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P0654Engine RPM Output Circuit

There is an electrical fault in the circuit the PCM uses to output the engine speed (tachometer) signal to other modules or the instrument cluster.

Definition

On many vehicles the PCM generates a conditioned engine RPM output signal for the instrument cluster tachometer, body modules, or add-on equipment such as PTO controllers. P0654 sets when the PCM detects an open, short to ground, or short to voltage on that output circuit. The engine itself usually runs normally because the PCM's internal crankshaft speed input is separate, but the tachometer may read zero or erratically and dependent accessories may not operate. The MIL may or may not illuminate depending on the manufacturer's implementation.

Common causes

  • Open or shorted wiring between the PCM RPM output pin and the instrument cluster or receiving module
  • Corroded or backed-out terminal at the cluster or PCM connector
  • Aftermarket tachometer, remote starter, or upfitter equipment tapped into and loading the RPM output
  • Faulty instrument cluster input circuit
  • Failed RPM output driver inside the PCM

Common misdiagnoses

  • Crankshaft position sensor replaced even though the engine runs fine and the fault is on the output side
  • Instrument cluster replaced before testing the signal wire
  • PCM replaced without checking for an aftermarket accessory loading the circuit
  • Ignition coil or ignition module replaced on older platforms

Troubleshooting steps

  1. 1.Confirm the engine runs and note the symptom

    Verify normal starting and running; a dead or erratic tach with a normal-running engine supports an output-circuit fault rather than a crank sensor problem.

  2. 2.Check for aftermarket taps

    Inspect for remote starters, aftermarket gauges, or upfitter connections spliced into the RPM output wire, and disconnect them to see if the code clears.

  3. 3.Scope or meter the output signal

    Backprobe the PCM RPM output pin with the engine running and look for a square-wave signal whose frequency tracks RPM; a flatline at 0 or 12 volts indicates a shorted circuit or dead driver.

  4. 4.Test the circuit end-to-end

    With connectors disconnected, verify continuity from the PCM pin to the receiving module (under 1 ohm) and no shorts to ground or voltage.

  5. 5.Isolate PCM versus receiver

    If the signal is good at the PCM pin but the tach is dead, diagnose the cluster; if the signal is absent with a proven-good circuit, the PCM output driver has failed.

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