tach....

The problem I'm having is the stock tach on my 93 GT is messed up. I know it worked correctly when I first got the car and then one day the problem just suddenly started. The car had no modifications. What happens is the tach needle will go in the worng direction. Sometimes, when I drive at slower speeds and shift around or under 3k, I can get the tach needle to actually go up in the right direction. But for the most part, especailly when I'm getting on the throttle, the needle goes down in the opposite direction. I posted this problem a long time ago when the problem first started but I only got a few responses. One person said If i have a msd box that could cause the issue, I had an msd box but I'm not 100% sure if the problem started before or after i installed that, like I said it just started out of no where. But, even after I took the msd out, it still did it. I think maybe my best bet is to open the cluster up and see if any connections are loose or disconnected. Anyways, istead of finding the problem, I installed a 5" autometer tach, which works perfectly and is obviously better than the stock tach. I usually try not to pay attention to the stock one, but it gets annoying to see the needle go the wrong way not to mention looks pretty lame. Does anyone have any ideas?

Couldn't figure out where that wire is..........I did come across something that said the stock tach wire is t/y, im guessing tan and yellow? So, I'll go check that out. Where would this wire be located exactly on the ignition coil? For 1987-89 mustangs, the stock tach wire is green with the yellow stripe. But, thanks for the idea of just disconnecting it, that would be much better...