My 79 Grand Am Speedo Not working & Tach jumping all over the place I had to replace Speedo from a 82 GP to the 79 Had to use the ground from the back of the 79 and place it on the 82 and then I saw the shift cable was cutting into by the Shift indicator plastic fixed it with a light pull chain Bell at the end of the chain it worked as for the tach it was the clip on the back was moving from the 8 setting to the 6 setting making the tach all over the place and when you do this Check all Bulbs before putting back in
