1992 F250 Tach Problem
 

I have a 1992 diesel 5 speed, and the tach is reading low rpm. Tach is working and will move but reading low rpm (I am guessing 700 rpm lower idel rpm shows 200-300). I replaced the tach sensor, checked, cleaned, lubed all plugs on right side, left side firewall, and behind insturment panel. Anyone had or have the same problem. I found the Ford Part Number 17360 which is the tach assembly for about $60 not sure if I should by it?