Im trying to solve my thunderbolt tach problems on my '78 1400...the tach im trying to use on my 1400 works fine on my '72 1150, but when i connect it to my '78 1400 it draws the tach needle backwards and it flutters wildly...the ignition set-up on these two engines is exactly the same, but the tach won't work correctly on the 1400...NOW, on my '72 1150 i plug the tach into my mercontrol and the brown signal wire from the mercontrol is attached to the brown post on the switchbox at the engine...tach works fine....NOW i know that in the later years mercury moved the brown wire off of the switchbox and connected it to one of the yellow wires at the rectifier...HERE's my question: if you connect to the yellow stator wire, you are receiving AC voltage, if you connect to the brown post at the switchbox you are receiving DC voltage to the tach...i have read posts here on iboats that directed you to remove the brown signal wire from the switchbox and connect it to the yellow stator wire...that doesn't make sense to me because then you are changing from DC voltage to AC voltage...i don't get it...!!
