The starter motor: the highest-current circuit in the car

An engine cannot start itself. It has to be turned fast enough to draw in mixture, compress it and get the first combustion to happen — a couple of hundred rpm on a petrol engine, more on a diesel. The starter does that, and to do it draws more current than everything else in the car put together: 150 to 400 amps on a car, briefly, from a battery whose ordinary job is measured in milliamps.

That number explains almost every fault in this article. At those currents a connection that would be perfectly adequate anywhere else in the vehicle is not adequate here, and a resistance too small to measure with an ohmmeter is enough to stop the engine turning.

What is in it

Three things in one housing, and knowing which is which is most of the diagnosis.

The motor — a series-wound DC motor, chosen because that type produces its highest torque at zero speed, which is exactly what is wanted against a cold engine.

The solenoid, which does two jobs at once and that is the part people miss. It throws the pinion gear forward into the flywheel ring gear, and at the end of its travel it closes a pair of heavy contacts that connect the battery directly to the motor. So the small current through the ignition switch controls the very large current to the motor — the switch on the column could not carry it, and no ordinary switch could.

The one-way clutch, on the pinion shaft. Once the engine fires it is turning far faster than the starter can survive, and the ring gear is much larger than the pinion, so the engine would drive the starter to destruction. The one-way clutch transmits torque only in the direction the starter drives, and freewheels the other way.

Diagnosing it by what you hear

The sound tells you which of the three has failed, and this is the most useful section of the article.

A single loud click, and nothing turns. The solenoid pulled in — that is the click — but the motor did not run. Either the main contacts inside the solenoid are burnt, or the motor is dead, or the battery cannot supply the current. Burnt solenoid contacts are extremely common and are often the whole repair.

Rapid clicking or chattering. The solenoid is pulling in, the voltage collapses, it drops out, the voltage recovers, it pulls in again. That is nearly always not enough current — a flat battery or a bad connection — rather than a failed starter.

Slow, laboured cranking. Voltage is reaching the motor but not enough of it. A weak battery, a poor connection, or a starter drawing excessive current because its bushes or armature are worn. It can also be mechanical: thick cold oil, or an engine with a real problem turning.

A whirring or spinning noise with the engine not turning. The motor is running but the drive is not reaching the engine — a failed one-way clutch, or a pinion not engaging. Also possible, and worse, is a ring gear with teeth missing, which will let it start sometimes and not others depending on where it stopped.

A harsh grinding. The pinion and ring gear are meshing badly — worn teeth on either. This damages the ring gear further every time.

Nothing at all, no click. The solenoid is not being energised. Now you are looking at the ignition switch, the starter relay, the immobiliser, the neutral safety or clutch switch — or a car in gear.

Measuring it properly

The test that separates a bad starter from a bad connection is voltage drop, and it is the one most people skip because it needs the circuit under load.

With the meter on DC volts, measure across each connection while cranking:

• Battery positive post to the starter's main terminal — should be under about 0.5 V.
• Battery negative post to the starter body — should be under about 0.2 V.
• Across the solenoid's two heavy terminals while cranking — should be small. A significant drop here is burnt contacts, and it is a common and cheap fix.

Anything much larger is resistance in that segment, and the scale involved is smaller than it sounds: those limits are under two milliohms on the supply side and under one on the earth side, and ten milliohms — still far too little for an ohmmeter to distinguish from a perfect joint — drops three volts at 300 amps. That is the whole reason the corpus says a continuity check proves nothing here. It is also why a starter circuit fault so often turns out to be a corroded earth strap, and why cleaning terminals fixes so many no-start complaints that were about to become a new starter motor.

Those two figures are the same ones used in using a multimeter, where the general voltage-drop table is; note that on a starter circuit they apply to the whole of each side rather than to each joint, because the cable runs are long and the current is enormous.

Watch battery voltage at the same time. It should dip while cranking but stay above about 9.6 V; below that the battery is the problem, not the starter. See the lead-acid battery for why voltage at rest does not tell you this.

The ten-second wait before starting

The original article ends on a piece of Iranian workshop advice — turn the key to the on position and wait about ten seconds before cranking — with three reasons. Two are sound and one is not.

Sound: the fuel system primes. Turning the key on runs the fuel pump for a couple of seconds to pressurise the rail, and you can usually hear it stop. Cranking before it finishes means the first compressions happen at low fuel pressure. On a car whose pump check valve leaks down overnight, this genuinely is the difference between starting on the first turn and cranking for several seconds. Waiting for the pump noise to stop is the real advice, and that is two or three seconds, not ten.

Sound, and more useful than it looks: you get to see the warning lamps. The bulb check happens with the key on and the engine not running, and it is the only moment you can confirm the lamps work. That matters more than it sounds — a blown charge warning bulb can stop the alternator charging at all, and the symptom is a flat battery with no warning light, because the bulb that would have told you is the failed part. See the alternator and charging system.

Not sound: that the delay gives the ECU time to check its circuits and avoid setting errors. Engine ECUs are designed to be cranked immediately — that is the normal case they are calibrated for — and cranking does not create fault codes by interrupting a self-test. Low voltage during cranking can make modules log codes, but the answer to that is a healthy battery, not a pause.

Before replacing one

Starters get replaced on suspicion more than almost anything else, and often the fault was elsewhere:

• The battery, load-tested rather than voltage-checked.
• The connections, voltage-drop tested under crank — both ends, including the engine-to-body earth.
• The solenoid contacts, which are frequently rebuildable and sometimes sold as a separate kit.
• The safety interlocks — a clutch or neutral switch out of adjustment gives a perfect no-click no-crank.
• The ring gear, inspected through the starter aperture. Fitting a new starter to a damaged ring gear destroys the new starter's pinion.
• Whether it is the right starter at all, on a car with stop-start. A conventional starter might see thirty thousand operations in its life; a stop-start car can reach that in a year, so those cars carry a reinforced unit matched to an AGM or EFB battery. Fitting an ordinary one of either is a common shortcut that fails early — the lead-acid battery has the battery half of it.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.