how to test the alternator with a voltmeter

How to Test the Alternator with a Voltmeter, Screwdriver & Multimeter

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Are you wondering if your alternator is in order? ANd you like to see how to test the alternator on a car using a Voltmeter, Screwdriver or even a Multimeter we have got you covered. Sometimes it’s very difficult to say, especially if you do not have an idea on what you should look-out for.

However, the best method to check is by making use of a voltmeter and this is why we are going to show you how to test the alternator with a voltmeter.

How to Test the Alternator on a Car Using either Screwdriver, Voltmeter or a Multimeter

We will also consider the following subtopics on alternator how to test  and all relating questions below:

  • how to find alternator amperage
  • how to test alternator with screwdriver
  • how to test alternator with multimeter
  • testing alternator amp output with multimeter
  • how to test alternator by disconnecting battery
  • how to check alternator by disconnecting the battery

So you have a new battery recently changed your car batteries and yet your vehicle could not start? Then, it is most likely that your alternator is bad.

Today, we will share with you how to test the alternator with a voltmeter. Below are the steps to follow :

How to Test the Alternator with a Voltmeter

  • Connect your meter leads on via battery terminals by looking for 13.8 to15.3 volts
  • Please you shouldn’t disconnect the negative battery cable just to test an alternator. If there is no battery in the circuit to act as a buffer, it simply means the alternator will produce up to 150 volts, which depends on your engine AMPs.
  • You should buy a voltmeter instead of disconnecting your battery cable. Then put off your engine, set your battery voltage at 12.5 to 12.8 volts. Note that if your battery is lower than that, then it needs to be charged first. And after which, turn on your engine and observe an increase in voltage reading, then if there is an increase in reading, it means the alternator is in perfect condition.

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The word amperage (AMP) happens to be a word used to describe the rate in which electric current flows. One of the best ways to know the problems of the alternator is to examine the base and have an idea of the AMP the alternator generates.

For you to know how to find alternator amperage, you need to have an understanding of how AMPs and volts works. AMPs are known to be current’s power, while volts are the capacities of any current.

To know an alternators amperage, you should note that alternators of bigger vehicles are likely to increase the amperage output which is around 200 to 350 AMPs. While small vehicle’s produces around 100 AMPs with a battery that runs on a 12 volts.

You can examine the fuse rate of your vehicle’s electric components, then you can calculate the AMP your alternator runs on. Alternatively, check for a diagram on your fuse cover, you will see AMPs numbers your manufacturers have set, just sum the numbers and you will know the AMP the alternator requires to power your vehicle’s electric components.


In you want to learn how to test alternator with screwdriver, then follow these steps:

  • Please insert vehicle key on the ignition switch without starting the engine.
  • Before accessing the alternator, open vehicle’s hood so you can test alternators current.
  • You are to test the magnetism of the alternator, so put the metal screwdriver close to the bolt on the alternator pulley.
  • You can then turn the key of your car to the on position without starting the car. This will activate the voltage regulator, put the metal side of your screwdriver close to the nut on the alternators pulley, you will observe a high magnetism which should pull the screwdriver. If this happens, it means the alternator is perfectly working, but if it didn’t, it means your alternator is not working perfectly.

How to test alternator with multmeter

Before you will test alternator with multmeter, ensure your car battery is completely charged. Then remove the negative lead from the multmter and place on the negative post of the car battery. After which, insert the meter’s positive lead on the battery positive lead, and set the meter to DC voltage making use of 20 scales and then observe the display to make sure the battery is completely charged.

You can then start your car to find out what your alternator is generating. Note that that reading should increase, but if not, then it means your alternator is not generating current. You can now turn on your heater blower and light to examine, if the alternator is not strong enough, then allow your vehicle to run for some time and once it reads, it means your alternator is in perfect condition.


  • Examine the AMPs the alternator generates if it works perfectly. Then examine the specification are of your vehicle’s manual where you will see alternators details. This will inform you about the maximum/minimum AMP of your alternator. Note that the maximum is anytime you put on something that consumes energy in your car, while the minimum is when you put on something like a car stereo. So ensure you write them down for record purposes because you will need them again.
  • You will need to get closer to your vehicle’s battery so you can examine the AMP output via the alternator, but first, ensure you put on your glove.
  • After which, open your vehicle’s hood, then start the engine and allow it to be idle for some time. Please make sure there is nothing that consumes energy like car air condition or light because you need to examine the minimum alternator AMP output.
  • You can now bring out your multmeter, setting it to read the AMP. Now insert the metal on the ending side of the black wire which extends the multmeter to the negative battery terminal. Note that this negative battery terminal comes with a black cable which is labeled NEG. The place the other metal on the ending side of the red wire on the positive battery terminal. Note that the positive terminal comes with a red cable which is labeled POS.
  • Examine the multmeter, it should be reading the minimum AMP you jotted down before. Though it does not matter if it’s 10% higher or lower than the minimum specification, but if its 10% lower, then get an expert to examine the alternator. And if it’s 10% higher, examine to ensure you didn’t turn on any electric component, and if you didn’t, then contact an expert for help.
  • Put on energy consuming component like your air conditioner, light and wiper. This will increase the AMP because they are components that consume energy. Then your alternator will adjust the output, that is if the alternator is working well.
  • Try to measure the alternator AMP output making use of the multmeter as you did before. Note that the reading will be near the maximum AMP you wrote down earlier. In case, the reading is 15% lower than the maximum, ensure you turn on as many components as possible. And if you did, then examine your alternator. But if you didn’t turn on many components, then do so and examine the reading again. After which, if you observe little or no change, then contact an expert to examine your alternator.

Once done, you can now turn off your car and close the hood.

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