2000 Powertech diesel not cranking fast enough to start.

Ok. Warmed up gen and changed oil and filter. Put in 10-30. No difference. Did notice starter was behind air filter canister. Putting in new one today. All connections look good as far as ground and positive hook up inside gen cabinet. Need my scope to look at exterior connections. Will keep you all posted today when I put in new starter. Somehow I don't think that is the prob. I noticed tear marks in the insulation from a wrench as someone has replaced the starter once. The mystery continues.