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Tachometer Inoperative/Inaccurate

  1. Tachometer is driven directly by an output signal from the HEI module to IPC. Start engine. If tachometer is inoperative, go to step 4). If tachometer functions, enter self-diagnostics. See ENTERING SELF-DIAGNOSTICS under SELF-DIAGNOSTICS. Select PCM parameter ED11 (RPM). Compare gauge reading and ED11 value while varying engine RPM.
  2. If both are within 200 RPM of each other, no problem is found. Check for intermittent conditions such as tachometer White wire too close to secondary ignition wires, faulty terminal connections, or broken White wire somewhere between IPC Black connector terminal B1 and HEI module. See Figure.
  3. If both vary by more than 200 RPM, ensure good connection at IPC terminal B1. Remove IPC. See INSTRUMENT PANEL CLUSTER  under REMOVAL & INSTALLATION. If terminal contact is good, replace IPC.
  4. If tachometer is inoperative, turn engine off. Remove IPC. See INSTRUMENT PANEL CLUSTER  under REMOVAL & INSTALLATION. Using DVOM set to AC voltage scale, ground negative probe. Backprobe positive DVOM probe to IPC connector terminal B1. If voltage is greater than 8 volts AC and changes as RPM changes, ensure IPC terminal B1 connection is good. If terminal has good contact, replace IPC.
  5. If voltage is less than 8 volts AC and/or voltage DOES NOT change with RPM, turn ignition off. Disconnect IPC connector. Start engine. Using DVOM set to AC voltage scale, ground negative probe. Backprobe positive DVOM probe to IPC connector terminal B1. If voltage is greater than 8 volts AC and changes as RPM changes, ensure IPC terminal B1 connection is good. If terminal has good contact, replace IPC.
  6. If voltage is less than 8 volts AC and/or voltage does not change with RPM, repair open in tachometer White wire between IPC connector terminal B1 and HEI module.
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