Tachometer stuck at 3500.

I have had this probelm for a little while now, and have attempted to do a little research myslef, but all my attemps havent really ammounted to much.
About a 5 months ago, before my timing belt broke, transmission took a crap, and alternator went out....my tachometer quit working.
Kinda.
When a turn the car on the tachometer will point to about 3500, this is with the car NOT running.
It says that even with the car running at idle though.
But its not like the needle is stuck there, if i rev the engine up, the needle will move all the way u threw the red.

I have read that the tachometer gets its readings threw the negative cable on the ignition coil...where is this cable?
And how would I test it to see if my tachometer is screwed up, or if its the ignition coil thats bad?

I work as an intern at a local shop, so I have access to...prettymuch everything.
Your help is greatly appreciated!
 
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