The tachometer sensor may fail; if the tachometer needle jumps around and the transmission has harsh shifts or does not upshift, this sensor should be checked. This sensor is mounted in a cover at the front of the motor.
Average mileage: 154,375 (108,500–200,000)
Drive Trains affected: 2WD, 4WD, 7.3L V8 Diesel, Diesel
16 people reported this problem
I have the same issue right now. Before the tach wasnt working at all. Then I screwed around a bit with the sensor and it started to work, but a month of two later, it started to jump around allot causing it to shift quickly between 2nd and 3rd gear very quickly over and over again for 10 minutes on the road. I just replaced the sensor with a brand new OEM one, and so far its stopped. So all hopes that it just needed a sensor.