Starting System Basics
The engine can’t “start” rotational movement on its own. It needs an electric motor to get it up to a minimal RPM to run, then the engine can run under its own power. The starter is the biggest load on the vehicles electrical system. We cannot simply run all that current through the ignition switch, in most systems a relay is used to to activate the starter solenoid, and the starter solenoid itself acts as another relay to engage the starter motor (explained later). Before electric starters, automobile owners needed to crank the engine over themselves! This was not ideal for any kind of quick getaway.
The starter solenoid sits on top of the starter motor and performs two main functions, it acts as a heavy duty relay for the starter and it engages the starter pinion gear to the ring gear on the flywheel/flexplate/torque converter. The solenoid has 3 terminals; a B+ terminal, an S terminal and an M terminal. The B+ terminal is connected directly to battery positive at all times. This wire is unfused meaning that if there is a short to ground on this wire, there will be sparks until the battery is drained. The wire from the battery to the B+ terminal will be very thick because it needs to carry call the current necessary to turn the starter motor and overcome engine compression. The S terminal receives power from the ignition switch either directly or indirectly with a relay. The S terminal connects to two windings, the pull in winding and the hold in winding. These windings are simply coils of wire wrapped around a plunger, which when energised produce and electromagnet. The pull in winding is made up of thicker windings and creates a strong electromagnet. It is grounded through the M terminal and starter motor. The hold in windings are smaller and create a weaker electromagnet. It is grounded directly to the starter case. The plunger sits in the middle of the windings and is held in place by a spring. The plunger gets pulled/held in by the windings when they are energised. At one end it is connected to a lever which forces the starter pinion gear to mesh with the ring gear. At the other end, when the plunger reaches the end of its travel, it pushes a contact disk which connects the B+ terminal to the M terminal which is connected to the starter motor. This energises the starter motor and also causes the pull in winding to stop flowing power. This is because once the contact disk connects B+ to M there is 12v on both sides of the pull in winding and no ground. The hold in winding continues to flow electricity and holds the plunger in place until the key is returns to the run position. The solenoid needs both windings to pull the plunger in but only the hold in winding to keep ot there. It takes much more effort to move the plunger to engage the starter than it does to hold it there. Since the pull in winding is no longer necessary, it would only waste electrical power to continue to power it.
The starter motor converts electric energy into rotational motion, using electromagnetism or electromagnetic repulsion. Most starters used in automotive today are permanent magnet starters. These starters have several permanent magnets placed inside the case around an armature. An armature is used to make an electromagnetic field of the same polarity as the permanent magnets, causing the armature to repel the magnets. Power from the M terminal, and ground from the case is supplied to the commutator strip through the brushes. The commutator strips Are connected to each other through the armature windings, this causes an electromagnetic field to form around the armature strips that are flowing power. If power is fed to commutator strip 1, ground is on commutator strip 5, power will have to travel through armature strips 2,3, and 4 to get to commutator strip 5. This will create a magnetic field around armature strips 2,3 and 4. To get the armature to rotate, a permanent magnet is placed near, but not right on top of where the electromagnetic field is formed. When the two like polarities repel, the armature begins to rotate. As the armature rotates, the brushes will contact the next commutator strips, keeping the electromagnetic field in one place (just next to the permanent magnet) but allowing the armature to spin. This is what creates the rotational movement necessary to start the engine. Starters may also have a planetary gear-set to reduce RPM and increase torque to the ring gear. Heavy-duty starters use field coils instead of permanent magnets. Basically they make both magnetic fields using electromagnetism instead of relying on permanent magnets. These starters are much more powerful than a permanent magnet starter but they take up more space, are much heavier and cost more to produce.
Starter Drive Pinion
The starter drive pinion is held out mesh with the ring gear by a spring until the starter solenoid engages and moves the lever, pushing the starter pinion into mesh with the ring gear. When the engine starts, the operator allows the key to return to the run position. This cuts power to the starter solenoid, which allows the spring to push the plunger back to its normal position. The plungers lever will pull the starter drive pinion back, out of mesh with the ring gear. It is important that the starter drives the flywheel and not the other way around. This is why starter drives have a one-way clutch. The one way clutch allows the starter to turn the flywheel, but if the flywheel starts to cause the starter pinion to turn faster than the armature, the one way clutch will slip. This protects the starter from spinning too fast.
Activating the Starter
Battery positive is supplied to the ignition switch, terminal 30 of the starter relay and the B+ terminal on the starter solenoid. When the key is turned to the start position, the ignition switch will activate the starter relay connecting terminal 30 to terminal 87, supplying power to the starter S terminal. The neutral safety switch can be located anywhere between the ignition switch and the relay, or between the relay and ground. The neutral safety switch opens the circuit unless the transmission is in park or neutral on an automatic, or the clutch pedal is depressed on a standard transmission. Some systems will not use a relay, this means that 100% of the current needed by the solenoid (not the motor) will need to travel through the ignition switch. Once power has made it to the S terminal, the plunger will engage the contact disk which connects the B+ terminal to the M terminal and forces the the starter drive pinion to mesh with the ring gear. Once the B+ terminal is connected to the M terminal, the armature will begin to rotate and turn the engine. Since the starter is bolted to the engine or transmission,and the engine/transmission is in some way bolted to the frame, the starters case is where the circuit gets its ground or battery negative source.