lift pump wont shut off

I have a 94 gmc 6.5 diesel that has a problem, the lift pump refuses to stop running when I shut the key off. I realize that it is made to run a few seconds until oil pressure cuts off. I have owned this pickup for ten years, so I am certain that pump is not wired direct, I have replaced lift pump and opsu, and does not change. The only way to stop it is by unplugging harness at pump. I also found that if I unplug opsu, it stops running. can my new opsu be bad or is it another problem I am not thinking of? I appreciate any help!:bang

Sounds like a bad oil pressure sending unit. If unplugging it shuts it off, then you found the problem. ONLY use a genuine AC DELCO for it unless you want to have repeated failures.