hey fellas...ok well I did a search and most of the tach probs I say were not too bad...
mine is weird....
this just happened last week....i have used that tach since I bought the car.....5 years ago...I have been turbocharged for about 4 months...
ok....so the car runs normally.....tach works well until about 6 rpm....then it starts to stutter....the car does not sound like it is breakin up....sounds normal and healthy...the rpm flutters about 6 then continues downward like normal....but by then the reading I think is off....i think it reads a whole 1000rpms off...eg...if i am at 8500 it reads 7500....at least that is what I think from the exhaust tone....
but hear this....when i rev the car in neutral...the rpm pulls all the way to 8500 without a problem...
wha do u think causes this?

 

just a guess, but im going to say a bad ground

 

it's between a bad ground, bad tach signal, and a broken tach.