The starter on one of my tractors (diesel) will not spin the engine. The solenoid rapidly kicks in and out, turning the engine a few degrees at a time. The engine runs OK if I "roll start" it. I've cleaned up all electrical connections. I've removed the starter from the tractor and tried testing it with direct battery connection--the solenoid kicks in and the starter motor spins with no load. Put it back on the tractor--same thing, rapidly kicking in and out, turning the engine just a little bit. Is this problem likely to be a faulty starter motor, or a faulty solenoid? Don't want to replace an expensive starter unnecessarily. Thanks!