the tach generator may run a different voltage than the rest of the system, the eps controller may only use 24v signal

have you tried checking the output of the new tach generator?

and on another note there may be a ramp setting in the truck setup for the steering gain (steering speed) and possibly a diode board it goes through, thats the way they had it on the OSO orderpickers, this being a reach truck may be different of course :o)