3406e tach wont work

Hi I'm new to this forum so thanks for any help up front. I'm working on a 97 w900 with 3406e. The tach has been intermittent since having the truck lately it hasn't been working much at all. I have installed the tach into other trucks and it works and have tried other tachs in this one and they don't work. I have ohm tested the wires from ecm to tach and they are good there is 12 volts and ground at the tach. I can get an rpm reading using my scan tool. There is no other problems with the truck. There is no tach sensor on the bell housing that I could find. My understanding is the cam sensors gets the rpm reading. I have called kenworth and made sure the switches on the tach are set right. The cat dealer has told me they need to scan it with their cat et. I am wondering if there are any other tests I can do before going to them. Again thank you.

 

I have tried wiggling wires and plugs to make it work and no luck. It seems to work on its own for no reason.