ODD RPM Guage Question

[Sea Raider I]

I am stumped, I am trying to hook up the RPM gauge to my 74 3 cyl Mercury 65hp motor. The gauge works only when I hold the pulse and ground wires in my hand. As soon as I let the wires go it stops working. I have the three wire setup connected to the controls. White wire to my ground buss, then the black and brown wires to gauge. Like I said it works fine when I hold the pulse and negative wires from the gauge..... Never seen this before, any suggestions welcome!!!

Thanks...