SN95 “jumpy” tachometer

[69 Rustang]

We have a 94 Mustang GT in the fleet as well as our other Mustangs.  I haven’t driven it a lot, maybe 200-300 miles since we bought it.  Due to some current circumstances I am driving it frequently at the moment.  Today I noticed the tach was jumpy.  On the freeway, it was bouncing up 500 rpm over where it should be.  The engine wasn’t revving or anything, just the tach needle.  The fuel gauge was also dropping about an 1/8 tank, then it would come up.  The other gauges were all fine the whole time.  5.0, 5 speed, 146k miles.  Has new plugs, wires, cap about 150 miles ago.  Any thoughts?

 

[midlife]

Bad ground?  Loose connection at the battery or fuse box?  You could also be losing your mind (if you find it, let me know because I lost mine some time ago and it might be hiding where yours is...)

 

[c9zx]

+1 on grounds. Chuck

 

[tashakes]

Not sure how the 94 works, but as above mentioned maybe a bad circuit card or connection/ground to the instrument cluster?

 

[Steve McMahan]

If it wasn’t for the fuel gauge fluctuation, I would say to look at the coil connector. You’ll see three wires on two contacts. The one with two wires is the tach feed. See if that connector is clean, has a tight fit, etc. The feed wire for the fuel gauge come up on the passenger side, under the sill plate. There is a ground under that plate, an orange wire I believe. That is the ground for the fuel pump and the sending unit.