Same thing happened to me after the motor was approx 4 years old. Had not completely stopped but was running very very slow. 13mm Grade L short link chain and 40kg Rocna
The rear seal had failed and let oil
into the motor which was full of carbon dust from the brushes
.( The motor is sealed up in a waterproof housing so there is no way for the brush dust to escape) I cleaned it all out and put it back together and still was not working properly.
Took it to old fashion Auto electrician that works on starter motors etc. The motor was put on a test jig and was drawing more than a 100 amps un-loaded. He re-soldered all the connections inside motor, dressed the brushes and cleaned up the commutator. On the test jig motor now ran at normal speed and was drawing less than 30 amps un-loaded. Replaced seal on gearbox
changed the oil
. All good has been working like new since October, cost $120 AUD, plus seal and oil.
It's an awkward mother to get at, motor held in place by 2 bolts and a lot of sealant
. Be careful there is a small key way on the shaft between motor and gearbox
that if you drop into the chain pile it's a bitch to find. Do not ask how I know. A cloth over the top of chain after it has been leveled off in the chain locker would help as well.