ok I have a 82 c20 with the 6.2...when i try and start the truck it cranks very slow not even turning the fam all the way around then acts like batteries are weak...i have replaced both batteries with atleast 650cca...replaced all battery cables with 2 gauge...took the starter to advance autoparts they tested it and the batteries thay are all good...I can roll start the truck down a small hill without even using the glowplugs...starts every time runs great all day almost 60psi oil pressure when driving 45psi at idle...I have been driving it every day just cant shut it off or it will not start...unless roll started in 2 gear...any ideals i'm out...need help

 

check the ground on the passenger side rear of the head if that checks out sometimes you will have to take out the starter and clean between it and the engine block this will also cause problems especially on old and greasy engines

 

I have already took a wire brush on a drill and cleaned up the starter mounting surfaces...haven't checked the ground on the back of the head will do that in the morning

 

It sounds like a bad ground in the system to me

 

I checked the ground on the passenger side the braided ground wire looked good cleaned up mounting surfaces...still cranks really slow...I have one battery grounded to the body and the motor and the other battery is just grounded to the motor...any other ideals I'm down to try just about anything...I'm tired of having to roll start

 

Try grounding directly to the starter this will at least rule out a poor ground issue

 

battery to starter or frame to starter or both

 

One to the frame the other to starter

 

I got grounds covered but my batteries were a little weak so I'm charging them up before I try her out...I hope this works

 

I have experienced this before, very likely it will be your starter. If you pull the starter and power it with no load it will spin just fine (which is what the auto parts store did) but if you apply a load, I used a chunk of 2x4 and forced it against the starter teeth. A good starter will chew the wood away instantly, a bad starter will sit there do nothing. Before pulling the starter off again, try turning the engine over by hand. You will know right away if the you have any internal engine damage causing slow cranking...