How to Determine If an Alternator or Starter Is Bad

Written by david mcguffin
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How to Determine If an Alternator or Starter Is Bad
Knowing a little about your car engine and its parts can help to determine wether or not you need to replace the starter or alternator. (car engine image by itsallgood from Fotolia.com)

If you are having problems with starting your car, there are multiple solutions depending on your situation. Knowing specifically what is happening when you try to turn your car on will help to diagnose which part may need to be replaced and whether you can replace it yourself or need to take it to a mechanic.

Skill level:
Moderate

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Things you need

  • Hammer
  • Jump leads or battery jumper pack

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Instructions

    Bad Starter

  1. 1

    Turn the ignition key. If the engine does not turn over, either your battery is completely drained or your starter is bad. Listen carefully for a click. Some starters will hum while they rotate and then click, while others will click shortly after you turn on the ignition. If you hear the click, your starter may be bad. However, there are some other tests you can do to make sure that it is bad.

  2. 2

    Open the bonnet. Wiggle the connections on your battery. Turn on your headlights to see if they are dimmer than usual or if they are retaining their brightness. If the battery is not low and your car is not starting, that is another indicator that it the problem is in your starter. Try jump starting your car to see if a boost helps it to start up. If so, then that is an indicator that the problem is not your starter. If a jump does not help at all, then chances are that you are looking at a bad starter.

  3. 3

    Lightly tap the starter with a hammer a couple of times. Try starting the car again. Sometimes this will help the starter to re-engage, but it will still need to be replaced within the next couple of starts. If tapping the starter leads to the car starting up the next time you crank it, that is another indication that your starter is bad.

  4. 4

    Take the bad starter to the parts store when you replace it. They will be able to give it a final test to see if it is bad or not. If it is not, then you will know that you will have to replace a different part. Consult the experts at the car parts store or a mechanic if the starter seems to be OK when tested.

    Bad Alternator

  1. 1

    Turn the car on. If the car struggles to turn over then that indicates a weak battery or a bad alternator. You can still drive around for a short while with a bad alternator if the battery is strong. Check the battery gauge on your dashboard if you have one. If it is low, then that indicates that your battery may not be getting enough electricity from the alternator.

  2. 2

    Remove the negative battery cable while the car is running. If the engine dies, that indicates that the alternator is malfunctioning. This is because much of the car's electrical power comes from the alternator while the car is running.

  3. 3

    Take the car immediately to an auto parts store. They will be able to give the alternator an electrical test to see if it is bad or if the battery itself is bad and needs to be replaced. It is much easier to replace the battery than the alternator. Having an expert test the alternator and battery together is the only way to know for sure if either or both parts need to be replaced.

Tips and warnings

  • To jump start your car, you will need another car that is running and a set of jump leads. Connect the cables first to the battery terminals of the car while it is running, and then to the terminals of the car that will not start. Once connected, try starting the car. If the battery is dead then the jump will help it to get the initial start-up charge that it needs to get your car going.
  • If you try to jump start the car, make sure that you do not touch the metal tips of the jump leads. If someone is helping you jump your car, make sure that you are communicating with them also about turning on your engine to make sure that they are ready for it and are not touching the cables or your battery.

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