I hooked up a factory tach and when I start up, the needle goes past 5000 rpm and points straight down and doesn't move when I rev it up. I hooked up an aftermarket tach to the same leads and it reads correctly. I do not know what year the factory tach is out of, but mine is an '89. Has anyone had a problem like this or know what may be wrong? The guy I got it from swore it worked when he took it out the original truck.