It is possible to just add a tach to your instrument cluster. If you have a truck with idiot lights instead of gauges this might be the best route to go.
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What you need to do is wire it up like this. On the back of the cluster, around where the 4 "bolts" are that hold the tach into the panel. These bolts are also the electrical connections for the tach. There will be 2 letters and 2 numbers. To wire the tach up, you need to do it like this.<O</O

1 or C = Hook the "Tach" (neg side of coil) wire here.
B = This goes to 12v switched power.
G = Ground if your truck has an Inline 6 or V8. <O</O
And also
8 = Ground if your truck has a V8. If you don’t ground this the tach will not read correctly if you have a V8.
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You should test your tach first using jumper wires to be sure it works before you start ripping and tearing your instrument panel apart. My tach had this P/N on the back of it. P/N EOTH-17380-AA
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"G" is the upper right
"B" is the lower right
"C" is the upper left
"8" is the lower left






This is what the back side of the instrument cluster looks like on a factory instrument cluster with a tach. You will not have the red flexible wire circuit material where your tach goes on a factory non-tach installation and will have to hard wire the needed posts.



Photos are from my Photo Albums located here.
https://www.ford-trucks.com/forums/a...p?albumid=1453


Jim