Sensor or gauge? The tach drops to "0" when ever engine speed is below 800 rpm, assuming the reading is accurate. The truck is my neighbors '91 F-250 SC Lariat, 7.3, auto w/OD, 4x4, around 125,000 reasonably trouble free miles. I think the tach runs off the two wire plug in the top front of the engine, by the injector pump, is this correct; and can it be tested? Is it cheap enough to just replace without bothering to test? How would you test a gauge? Thank You.

 

More than likely the cause is the sensor you are speaking of, check the wires, and make sure they are not broken, the sensor runs off the injector pump timing gear.

Dave

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