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Test Kg: Tachometer (Excursion & F250-750): Testing

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    NOTE: For Excursion and F250-550 models, go directly to step  8).
    NOTE: For additional testing information, see DIAGNOSTIC AIDS .

  1. 1) Tachometer Inoperative: Check Tachometer PWR Fuse
    Remove and inspect fuse No. 29 (10-amp) from central junction box. Central junction box is located behind lower left side of instrument panel. If fuse is okay, reinstall fuse and go to next step. If fuse is blown, locate and repair short to ground in Tachometer PWR circuit (Red/Yellow wire) between central junction box and instrument cluster. Replace fuse and retest system.

  2. 2) Check Tachometer Ground Circuit

    Turn ignition off. Access connectors behind instrument cluster. Disconnect instrument cluster Black 12-pin connector C220a and inspect for damaged pins, corrosion and loose wires. Repair connector as necessary. Using DVOM, measure resistance between chassis ground and terminal No. 12 (Black wire) at instrument cluster Black harness connector. See Figure. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, locate and repair open in Black wire between instrument cluster and ground G201 located at center of dash, below instrument panel. Restore vehicle and check system for normal operation.

  3. 3) Check PWR Circuit To Tachometer

    Disconnect instrument cluster White 12-pin connector C220c and inspect for damaged pins, corrosion and loose wires. Repair connector as necessary. Turn ignition on. Using DVOM, measure voltage between chassis ground and terminal No. 12 (Red/Yellow wire) at instrument cluster harness connector. See Figure. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, locate and repair open in PWR circuit (Red/Yellow wire) between instrument cluster and tachometer PWR fuse No. 29 in central junction box.

  4. 4) Check Tach Signal Circuit To Instrument Cluster

    Connect scan tool to Data Link Connector (DLC). Connect a jumper wire between instrument cluster Black 12-pin harness connector terminal No. 12 (Black wire) and COM port at scan tool. Connect another jumper wire between instrument cluster White 16-pin harness connector terminal No. 12 (White/Pink wire) and SIG port at scan tool. Start engine and allow it to idle. Using scan tool, perform the following:
    • Select VEHICLE and ENGINE.
    • Select DIGITAL MEASUREMENT SYSTEM.
    • Select FREQUENCY METER.
    • Change level to 4 VOLT DC.
    • Press LINK button to access PIDs.
    • Select PID/DATA MONITOR.
    • Select RPM PID.
    • Press START.

    Observe RPM PID reading at specified engine speed. See RPM PID FREQUENCY SPECIFICATIONS  table. If frequency is as specified, inspect instrument cluster printed circuit board for damage or open circuits. Repair as necessary. If printed circuit board is okay, replace tachometer and retest system. If frequency is not as specified, go to next step.

    RPM PID FREQUENCY SPECIFICATIONS

    RPM Hz
    650 33-53
    1500 90-110
    2500 160-180

  5. 5) Check For Open In TACH Signal Circuit

    Turn ignition off. Disconnect PCM connector and inspect for damaged pins, corrosion and loose wires. PCM is located in left rear of engine compartment, near brake master cylinder. Repair connector as necessary. Install Rotunda Breakout Box (014-00950), leaving PCM disconnected. Using DVOM, measure resistance of TACH signal circuit (White/Pink wire) between instrument cluster White 16-pin harness connector terminal No. 12 and test pin No. 19 at breakout box. See Figure. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open in White/Pink wire between PCM and instrument cluster and retest system.

  6. 6) Check For Short To Ground In TACH Signal Circuit

    Using DVOM, measure resistance between chassis ground and instrument cluster White 16-pin harness connector terminal No. 12 (White/Pink wire). See Figure. If resistance is more than 10 k/ohms, go to next step. If resistance is 10 k/ohms or less, repair short to ground in White/Pink wire between PCM and instrument cluster and retest system.

  7. 7) Check For Short To Voltage In TACH Signal Circuit

    Turn ignition on. Using DVOM, measure voltage between chassis ground and instrument cluster White 16-pin harness connector terminal No. 12 (White/Pink wire). If voltage is more than 10.5 volts, locate and repair short to voltage in White/Pink wire between PCM and instrument cluster. Retest system. If voltage is 10.5 volts or less, replace PCM and retest system.
    8. NOTE: For additional testing information, see DIAGNOSTIC AIDS .

  8. 8) Perform Tachometer Active Command Using Scan Tool
    If any other gauge or indicator on instrument panel is faulty, diagnose instrument cluster concern before proceeding. See appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. Turn ignition on. Perform QUICK TEST  procedures to ensure all electronic engine control systems are working properly and DTCs are not present. If DTCs are present, service appropriate DTC. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no DTCs are present, use scan tool and access Instrument Cluster Tachometer Active Command. Select tachometer, press "Trigger" and scroll in increments of 10 percent. Monitor vehicles tachometer reading. Tachometer should increase in increments of approximately 600 RPM for each 10 percent change. If tachometer reading does not increase as specified, go to next step. If tachometer reading does increase as specified, replace PCM.

  9. 9) Check For Correct Instrument Cluster Operation

    Disconnect all instrument cluster connectors. Check connectors and inspect for damaged pins, corrosion and loose wires. Reconnect all instrument cluster connectors and make sure they seat correctly. Operate system and verify concern is still present. If concern is still present, replace instrument cluster. See appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. Test system for normal operation. If concern is no longer present, concern was intermittent and system is operating correctly at this time. Concern may have been caused by a loose or corroded connector. Retest system.
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