starter question

[gf899]

I took the old starter up to the auto parts place (Advance Auto) and it checked out OK
on the machine, so I tried new (+) battery cables but no go. (just clicks sometimes).
I also cleaned the grounding surface on the block.

I know the ignition switch and relay are OK because I hooked the starter up outside the truck and
it spun over.

I am thinking that it is weak and not working under load is why it worked on the tester.
Anyone else have a similar experience or any ideas?

Any help appreciated,

Greg

1978 Ford F150 auto 2 WD 351M

 

[gf899]

UPDATE,

New starter fixed problem.