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-350 The tachometer sensor may fail
Ford F-350 Problem
Drive Trains Affected: 2WD, 7.3L V8 Diesel, Automatic Transmission, Diesel
Average Mileage: 154,865 mi (137,596 mi - 167,000 mi)
Visitor, 1990 Ford F-350, Automatic Transmission, 137,596 mi
Tachometer jumps around, even going to zero, causing irratic down and up shifting of the automatic transmission. Very disconcerting.
Visitor, 1993 Ford F-350, 2WD, 160,000 mi
I have a 7.3L and the tachometer started dropping down and becoming erratic (after the engine heated up)then 1800-2000 rpm sometimes it becomes rock solid. So I bought a new sensor from Ford Dealer and I'm about to install it today. This circuit uses a HET (Hall Effect Transister) type pick up and is probably breaking down under heat. It does seem most people have the problem begin around 160,000.