How To Test An Alternator With A Multimeter

The alternator works to charge the battery and other connected parts of the car when it running. If the alternator is not working well causes weak lights operation and the vehicle engine does not work well. There are some tests used for alternator testing, like as using of multimeter to find working conditions. Here we will cover details for alternator testing with the use of a multimeter.

What is an alternator?

The alternator, also called a synchronous generator, is used for producing AC power at a certain frequency.
The alternator comes with coils wrapped over an iron core for generating current through the phenomenon of electromagnetic induction.

The coil produces current in two techniques, the first one is a rotating coil, and the magnetic field is static, and the other is a rotating magnetic field and the coil is static
.The alternator is the main component of automobiles for charging the battery at time of driving. it also used for energy-producing systems such as nuclear, thermal, and hydropower stations.
Car battery comes with an open circuit voltage of 12.45 to 12.6 volts. The car alternator operates at a high voltage compared to the battery voltage. Since voltage losses cause the voltage at the alternator to reduce when it moves through longer wiring of vehicles.
So the voltage at the alternator is 14.5 volts. The voltage at the battery is 14.2 volts, and at the closest tail light bulb, the voltage is about 13.5 volts since voltage losses occur.

Why Test the Alternator?

If you have an accurate working alternator, that will provide some good functions, so it needs proper testing for good working.

Battery Charging:

A good working alternator provides a battery to properly charge and provides the required power for starting the vehicle and the operation of other components.

Power Supply:

It provides current to car power control circuits such as lights, the AC system, and other components.

Voltage Regulation:

The alternator regulates voltage to avoid overcharging the battery, which can affect the battery or can make the battery dead.

Electrical System Stability:

The bad alternator causes some problems for the electrical system, like erratic dashboard lights and intermittent electrical errors.

How to Test an Alternator?

Follow these points for testing the alternator with a multimeter.
- First of all, set the meter value to voltage and confirm that it is set at 20 DC volts. If your voltmeter does not have a setting for incremental set at DC volts.
- Check the alternator at the positive and negative terminals are clean.
- Connect the meter lead with the required point and the negative with the negative. Normally, the negative terminal is black and the positive is red.
- Just check the voltage of the battery if the car is not running. Note values, that is, about 12.6.
- Now start the car and check the voltage at the battery when the car is running. That is in the range of 14.2 to 14.7. So the alternator is generating voltage.
- A value more than 14.7 shows the battery is overcharged, and a value below 14.2 shows the alternator is undercharging the battery.
- Onthe light and other electronics to confirm that the value of the meter is not less than 13.
- When the car is off reading is over 12.6.

Bad Alternator Symptoms

Visual Inspection

Check the alternator visually. In this inspection, check any loose wire or any connection. With that, find a crack or damage on the alternator casing that helps dust to move in the internal circuit. Also check the belts of the alternator that are worn. If you find any sign, that means the alternator is not working well.

Voltage Test

With use of a multimeter for testing voltage coming directly from the alternator with the engine off and on. For this, connect the red probe with the output terminal on the back side of the alternator and the black probe with the ground. When the engine is off, the reading is 12 volts, and then when the engine is started, the voltage reading increases to 13.5 to 15 volts. If the voltage value is about 12 volts, it means the alternator is not charging well.

Battery Testing

If the alternator is bad not charge the battery at a time when the engine is working well. For this test, a multimeter was used for checking battery voltage at engine-off conditions. After that, start the engine and again perform testing.
In the engine-on state, voltage increases. If it is the same or lost, that means the alternator has an issue. Also on headlights, check if there is voltage less than 12 volts.

Removed Battery

It is an easy testing method for checking the alternator output. Through engine idling, remove the negative battery pin. If the engine stalls out, that means the alternator is not producing current for the running engine, and it means failing. If the engine is running, the alternator is charging accurately. So reconnect the battery cables again.

Warning Lights

Different cars come with built-in voltage meter lights that are based on a sharing system. The battery warning light comes on if the ignition is on, and that goes out when the engine starts. If the engine lights remain on, it means there is a bad alternator.

How to Test an Alternator with a Multimeter?

Measure the battery’s voltage in standby and running modes.

The main function of the alternator is to charge the battery. So the basic method to test an alternator is to check the car battery voltage.
- First of all off the car engine, then move the matter dial and set the knob for measuring DC voltage to a value of 15 to 20 20V. The main rule is to choose a value for the meter that is a low number but larger than the voltage measured.
- Make a connection of positive and negative pins with positive and negative pins of the car battery.
- Check meter display and check voltage measured. This value is 12 to 13 volts. If voltage is less than 12 volts, charge the battery with a battery charger.
- Start the car, and with a meter, check the battery voltage when the car is running. If the voltage value is between 14 and 15 volts, the alternator is working normally.

Car’s electrical system loading.

Set the car running and use the meter for checking battery voltage. On all heavy connected loads, like on lights, radios, fans, and heater blower motors.
Make a connection of positive and negative leads of the meter with respective leads of the car battery and measure the voltage.
Make compression of voltage with the value of extra load off. If the value is less than 13 volts, the alternator is keeping up with the electrical system’s demand.

Check Alternator

First of all, find the alternator location in the car. Rotate the meter dial and set the knob for measuring DC voltage value in the range of 15 to 20 V.
On a car engine, check the alternator belt is spinning well without slipping.
Make connection red lead and the red lead of the alternator.
Black with meter of any metallic component of car frame
Check values on the meter display; if the alternator is working well, the meter reading will be in the range of 13 to 14 volts. IF the value is higher, that means the alternator voltage regulator is bad.

How to test the alternator with a screwdriver?

The alternator testing process with a screwdriver is an easy and simple process that anyone can perform. This process is easy if you are off-road and parked on level ground. Follow these steps for alternator testing with a screwdriver.
- First of all, put the key into the ignition. Not starting the car, just inserting it into the slot and leaving it.
- Now open the hood, detect the latch on the underside of the hood, and pull it up. Now you can easily access the alternator.
- The alternator comes with a coil-shaped design with a belt connected to other engine components. According to the vehicle, there are extra belts and pulleys. The main component is hooked up to the battery with a cable.
- After finding the alternator, use a screwdriver for testing. Put the screwdriver close to the alternator if the key is in the ignition. Hold the screwdriver close to touch but avoid damaging any engine parts. Connect the tip of the screwdriver blade with the alternator nuts close to the pulley. The alternator is working well if you feel magnetism for a screwdriver, and if you don’t feel that means there is a problem with the alternator.

FAQs

How to test a bad alternator?

Disconnect the negative cable of the battery when the car engine is running. If a car or vehicle stalls, the alternator can be bad. The use of a multimeter for checking voltage produced with the alternator helps to find that it is working well.

What voltage value should an alternator test at?

13.5 to 14.5 volts

What voltage should the alternator output when the engine is running?

The alternator output value is in the range of 14 to 14.5 volts for the engine in running conditions.

How do I know if my battery is causing charging problems?

If the battery is completely charged but not starting or running, it means problem with the alternator. If the battery does not hold a charge, then the battery has an issue.

Can a faulty alternator drain a car battery?

The bad alternator can drain the battery through recharging when the engine is running.

What to do if the alternator tests pass but my battery still isn’t charging?

Check wiring, fuses, connectors, and voltage regulators for fault detection.

What are the signs of a failing alternator besides a battery light?

The common signs are flickering lights, burning smells, and electrical faults.