1989 gmcJuly 23, 2015oil pressure is reading low, oil is full. rev motor pressure is up. Keywords:oil pressure 1 thought on “1989 gmc” Freeautomechanic July 23, 2015 at 10:03 am This is common when the oil pressure sending unit is failing or leaking. They are plenty cheap enough to just install a new one. This of course is assuming that you have checked the oil level and it is full . Comments are closed.