I have a 1966 a-code with an original rally pac tach and clock. Last instrument I need to get working properly is the tach. for what ever reason, it is reading VERY high. at idle it starts at 4500-5000rpm when its running at about 600-700 in reality. the needle goes up if you rev it up, so its receiving a signal. I was wondering if there was anybody that know how to trouble shoot the install of one of these. Lots of info on the aftermarket ones, but they operate on a totally different principle.