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Q: Wheres my tach. sensor located on my 91 gmc 1500 sierra? on 1991 GMC 1500 Pickup

It jumps around
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what else is jumping besides your tach? Engine? In other words is tach a reflection of how engine is really running or is it doing it's own thang? That answer will determine what needs to be serviced/replaced.
Now that we have that out of the way , it's time for you to 'make nice' with a parts guy at your local GM dealer and see if you can get some info from him or his favorite tech in shop to find out any common issues they had with the tach or signal circuit (possibly a printed circuit board issue) Either way , given the age of the truck , and if it's running good ,I'm not sure how much the tach would concern me (relative to expense of repair). You could try removing and un-plugging then plugging the instrument cluster back in check all pins for corrosion.might just be worn contact after 21 years. good luck! hope I've been a little help.
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