90 ranger tach

Does anyone know where the factory tach on my 90 Ranger 2.9L is tied into? I've heard both "alternator" and "coil". Mine will intermittantly read higher rpm's than normal, and then will return to normal. None of the electrical schematics I have reference the tach wiring. Thanks in advance for any help!

Mine does that occasionally in cold weather. Its a 90 w/2.9 also. I havent figured out what it is though.

i have a 91 ranger with a 4.0 same problem and speedometer is off when its cold out