New Holland Tachometer problem

KMACS · Aug 3, 2021

Have TC 35DA with electronic tach. The tachometer needle does not drop past the PTO mark on the gauge when tractor is shut off. Once cranked it pegs past 3000RPM regardless of engine speed. Doesnt help to tap it and have even opened the cluster and turned it back by hand, but it returns to this state once tractor is started.
Recently had tractor in for repairs. 2 different shops as first shop failed to diagnose the problem. Second shop
replaced fuel injectors and tractor runs great. Tach was working when it went into first shop.

I have read that this tach has a wire that goes to alternator but I cant locate it so far to check it.
All else working on instrument cluster. Question if instrument was shorted while in shop?
Open to any thoughts on how to diagnose and fix the problem. New instrument cluster very expensive!!

dlctcg · Aug 3, 2021

Did you trying putting a call into shop that did the work? They may be able to put you in the right direction quickly...

But here is some info that may help your diagnosis.. I'm going off our TC40D... so can not confirm all is the same on yours...

If I remember correctly fuse #4 supplies power to the tachometer circuit as well as the rest of the instrument panel so if the other instruments are working fuse #4 is good.

The Tach is connected to the alternator via one wire... it will be a single wire connector at the back of the alternator & will run pin #20 on the instrument panel connector.
You can check & make sure it is connected to the alternator first.... then you could check it for continuity with an ohm meter. With the connector plug removed & facing you Pin #20 will be on the bottom row, it will be the 5th pin from right....

Pic of the Plug