Here is what I know: I was trying to start my 91 ecodiesel after swapping to a turbo pump, when one day, while cranking, the cranking speed suddenly dropped. I was confused, as I just charged the battery overnight. I checked all my connections, nothing loose, so I pulled the starter and took it to a shop and had it rebuilt. They charged me for brushes and a starter drive. Took it home, put back on, same thing, slow crank. I did not crank it for more than 5 seconds before looking online and seeing people say to replace starter bushing. Did that. Same thing. I put jumper cables directly from negative on the battery onto starter body as a ground. Then the positive onto the solenoid. Still nothing. I put a mk3 tdi starter on, still slow cranking, except I did not try jumping the tdi starter directly to the battery. Where do I go next? I can turn the engine over by hand with a ratchet on the crank, and it turns nice and easy, it's not locked up or anything. I checked timing, and rechecked timing, it's spot on. What do I check next?
IF YOU DID THIS
I put jumper cables directly from negative on the battery onto starter body as a ground. Then the positive onto the solenoid. Still nothing.
And haven't taken it back to the shop to demo the fact it isn't working then I would ask why not. You paid for a serviceable starter and you don't have one.
Time to go back where you spent your money and get what you paid for.
It is better than before, but still nothing like it once was.
Check and/or replace your battery cables. I had a similar situation after last clutch change, everything worked just fine prior to pulling it apart to replace clutch. After reassembly, had very slow cranking. I jumped with cables as you have from the battery to frame and starter. Performed voltage drop check everything checked out normal. Cleaned all grounds and connections. I was pulling my hair out, it was driving me crazy that I couldnt figure out the problem. Finally as a last ditch effort before towing it to a shop I replaced both cables from the battery and that solved all of the problems.
Crank and crank or click and click for quite awhile.
Then go feel for hot spots on the cable connections,.. both ends of cables.