Belarus 250/310 Starter Solenoid Wiring.

I have a Belarus 250/310 that I have to play with the starter
solenoid every spring. The wiring inside mystifies me. I know
how to wire the solenoid to the tractor, and how it works, but
every spring I have to play around with it, and eventually I get it
to work, but I'm not sure how/why.

The solenoid, which is mounted on the starter has two big
electrical connection bolts. One goes to the tractor battery, the
other is connected to the starter itself. Inside the solenoid one
bolt has no connections and one has a wire that goes into the
solenoid coil. When the solenoid coil is energized, a disk makes
the connection from one bolt to the other, turning the starter
motor over, and also pushing a gear to connect to the flywheel.

The only other connection is the stud which is connected to the
starter switch on the dashboard. This stud has two wires
which go into the solenoid coil. When the starter switch sends power
to this stud, it should energize the solenoid coil connect the
starter motor to the battery power, and connect the starter
motor bendix gear to te flywheel.

There is of course also a ground circuit to the starter and
solenoid...

The problem is that it doesn't always work. Why are there three
connections to the solenoid instead of one ( plus ground)

Thanks, Jim.

A typical solenoid like you describe uses the two large terminals to provide a path from the battery to the starter motor with the disk in the solenoid acting as the switch.

The two smaller terminals, one provides a means to energize the switch and the other provides temporary power out of the solenoid. The terminal that has the wire connected inside is normally connected to two electrical coils in the solenoid. One coil is grounded to the solenoid case and the other is connected to the large terminal that goes to the starter motor. The Pull In coils is the one that is connected to the large terminal. The Hold In coil is connected to the case ground. When you apply power to the small terminal both coils are energized and pull the solenoid plunger to engage the drive in the ring gear and then also push the contact disk to close the circuit from the battery cable terminal to the starter motor terminal and also the temporary power terminal. Once the circuit is closed between the battery and the starter motor the pull in coil has the same voltage on both ends of the coil and no longer functions as an electromagnet. The solenoid plunger is now held engaged only by the hold in coil. When power is removed from the small terminal the electromagnetism drops off in the hold in coil. As the power is removed the collapsing of the lines of force in the winding create a small bit of current in the coil. This current is now applied to the pull in winding as the circuit between the battery terminal and starter motor terminal is open and the pull in coil is once again grounded through the starter motor. The magnetic lines of force in the pull in coil are opposite those of the hold in coil and cancel each other out insuring there is no longer any magnetic pull to keep the solenoid plunger from returning to the original position.

The other small terminal that provides temporary power while the solenoid switch is closed is normally uses for gasoline engines to provide battery voltage to bypass an ignition resistor. On diesel engines it is sometimes used to allow an electrical ether assist to be used only when cranking the engine. On propane engines it is sometimes used to energize a primer valve while cranking.

Owen described the solenoid function well.

Now the question of why it doesn't always work... Assuming the battery is sufficiently charged and the cables are clean and tight, see if this helps.


You didn't describe the symptoms, but if you supply voltage to the "S" terminal, and nothing happens, as in no clunk of the plunger, typically there are 2 causes:

1. The brushes are worn and failing to complete the ground circuit. Also oil fouled brushes/commutator or any open circuit in the starter can cause the same open ground circuit.

The solution, repair the starter.

2. There is sufficient voltage drop between the battery, starter switch, and neutral switch to not have enough power to pull the solenoid plunger in. If the starter switch and neutral switch check out, the problem is too much resistance in the wire.

The solution, mount a heavy duty relay (Ford fender mount starter solenoid) near the starter. Using 10ga wire, connect the solenoid power terminals between the battery terminal of the starter and the "S" terminal on the solenoid, and connect the wire removed from the "S" terminal to the coil terminal of the relay. This works because the relay is much easier to pull in than the starter solenoid.

If the starter "clunks" but does not spin, the solenoid internal contacts are worn.

If the starter spins but the engine does not, the starter drive is worn or oil soaked.