i posted this in the fergie forum......
I had a similar issue with an aftermarket starter(s). very frustrating,and i'll try to keep it as short as i can. Same as you, just doing some routine maintenance, and ended up with a starter problem. the starter was an after market starter, one year old. it wouldn't do anything on the tractor, wouldn't click, wouldn't crank, nothing. but worked fine when i put twelve volts to it on the bench. every thing was in order on the tractor. i triple checked everything, and was certain there were no issues with the wiring, switch, ect...i found a brand new one on craigslist. I took a battery, and jumper cables along with me to verify it worked, and it did. got it home and put it on the tractor, and nothing!! took it off the tractor, bench checked it, and it worked fine. so the brushes were intermintit...took the starter apart, polished the commutator with some scotch brite, checked the internal connections, took winding resistance readings. I put it back together, bench tested it,and worked fine.I then took it to the auto parts store to have it tested on their machine, it tested good. put it back on the tractor..this time it worked, but very sluggish, it barely turned it over.
so, i set out to solve this starter riddle. I ordered two "twelve volt" starters of the internet, when they arrived i put one of them on the tractor, and same as the others, weak, sluggish, wouldn't turn the engine. at this point, any sane person would have think it's the tractor, or the battery. all of which i ruled out. including using different batteries, different jumper cables, two battries in parallel, direct ground to the starter frame. the tractor WAS NOT THE ISSUE!! I was able to locate auto electric shop that rewinds, rebuilds starters. I called em up and asked if they could test these starters for me, they said yes, so i dropped em off. they called a day later, and said they tested fine. I went after work to pick the starters up. they happened to have one of thier own, rebuilt starters on the shelf that was the exact starter for my ferguson TO 30. I asked the guy at the shop if we could test his rebuilt starter. I wanted to see what the current was being drawn by the different starters. he and I put all the starters on his test rig, the two after market starters each drew about fifty amps, his drew one hundred amps. he told me the current doesn't matter much on these things, and told me i have a problem somewhere on the tractor. he told me to trust his thirty five years of experience,the starters are FINE!!
Ohms law was telling me otherwise. his starter was putting out twice the power as the aftermarket ones, he just didn't realize it. so, i bought the rebuilt starter from him, in spite of him telling me i had good starters. so, i brought it home, put on the tractor, and it worked just like it should. it turned the engine effortlessly, and i haven't had a starter issue since. I know its a long story, but i wanted to share my bad experience with the forum, so others may benefit from my misfortune, and the hard lesson i learned about the chinese junk that is being sold.