Tractor starter

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!

Check the teeth. You may have a dead spot on the flywheel, or on the starter gear. I had that problem with my Oliver. I took everything apart, checked wires, everything. Put it back together and it worked again.

Otherwise you can check with a local alternator shop that should be able to repair it if the starter motor is indeed bad. I would hook power up to it while it's disconnected and see if it spins. If so, then chances are it's not the problem.

battery. Use jumper cables and see if it starts.

If the solenoid is kicking in and out rapidly (what I call machine gunning) it's not getting enough power. Look for low battery, or bad connection including the ground.

A first guess is a bad battery that will not provide starting current. (1/5th of which is applied on a bench test)
If the battery will easily start another vehicle, then the starter may be dragging internally (bad bushings) and drawing unrealistic current from an OK battery. Jim

Thanks, Jim. I don't believe it's a bad battery. I have tried jumping from a known good battery--no difference. And, just to be sure, I connected my 200 amp charger to it--same result. Also, since the problem appeared a few days ago, on three occasions repeated efforts to start it did eventually result in it catching and spinning the engine normally and starting it. The thing that it's doing isn't like the clicking a low battery causes. The solenoid kicks in and out "hard." I'm beginning to believe it's a faulty starter motor.

Since you have ruled out the batteries and connections the next place to look is the starter assembly.

A failed hold in winding in the solenoid will cause those symptoms.

The solenoid has two coil windings used to pull the solenoid plunger and close the switch. The pull in winding is usually the heavier of the two. It is connected from the S terminal to the starter terminal and is grounded through the brushes and armature. The hold in winding is connected from the S terminal and grounded the the solenoid case.

When you apply power to the S terminal both windings work together to pull the solenoid plunger into the solenoid. Once the plunger mover far enough to engage the switch both sides of the pull in winding will have battery voltage applied to them and will no longer exert any electromagnetic force. The hold in winding is left to keep the solenoid plunger in the engaged position. If the hold in winding is not working the plunger spring will push the plunger back out of the solenoid as soon as the switch closes. When the switch opens the plunger will again be drawn into the solenoid.

Th check the solenoid winding disconnect the solenoid terminal that is connected to the starter. Then check for continuity between the S terminal and the stater terminal and also between the S terminal and the solenoid case.

If you are sure the battery is ok,then you may have bad cable.There would either be a break internally,or,corrosion inside a cable end where you can't see it.Check voltage,if you have a meter,at battery,and then at starter,and then crank it and read voltage at starter,and at battery.If it drops way off at starter,you have a bad cable.If it drops at battery,then the battery is bad.Or,as someone below said,bypass battery cables with jumper cables.Does your ground cable connect directly to engine,or to frame or engine block;could be another source of lost connection? Also try jump starting directly at starter terminals.Mark

Thanks, Owen! Your explanation of the inner workings of the solenoid begins to make sense for the way it is mal-functioning. I will check that out in the morning. Thanks again!

The head of the bolt in the solenoid is burned off enough it doesn't make a good connection to start. Try with a piece of copper on the bolt to the starter motor Fasten it on so you don't burn the bolt threads. Hook your charger to that with the battery to kick in the solenoid. If it starts it is the solenoid if not then probably the starter.