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.Google+
Car Problem Reports
Ford F-250 The tachometer sensor may fail
Ford F-250 Problem
Drive Trains Affected: 2WD, 4WD, 7.3L V8 Diesel, Diesel
Average Mileage: 149,125 mi (124,000 mi - 188,500 mi)
Visitor, 1993 Ford F-250, 4WD, 149,000 mi
This I think is also causing the engine to hover when at a constant speed.
Visitor, 1993 Ford F-250, 4WD, 124,000 mi
When at idle tach drops to zero then when accelerator is pressed it jumps to appropiate engine speed
Visitor, 1993 Ford F-250, 2WD, 188,500 mi
just started had to order part
needle jumps below 1400 rpm and shifts incorrectly above this its ok. im going to try the sensor.