Deere 410b tach not working

[badbowtie]

I just bought a deere 410b and have fixed a couple things and still have couple cylinders to repair. But the hour meter is working but the tach is not can someone help me diagnose the problem.

 

[TractorGuy]

I think most industrial tractors get the tach signal off the alternator. If someone changed the alternator to a car unit you won't have a tach.

 

[badbowtie]

Okay I am still trying to figure out the tack doesn't look like alternator has been replaced. I need some more help to locate problem.