Pre-engaged Starter Solenoid

The solenoid has two windings - the pull-in winding 'A' which is about 0.4 ohms, and the hold-in winding 'B' which is about 1.2 ohm. One end of both is connected to the solenoid operate spade '1', but whilst the other end of the hold-in winding is connected to earth directly, the other end of the pull-in winding is connected to the starter terminal '3' which has an almost zero resistance path to earth via the motor stator and rotor windings. This puts the two windings in parallel measuring about 0.3 ohms.

At the instant the starter relay connects 12v to the solenoid operate terminal '1', about 30 amps flows through the pull-in winding, plus about 10 amps through the hold-in winding. This total current of about 40 amps pulls the solenoid plunger in very smartly. Because the pull-in winding is passing about 30 amps through the motor, the pinion starts to turn slowly as it moves towards the flywheel, which aids its engagement.

The solenoid plunger moves, and bridges the three contacts above it, connecting battery voltage to the Starter terminal '3' and the coil ballast resistance bypass terminal '4' (18V engines and V8s). Not only does this connect 12v directly to the motor, so spinning it at full power to crank the engine, but it also effectively bypasses the solenoid pull-in winding. As this now has 12v (nominally, in practice more like 10v once the load of cranking the engine has been applied to the battery) both sides it passes no current, whereas the hold-in winding is still passing current directly to earth. So the total current through the solenoid drops from about 40 amps to about 8 amps at a cranking voltage of 10v.

This near-instantaneous (normally) reduction in current prevents heat damage to the solenoid windings. Note that with solenoid contact problems where the solenoid clonks but the motor does not spin, the high current will continue to flow, and leaving the ignition key turned to 'Start' for any length of time will almost certainly damage the solenoid.

Bob Davis noticed that the two windings in the drawing were shown as being wound in opposite directions. This was an error in the drawing, the two must be wound in the same direction or the magnetic fields will be in opposition, and has been corrected.

The solenoid windings: 'A' is the end of both windings that is connected to the operate spade - the hold-in winding being relatively thin, and the pull-in winding thick. 'B' is the other end of the hold-in winding, welded to the body of the solenoid i.e. at earth potential. 'C' is the other end of the pull-in winding, which goes to the output stud of the solenoid i.e. the one to which the motor cable is bolted.