Dolphin Magic motor testing

Magic Dolphin had broken one brush cylinder.Power pack failed( as consequence? ) Repaired with new thermal protector switch and rectifier.Tested motor unit before reassembly in body of cleaner.Result was that both motor shafts rotate whenever I switch on but for 4 secs only.Question is are the motors stopping due to some safety cut out ( perhaps to prevent damage when out of the water)? If so I'll go on with reassembly with the new brush cylinder and re-test in the pool.Please advise.