Always check wiring
The modern motor car embodies an amazing amount of ingenuity, accumulated over more than a hundred years. Nowhere is this illustrated more clearly than in the design of the starting system.
Cranking an engine is hard work, especially with a high-compression engine. An electric motor capable of continuously cranking an engine for long periods without overheating would be too bulky to fit in the available space in the engine compartment.
The first stroke of ingenuity came when engineers realised they might get away with a smaller, lighter electric motor which would operate on overload while cranking the engine and will therefore get hot, but before it gets so hot it will be damaged, the engine will start and the starter can stop working and cool down again.
They found they could get away with it and they have been doing so to this day.
That means, however, that a starter should never be asked to work continuously for longer than 10 seconds. If the engine fails to start within 10 seconds, one has to wait at least 30 seconds so that the starter can cool down and the battery can recover. Otherwise there is a very real danger of seriously damaging the starter motor through overheating.
The current which a starter draws from the battery while operating may well be in excess of 150 amperes and to switch such a current on and off you need a heavy-duty switch.
This function is performed by the "solenoid", which is the tube-shaped component usually mounted on (but sometimes inside) the starter housing.
When the starter is activated by the driver, either by the ignition key, or with a push-button on the dashboard, a smallish current flows through windings inside the solenoid, creating an electromagnet which pulls in a spring-loaded plunger so that substantial, high-grade contacts are bridged, and this allows the heavy current to go through to the starter motor. In this way the solenoid functions like a heavy-duty relay.
When the driver releases the key or push-button, the plunger spring pushes the plunger back to its initial position and the starting current is switched off.
Actually, the starter solenoid is a brilliant dual-purpose design because the movement of the plunger is also used (via a lever arm) to move the pinion gear along the armature shaft so that it meshes with the ring gear on the flywheel a split second before the contacts in the solenoid are bridged and the armature starts to rotate.
This is the "pre-engaged" starter, an improvement on the so-called "inertia" type that used a Bendix drive, itself an ingenious design, for pinion engagement.
There have been two main subsequent advances in starter technology. The first was the introduction of a reduction gear in the starter which reduces the rotational speed of the output shaft while at the same time increasing the torque. This permitted the use of a higher-speed, lower-current, more compact starter, while increasing cranking torque. It was pioneered by Chrysler in 1962 and, when the popular Chrysler Valiants appeared here in the mid-60s, their starters had a distinctive sound.
Nowadays reduction gear starters are used on virtually all vehicles.
The other main advance was the use of strong permanent magnets instead of field coils to create the required magnetic field inside the starter motor.
This has allowed starters to be made both smaller and lighter without sacrificing power output.
Engineers have always been aware of the fact that, if a pinion fails to disengage from the ring gear as soon as the engine starts, it poses a serious danger to the starter motor. Because the pinion gear is so much smaller than the ring gear, it rotates about 15 times as fast as the ring gear when they are in mesh.
This means the armature shaft on which the pinion is mounted must rotate at 3000rpm to crank the engine at 200rpm which is sufficient for starting.
But when the engine starts, the engine speed might increase instantly to, say, 1000rpm, and if the pinion remains in mesh with the ring gear, the armature shaft will be forced to spin at 15000rpm. At this speed the armature would simply disintegrate. The same thing will happen if the starter is accidentally operated while the engine is running.
To guard against this ever-present danger, all sorts of safety mechanisms are in use. These include an over-running clutch which allows the pinion to spin freely when the flywheel is "over-running" the armature, a neutral safety switch, a blocked restart with engine running, a timed block restart after a failed start, and so on.
When a starter cranks the engine sluggishly, haltingly or not at all, it's an indication something is wrong in the starting circuit. This comprises the battery, the solenoid, the starter motor, various cables, the ignition switch and probably various safety devices.
The fault could lie in any of these components, not necessarily in the battery or the starter motor.
The battery is usually the prime suspect, but often this is completely unfounded.
Only a trained auto-electrician has the knowledge and equipment to pinpoint the trouble.
All that the rest of us can do before calling in an expert is to check that all the wiring is intact and firmly connected.