Have Transmission Questions? Ask a Mechanic for Answers ASAP
The best way to diagnose this one is with a compatible ford scan tool. The scan tool will tell you whether the signal coming from the speed sensor on the transmission matches up to what the PCM is reading. That would tell you whether the speed sensor/sensor wiring is the concern or if the PCM is the problem. Sometimes we see these PCMs lose programming and the speedometer acts up until you have the PCM reprogrammed.
Also, try unplugging the alternator and retest drive the vehicle. If the engine runs good with the alternator disconnected, suspect a bad diode in the alternator causing RF interference with the OSS sensor. This would also cause it to shift incorrectly.
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