I have a 1986 F350 with a 6.9 liter diesel and a C6 tranny. The tach recently started jiggling real quirky. Sometimes it'll work OK but mostly not, up an down. Anybody identify this problem and advise how to fix? Thanks

 

Check the wires on the sensor. It's up front on the engine and reads the teeth on the gear that drive the injection pump.

 

Thank you Dave, I'll take a look and see, Bill

 

Also read up in the How-To and FAQ sections. There are a few threads about the tach's acting up and fixes there.

 

Thank you, I'll do that too, Bill