Installed a tachometer on my 2006 C90 Boulevard this weekend. Seemed to be working in garage. Took to work this am, at 80 mph was only reading 1600 rpm's. Any ideas? 4 wires, 1-ground, 1-backlight, 1-hot, and 1 - neg wire to coil. All hooked up just as diagram shows. Generic tach that was bought over a year ago, cant remember maker.