[Stutzdriver]

The tach I removed had a ground lug spot welded the the back of the tach housing and a pair of brown (ground) wires attached to the lug. Since the replacement had no lug I cleaned th back of the housing a silver soldered a spade lug as was there on the old tach in the same spot of the housing. Now that I think about it might the replacement tach been form an older (newer) 944 and it is internally grounded? The old one might have not had the housing internally grounded?

I'll test the old tach as to the ground as you describe and let you know what I find.