1998 Ford Mustang SPEEDOMETER

SPEEDOMETER/ODOMETER INACCURATE
Road test vehicle. If speedometer is operating, go to next step. If speedometer is not operating, go to step 5).
Drive vehicle over a known, measured distance. If odometer is reading accurately, go to next step. If inaccurate, go to step 4).
Drive vehicle at a steady speed over a known, measured distance and record time taken to travel that distance. If speedometer meets Speedometer Calibration Tolerance specifications, system is okay. See SPEEDOMETER CALIBRATION TOLERANCE table. If speedometer fails to meet speedometer calibration tolerance, replace speedometer assembly.
SPEEDOMETER CALIBRATION TOLERANCE (1)
Actual Speed - MPH. Allowable Range - MPH
30.27.8-34.7
60.57.7-64.5
(1)Odometer measured over actual 10 miles: 9.6-10.3 miles.

Check for correct driven gear on Vehicle Speed Sensor (VSS). If driven or drive gear is suspect, refer to manufacturer's parts department for correct specifications in relation to final drive ratio and tire size. If parts are correct, go to next step. If parts are not correct, install correct parts and retest system. See Fig. 5.
Raise drive wheels off ground and support vehicle. Disconnect vehicle speed sensor connector. Connect and AC voltmeter between output terminals of VSS. Operate vehicle in Drive at 20-40 MPH and observe voltmeter. If voltage is 1.3-6.1 volts AC, go to step 8 If voltage is not correct, go to next step.
Disconnect VSS from automatic transmission. Verify presence of speedometer driven gear with all teeth in good condition and presence of retainer clip. If driven gear teeth are okay and clip is present, go to next step. If gear is damaged or clip missing, replace with proper speedometer gear and clip and retest system.
Verify presence of speedometer drive gear on output shaft of transmission. If drive gear is present, go to next step. If drive gear is not present, repair transmission as necessary. See appropriate TRANSMISSION repair in TRANSMISSION SERVICING - A/T in the AUTOMATIC TRANS SERVICE section or TRANSMISSION SERVICING - M/T article in the MANUAL TRANS SERVICE section.
Disconnect negative battery cable and remove instrument cluster. See INSTRUMENT CLUSTER under REMOVAL & INSTALLATION. Disconnect instrument cluster connectors C250 and C251. See Fig. 3 and Fig. 4. Reconnect negative battery cable and turn ignition switch to RUN position. Check for battery voltage at instrument cluster connector C250 terminal No. 4 (Light Green/Yellow wire). If battery voltage is present, go to step 10). If battery voltage is not present, go to next step.
Remove fuse No. 8 (10-amp). See Fig. 2. Measure resistance between ground and instrument panel connector C250 terminal No. 4 (Light Green/Yellow wire). If resistance is more than 10 k/ohms, repair open in Light Green/Yellow wire and retest system. If resistance is less than 10 k/ohms, repair short to ground in Light Green/Yellow wire.
Turn ignition off. Measure resistance between ground and instrument cluster connector C250 terminal No. 6 (Pink/Orange wire). If resistance is less than one ohm, go to next step. If resistance is more than one ohm, repair open in Pink/Orange wire and retest system.
Reconnect VSS. Using an ohmmeter, measure resistance between ground and instrument cluster connector C250 terminal No. 5 (Gray/Black wire). If resistance is less than 10 k/ohms, repair short to ground in Gray/Black wire and retest system. If resistance is more than 10 k/ohms, go to next step.
Using an ohmmeter, check resistance of Gray/Black wire between cluster connector C250 terminal No. 5 and VSS connector (circuit No. 679). If resistance is less than one ohm, go to next step. If resistance is more than one ohm, repair open in circuit No. 679 (Gray/Black wire) and retest system.
Check resistance between ground and circuit No. 676 (Pink/Orange wire) at VSS connector. If resistance is less than one ohm, go to next step. If resistance is more than one ohm, repair circuit No. 676 (Pink/Orange wire) for open and retest system.
Turn ignition off. Check for continuity between cluster connector printed circuit connections and gauge. If there is continuity through instrument cluster printed circuit, replace speedometer assembly and retest system.

Check and replace vehicle speed sensor on left rear of tranny