76 912E basically a 911 tach question...
 

I have a tach out of my 912E a 76 has 3 prongs on the back, Im assuming these are ground, the pickup, and the other one im un-sure about. It goes to a box under the cowl that has VDO on it, however it also has EGR on it. Do I need this box to run the tach? And I had accidentally pulled the connectors off of the tach when pushing it through the dash. Which prong goes to where. They are numbered 1,2, and 3. And how do the 912s or basically 914s pick up there tach signal? Im new to porsches and have owned a BMW for the last couple years so thats where my knowledge is. Thanks

I'll take a guess and say the tach pics up it's signal from the ignition coil, thats how 911's are at least.