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Tachometer Output Failure

  1. Check CTO Circuit For Short To Power -  Turn ignition off. Disconnect PCM 104-pin harness connector. Connect Breakout Box (49-UN01-130) to PCM harness connector only. See Figure . With KOEO, measure voltage of CTO circuit between breakout box terminal No. 48 and ground. Turn ignition off. If voltage reading is less than 0.5 volt, go to next step. If voltage reading is 0.5 volt or more, repair short circuit. See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. Verify symptom no longer exists.
  2. Check CTO Circuit For Short To Ground In Harness -  Disconnect PCM 104-pin harness connector. Disconnect instrument cluster 16-pin harness connector. Measure resistance between breakout box terminal No. 48 (CTO SIG circuit) and terminals No. 51, 76, 77 or 103 (PWR GND circuits) and chassis ground. Record readings. Measure resistance between breakout box terminal No. 48 (CTO SIG circuit) and chassis ground. Record reading. If both resistance readings are more than 10,000 ohms, go to next step. If either resistance reading is 10,000 ohms or less, repair short in wiring harness. Verify symptom no longer exists.
  3. Check CTO Circuit For Open Harness -  Measure resistance of CTO circuit (Tan/Yellow wire) between breakout box terminal No. 48 and instrument cluster 16-pin harness connector terminal No. 15. If resistance reading is less than 5 ohms, go to next step. If resistance reading is 5 ohms or more, repair open circuit in Tan/Yellow wire between PCM 104-pin harness connector terminal No. 48 and instrument cluster 16-pin harness connector terminal No. 15. See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. Verify symptom no longer exists.
  4. Check CTO Signal From PCM -  Connect PCM 104-pin harness connector. With KOER, measure voltage of CTO circuit (Tan/Yellow wire) between instrument cluster 16-pin harness connector terminal No. 15 and ground. If voltage reading is 3-9 volts, see appropriate testing in INSTRUMENT PANEL article in ACCESSORIES & EQUIPMENT. If voltage reading is not 3-9 volts, an open or short is present in PCM. Replace faulty PCM. Verify symptom no longer exists.
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