Tachometer Fluctuation

[chardway]

As a newbie, one more item I am having trouble with on my 96 PS is the tach. The tachometer will fluctuate about 600 - 700 rpm when I am cruising. I can be travelling along at 70 mpg and will see the tach jump from about 2,200 rpm to about 2,700 rpm (I have 4.10 gear). Anyway, I am 100% sure it's not the tranny going in and out of lock-up or shifting in and out of gear.

Any ideas what may be making it "jump". Some days it will read on the low end all day, other days the high end, and some days it is irratic. I assume the tach is electric but wonder what signal drives it??

Thanks!

[circle5]

should be 2700 with 4.10 gears

[fordnut85]

The signal comes from the cam position sensor and they are notorious for going south on our trucks so that would be my first guess. Most guys (myself included) carry a spare in the glove box since they are so common to failure. If you can get access to a good scan tool you can plug in and watch the CPS signal for a spike or drop then you would know for sure that it is the CPS and not the Tach itself wich is still a possibility.