How to Test Alternator by Disconnecting Battery?[Easy Steps]

Usually, electrical problems in the car are attributed to the battery. However, the alternator may not be providing adequate power.

If the alternator does not work properly, the vehicle will give electrical failures, even stopping suddenly, even though the battery is fine. It is therefore advisable to check the performance of the alternator before replacing the battery.

While it is well known, there are several ways to test an alternator and one of the most popular ways is by disconnecting the battery. Here you may ask, how to test an alternator by disconnecting the battery?

Well, actually the answer is in the question, but nevertheless, before, we must review a few details.

How does a car alternator work?

The alternator is basically an endless generator of electricity in vehicles.

It takes a lot of energy to start an engine and meet all the electrical needs of a vehicle.

While different batteries can have huge capacities, eventually headlights, heater, radio, navigation and other components will drain the battery power.

An alternator charges the battery while the vehicle is running by converting mechanical energy from a rotating pulley into current.

The rotor, brushes and other internal parts of an alternator wear out over time, so replacing it is something that must eventually be done.

The following symptoms should give you an idea of what is wrong, but we recommend taking the vehicle to your trusted workshop for load testing and system starting.

Symptoms of a Failing Alternator

While the alternator is a relatively simple component, it plays a very critical role in the operation of the vehicle. It essentially converts mechanical energy from the crankshaft into electricity through induction. This is used to power automotive accessories, which can be anything from headlights to windshield wipers. The alternator keeps the battery fully charged, which is responsible for providing the electricity needed to start the car.

Since the alternator is connected to other critical systems of the vehicle, it can generate some mechanical difficulty and this further complicates the diagnosis of problems in the car. However, paying attention to the following six signs will make it easier for you to know if the fault is in the alternator.

So, here we are with 6 symptoms of faulty alternator:

1. Dimmed lights

The faulty alternator symptom that most drivers recognize is dim or flashing lights.

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This is really obvious when the headlights are on at night, but you can also see it in the dashboard lights and the interior light in the center.

If the lights come on as the RPM increases, it’s an even safer signal.

2. Service Engine Light

Another obvious signal is what the vehicle is trying to say through the dashboard.

This varies by model, but when the alternator starts to drop, you can turn on a “Check Engine”, or a battery icon indicator.

3. Strange noises

The serpentine belt may have stretched with age and may not turn the alternator pulley effectively, resulting in a lack of load.

This usually results in a squealing noise.

On the other hand, the internal bearings may wear out, causing a grunt or grinding noise.

4. Electrical problems

The vehicle’s electrical equipment, such as power seats or windows, can be slow to operate.

Without enough power, sometimes the device will not work or will stop working suddenly as if the radio were turned off.

5. Stopping the Engine

If the engine suddenly dies off while driving, it could be an alternator problem.

Fuel injection needs a good amount of electrical energy, and without it, the engine stops quickly.

6. Dead battery

Without an alternator running, the vehicle quickly consumes the full capacity of the battery, leaving it depleted.

The hood should be opened and the battery checked.

A new battery and a malfunctioning alternator will leave the vehicle stationary again.

Testing Alternator by Disconnecting Battery

So, we come to the question, how to check for the bad alternator by disconnecting the battery? Well, this will be a quick and easy process, follow the steps below.

Step 1

  • It starts the vehicle either by passing the current through wires or by charging the Battery.
  • Open the door and leave it open so that the roof light comes on and stays on.

Step 2

  • Disconnect the power leads or whatever you used to start and run the vehicle.
  • Go back inside the vehicle and see how brightly the roof light is shining.

Step 3

  • If the vehicle is still with the engine running, then look once more at the light on the roof and…
  • turn on the headlights. Observe the roof light once more.
  • The roof light should dim only momentarily when the headlights are turned on and return to normal brightness. If this happens, then the Alternator is not failing.
  • If the intensity of the brightness gets progressively weaker and DOES NOT RETURN to its previous intensity the Alternator is failing and is not charging the Battery.
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Step 4

  • Continuing and if the engine is still running ….
  • With the headlights still on, turn on the air conditioning or heating system and set the fan motor (of the fan) to the highest position.
  • The roof light should dim only momentarily when the headlights and air conditioning are turned on and return to normal brightness. If this happens, then the Alternator is not failing.
  • If the intensity of the brightness becomes progressively weaker and DOES NOT RETURN to its previous intensity the Alternator is failing and is not charging the Battery or providing the necessary current for the electrical needs of the vehicle.

Alternator Test Results

OK, here is a review of the results of the Alternator test we did above:

The Alternator IS FAILING: if every time something is turned on (like the headlights, the windshield wiper motor, the air conditioner, etc.) the intensity of the brightness of the roof light decreases progressively until it looks very weak.

The Alternator IS WORKING OK if the intensity of the roof light only decreases momentarily when several things are turned on one after the other and returns to its normal brightness.

Criticisms of Battery Disconnecting Test

how to test alternator by disconnecting battery

Actually, it is very easy to test the alternator by removing the battery cable. But this is very risky because where is the problem?

Experts today have shown concern about the effectiveness of this method. This method was effective for generators, however, an alternator is not a generator. So, when you try an alternator with the method described above (removing the battery cable), it considerably increases the possibility of killing that regulator, and that is the risk.

Now about the ineffectiveness of it-

Well, first of all, the vehicle will work if the alternator produces enough energy. But, there is also the possibility that the alternator is in poor condition, but produce the least amount of energy to run the engine and as a result, will give a false positive indicating that the alternator is fine when it is not.

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How to Test an Alternator in the Right Way?

Necessary tools:

  • Multimeter
  • Safety glasses
  • Code reader

How to diagnose an alternator in bad condition

If the vehicle has a “Check Engine” light, connect a code reader to the diagnostic port.

If the code P0562 appears, the alternator is most likely in poor condition (note that codes may vary by vehicle make/model/year).

If an alternator is suspected of failure but does not have a warning light, raise the hood and check the condition of the belt.

If it is glazed and looks slightly burned, it is an indication that it is slipping.

The belt is too loose, so it slides over the pulleys instead of running on them.

Metal friction on rubber heats the belt, wearing it out quickly.

The tensioner must be adjusted to obtain the right tension (too tight can also damage the bearings of an alternator), or the belt must simply be shaped like a serpentine.

If everything looks good under the hood, you can check it with a multimeter.

Make sure the vehicle is parked on a level surface with the parking brake on, and wear safety glasses.

Set the meter to 20 VDC and connect the cables to the battery: positive cable to the positive terminal of the battery and negative cable to the negative terminal. It should read about 12.6 volts.

Start the engine and check the voltage again carefully.

This time, it should be at least 14.2 volts if the alternator is working properly and is charging the battery.

If the voltage is good, we should turn on the headlights, interior lights, radio, heater, and any other electrical loads.

The meter should still show more than 13 volts.

If any of the voltages are below specifications, the alternator should be replaced.

All wires should also be checked for corrosion or a loose connection.

Final Thoughts

Well, now you know how to test alternator by disconnecting the battery. More specifically, you know the symptoms an alternator gives off when it is failing, the reason to avoid this test, and why to use the suggested alternatives.

Our goal is always to help you have your vehicle in the best condition, explaining the existing methods, the specific tests, and what methods to avoid.