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If Alternator Is Bad Will a Jump Start Work?

Close up of jumper cables on a car to jump start it

If you’ve ever had a car that wouldn’t start, you may have wondered if a jump start would work. The answer is…maybe. It depends on the alternator. If the alternator is bad, will a jump start work?

A jump start on a car with a bad alternator may work. It depends on if the battery has enough charge to keep the car running. However, the battery will continue to drain without recharging if the alternator is bad. So, it is best to get the alternator replaced before the battery gets drained.

In this blog post, we will discuss what an alternator does and how to know if it’s bad. Keep reading because we will also talk about what to do if your car won’t start and the alternator is to blame!

Will a Jump Start Work On a Car with a Bad Alternator?

A jump start may work on your car even if your alternator is faulty.

If your car does start, this is because the battery has enough charge left that the car can still run. However, because the alternator is bad, even if your car starts it may not run for a long time, since the battery is being continuously drained with no way to recharge it.

If you try to jump-start your car and it doesn’t turn on it is most likely because the alternator is bad and the battery is completely drained already.

Sometimes the car will run after a jump start and you will turn off the car and it will not start the next time you go to drive. This is a good indicator that the alternator is indeed bad and needs to be replaced.

How to Tell If Your Alternator Is Bad?

There are a few different ways to tell if your alternator is bad. There are two different ways to find out if an alternator is bad in your car.

The following is one way to check if your alternator is bad using a voltmeter:

  • Find the battery – Using your owner’s manual pop the hood of the car and locate the battery for your car. Generally, the battery is on the left if you are facing the front of your car.
  • Connect the voltmeter – Connect the positive wire on the meter to the positive battery terminal. Connect the other wire on the meter to a bare metal source for grounding.
  • Read the voltage – Using the meter’s digital display, look to see what the readout for the meter is saying. The average of a battery connected to an unstarted car is 12. If the reading is lower than 11, the battery needs to be replaced.
  • Turn on the car – Disconnect all the wires for the voltmeter and now it is time to start your car. Once the car is started reconnect the voltmeter to the battery.
  • Read the output – Your reading on the voltmeter should be higher now that the car is started. If your output is lower it means the alternator is faulty. It is time to repair or replace your alternator.

Another way to know if the alternator is bad is if your car dies after a jump start. This means there was enough juice in the battery to get the car started but the alternator was not working and so the battery could not recharge. Thus by the time you got to where you were going and shut off the car, the battery had used up its energy and would need another jump start.

Testing the altnernator to see if it's good or bad and charging the battery while the car is on by using a multi-meter on the battery

How to Tell If Your Battery Or Your Alternator Is Bad?

Oftentimes the easiest way to tell if it is your battery or your alternator is to get them tested. Now we already covered how you can test your alternator, just grab a voltmeter and you are off. But to test your battery the simplest option is to take it to an auto parts store.

Using a Voltmeter

Many auto parts stores will test your battery for free.

These stores will likely use a voltmeter, so if you have one of those, follow the same steps for checking your alternator with a few changes:

  • Start with the car off – Connect the voltmeter to your battery. The positive wire connects to the positive end of the battery and the other wire connects to a grounding surface.
  • Check the reading – Look at the output for the battery’s voltage. You want it to read around 11 to 12, if it is lower than 11 the battery needs to be replaced.
  • Disconnect the voltmeter – Now that you have checked the output you can disconnect the voltmeter from the car battery.

Now, let’s see the next signs.

Check Your Car for Unusual Behavior

Sometimes cars will give you signs that the alternator or battery is bad.

Use this list to check and see if one of these things is faulty with your car:

  • Dim lights – If your lights and internal dash lights are dim it is time to get your battery and alternator checked.
  • Changes to your lights – Sometimes if your battery or alternator is bad, the lights will get brighter and then gradually dim as the battery tries to keep power to all parts of the car.

Now, let’s see some signs you may have a bad battery specifically.

Signs of a Bad Battery

There are some distinct signs of a bad battery.

Be on the lookout for the following:

  • Corrosive battery – If there is a lot of corrosion on the battery it may be time for a replacement.
  • Simple instruments will not run – If you turn the key before ignition, the radio and other car instruments should turn on. If the wipers, lights, and radio do not turn on then the battery has no power left.

Now you know everything about whether your car will start using a jump start if the alternator is bad.

In Summary: A Jump Start May Work Even If the Alternator Is Bad

Sometimes you can get a car to start even with a bad alternator, but it won’t stay running for long. Your car may not start if you have a bad alternator because the battery is now dead and has not been able to recharge.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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