Diagnostic Aids

Perform this test only when directed by QUICK TEST or if no DTCs are present and tachometer is inoperative. This test is only intended to diagnose the following components and circuits.

• Wiring Harness Circuits (VPWR, GND & TACH Signal)
• Tachometer

• Excursion & F250-550
  
  Tachometer is controlled by instrument cluster. Instrument cluster receives engine speed information (RPM) from PCM through Standard Corporate Protocol (SCP) communications network. See Fig 1.
• F650-750
  
  Tachometer is controlled by TAC output circuit on PCM. See Fig 2.

Fig 1: Identifying Tachometer Circuits (Excursion & F250-550)

Courtesy of FORD MOTOR CO.

Fig 2: Identifying Tachometer Circuits (F650-750)

Courtesy of FORD MOTOR CO.