I recently bought a 97 F150 with the 4.6 engine and automatic tranny. It runs very well but I do have a small question. It has a factory tachometer. most of the time the tach works right (as far as I know) but on a regular basis it will drop to zero or it will stay at zero at start up and stay there for about a half hour of driving and then go back to normal operation. On one occasion the tach suddenly went all the way to max!! it stayed that way till I shut the engine down. the next time i drove it, it worked normaly. Is this a case of a bad tach (probably) or is it a sign of some computer or wiring problem? Any clues?