speeds. If RPM PID varies smoothly with vehicle RPM, vehicle is okay. If RPM PID does not vary or is erratic, see appropriate SELF-DIAGNOSTICS article in ENGINE PERFORMANCE to diagnose Clean Tach Output (CTO) circuit. If only tachometer is inaccurate or inoperative, go to next step.
  2. Turn ignition off. Remove instrument cluster. See INSTRUMENT CLUSTER  under REMOVAL & INSTALLATION. Disconnect instrument cluster electrical connector C253. See Figure . Turn ignition on. Measure voltage between ground and instrument cluster harness connector C253 terminal No. 12 (Black/Yellow wire). Voltage should be more than 10 volts. If voltage is more than 10 volts, go to next step. If voltage is less than 10 volts, repair short to voltage in Black/Yellow wire between instrument cluster connector and fuse. See WIRING DIAGRAMS  .
  3. Turn ignition off. Measure resistance between ground and instrument cluster harness connector C253 terminal No. 6 (Black/Green wire). See Figure . Resistance should be less than 5 ohms. If resistance is less than 5 ohms, go to next step. If resistance is more than 5 ohms, repair open in Black/Green wire between instrument cluster connector and ground. See WIRING DIAGRAMS  .
  4. Visually inspect instrument cluster printed circuit for damage, cracks or hot spots. If instrument cluster printed circuit is okay, go to next step. If instrument cluster printed circuit is damaged, replace printed circuit and gauge contact clips. See PRINTED CIRCUIT  under REMOVAL & INSTALLATION.
  5. Install EEC-V 104-pin Breakout Box (014-00950) following manufacturer's instructions. Measure resistance between breakout box terminal No. 48 and instrument cluster harness connector C253 terminal No. 5 (Light Green/Red wire). See Figure . Resistance should be less than 5 ohms. Measure resistance between ground and instrument cluster harness connector C253 terminal No. 5 (Light Green/Red wire). Resistance should be more than 10 k/ohms. If both resistances are as specified, see appropriate SELF-DIAGNOSTICS article in ENGINE PERFORMANCE to diagnose Clean Tach Output (CTO) circuit. If resistances are not as specified, repair open in Light Green/Red wire. See WIRING DIAGRAMS  .
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