Your starter motor performs the vital task of turning the engine over when it’s at rest until it can run on its own, so when it goes wrong you’re likely to notice as soon as you turn the key in the ignition. Fitting a new one is something you should be able to manage yourself, but it’s not always easy.
A car’s starter motor is a high voltage, high current motor designed to get your engine turning. Back in the bad old days this task would generally be achieved by some form of hand cranking, which was strenuous and often dangerous. Broken thumbs and other nasty injuries caused by kickback came as a built-in hazard. Starter motors save you this trouble by putting in a large amount of effort on your behalf for a very short time – usually just a couple of seconds. A small gear on the starter is engaged and spins very fast, turning the larger gear (ring gear) on the engine until it gets going.
Once the starter motor has done its job it disengages. If they ran continuously they would soon burn out, but they don’t need to – they’re only required for just long enough for the engine to continue under its own steam. Note that if your car stalls in traffic and you can’t restart it, you can sometimes roll it to safety using just the starter motor. Put the car in first and take your foot off the clutch, then turn the ignition and steer in the direction you want to go. It doesn’t make a pretty noise and it’s not good for the car, but in emergencies it can get you to the curb and out of danger when there’s no other alternative.
When starter motors fail
The first symptom you’re going to notice when your starter motor goes is that – predictably – the car doesn’t start. All you’ll get is a series of clicks when you turn the key. Persist a little longer and you’ll probably be rewarded with a screeching noise of badly-engaged gears as the starter fails to turn the ring gear. You’ll want to stop pretty quickly at this point, since damage to the ring gear will entail time-consuming and costly work to put right.
It will be apparent that the problem lies with the starter motor rather than the battery if your lights and electrics are all fine. You might manage to bump start the car, but you’ll have exactly the same problem when you next try the ignition. That and the clicking/screeching indicate that your starter motor is shot, not your battery. Before you start ferreting around under the car with spanners, though, check that all the connections are good – it might be a far simpler matter than having to replace the starter.
Replacing a broken starter motor
Starter motors are rather inconveniently tucked away and difficult to access in most modern cars. You should find it bolted to the flywheel housing near to the transmission. Chances are you’re going to need to jack the car up and get under it to do the work, but check the web or a manual for instructions and tips on your specific make and model of car. If the car’s been running recently, mind out for the exhaust – which will be hot – and wear safety glasses because you’re going to be dislodging a fair amount of dirt and rust.
Start by disconnecting the battery (DO NOT OMIT this step). Then disconnect the wires from the starter motor. There are usually two or three, so take photos or make notes so that you remember which is which when you come to reconnect them.
Most starter motors are held in place by just two bolts, but you will need a socket set and probably extensions to reach them. You may also need to remove other bits of car to be able to access them in the first place. Care should be taken with the last bolt because starter motors can be rather heavy and it will be directly above you. While you’re there, take a good look at the teeth on the ring gear that the starter engages with. Damage to these heralds an imminent decrease in your bank balance.
Take your starter motor in to a parts supplier or find a used starter motor one from a breaker. The one you get back may be much smaller and lighter than the ungainly lump of metal you dropped on your face earlier, thanks to improvements in design over the years. Follow the process in reverse with your shiny new starter motor, and don’t forget to reconnect the battery when you’re finished.