Otto Explains — Alternator or Starter
Alternator or Starter Motor: Which One Is Failing?
Short Answer
The clue is when the problem appears. The starter motor works only while you start the car, turning the engine over, so a starter fault shows up at that moment: a click, a whirr without the engine turning, or no response at all. The alternator makes electricity while the engine runs, so a charging fault shows up afterwards: a battery warning light, dimming lights or a battery that keeps going flat. A weak battery can imitate both, so it should be tested first. If the battery light comes on while you are driving, have the car checked at the earliest convenience, and stop safely if the temperature rises or the steering suddenly becomes heavy.
What does the starter motor do?
An engine cannot start itself. The starter motor is an electric motor that turns the engine over when you start the car, using power from the battery. A solenoid on the starter pushes a small gear, the pinion, into the ring gear on the engine's flywheel and switches the motor on. Its job is done once the engine is running. Because cranking draws a heavy current from the battery, it is normal for the lights to dip briefly while the starter is turning.
What does the alternator do?
The alternator is the car's generator. A belt from the engine turns it, and it converts that movement into electricity. While the engine runs, the alternator powers the lights, the radio and the rest of the electrical systems, and keeps the battery charged, with a regulator holding the voltage at the right level. The battery only helps out when the car asks for more electricity than the alternator is producing at that moment.
Which signs point to the starter motor?
Starter problems show up at the moment you try to start. Signs that can point to the starter include no response at all, a single click with the engine not turning, a whirring sound without the engine turning over, or an engine that turns slowly or unevenly. One useful clue: if the dashboard lights come on normally but you hear only clicking or whirring, the starter is the more likely cause, because the battery is still running other systems. That is a pattern, not a diagnosis, since a tired battery can light the dashboard and still be too weak to crank the engine. When the starter is at fault, the usual causes are corroded or loose connections, a faulty solenoid, wear or damage in the motor itself, or a worn drive gear. The full checklist for a car that won't start, including fuel and immobiliser faults, is in why a car won't start.
Which signs point to the alternator?
Alternator problems show up while the engine is running. Signs that can point to the charging system include the battery warning light; headlights that dim, flicker or change brightness with engine speed, often most noticeable at idle; electrical accessories that slow down or cut out; bulbs that fail more often than they should; unusual noise; and, as the battery runs down, rough running or stalling. A battery that keeps going flat, or a car that becomes hard to start, can also come from an alternator that is not keeping the battery charged. The opposite fault exists too: a faulty alternator or regulator can overcharge, making the battery hot. What the warning light itself means is explained in the battery warning light.
Why should the battery be tested first?
The battery is connected to both parts, so it can imitate a fault in either. A weak or discharged battery can give rapid clicking or a slow crank that sounds like a starter problem, and a poor battery connection can give no response at all. On the charging side, an ageing battery, loose connections, wiring faults or a worn or loose drive belt can produce the same symptoms as a failing alternator. That is why starter and alternator makers tell workshops to check the battery's charge and condition, and the connections, before condemning either part. Replacing a starter or alternator on symptoms alone risks paying for a part that was never the problem.
Can you keep driving if the alternator has failed?
Not for long. Once the alternator is no longer charging, everything electrical runs on whatever charge is left in the battery. As that runs down, the lights dim, accessories fail, and the engine can run roughly and stall. Nobody can say reliably how far a particular car will get, so this is not a fault to live with. If the battery warning light comes on while you are driving, have the car checked at the earliest convenience rather than carrying on with long journeys. On many engines the same belt that drives the alternator also drives the water pump, so keep an eye on the temperature gauge and the steering: stop safely if the temperature rises or the steering suddenly becomes heavy, and do not restart the engine if the temperature has climbed quickly, because the belt may have broken.
When should the car be checked, and where?
Have the starting and charging system tested as soon as you notice any of these signs, rather than waiting for the morning the car fails to start. AutoTronix Express tests the battery first and, if it is healthy, looks for the cause in the alternator or starter. It diagnoses, tests and replaces both, and checks the charging wiring, connections and drive belt before recommending a part. Where the fault turns out to be in the wider electrical system, such as damaged wiring, corroded connectors, earth faults or a battery that drains overnight with nothing left on, AutoTronix Lab traces it to its source.
Common Questions
- Can a failing alternator prevent the car from starting? Yes, indirectly. If the alternator is not keeping the battery charged, the battery gradually runs down until the car is hard to start or will not start at all. Charging the battery or fitting a new one can make the problem seem fixed until it runs down again, which is why the charging system should be tested at the same time.
- Why do the lights dim when I start the car? A brief dip while the engine is cranking is normal: the starter draws a heavy current and the voltage falls for a moment. Lights that stay dim, flicker, or rise and fall with engine speed once the engine is running are a different matter, and point towards the charging system.
- What if the starter keeps whirring after the engine has started? That is a fault: the starter is staying engaged after it should have let go. Switch the engine off straight away and have the starting system checked before using the car again.
- Can jump-starting damage the alternator? It can if something goes wrong. Connecting the leads the wrong way round, or a short circuit while a battery is being connected or jump-started, can damage the diodes inside the alternator, so follow the connection points and order given in your owner's manual. Moisture from high-pressure engine washing and oil leaking onto the alternator are other known causes of damage.