'Could be ...
IF YOU'VE LET MOST OF YOUR OIL LEAK AWAY WITHOUT REPLACING IT!

More likely the two rubber o-rings on the Oil Filter cap shaft are old,hard and brittle.

OR your Bearings are GONE and don't hold pressure anymore.

Most Likely ,However, is that you've knocked loose the OP sensor line [Wire].

__________________
'84 300SD sold
124.128