NosXr3i

I am trying to put a digital Rev counter into my MK4 Cabriolet which has the old Mechanical fuel injection system on it( the engine and loom are post 86). The old rev counter works fine but when I connect the digital counter it is fluctuating really badly and wont settle down, I can only assume the original has some sort of suppressor built in and this one dosent, Can anyone help me and let me know how to get the digital counter to work properly.