Tips For Testing Your Alternator With The Battery Connected

by Phil Borges // in Car

You may be wondering what the difference is between testing an alternator with or without the battery connected.

The answer is that there really isn't any difference, but it's a good idea to test both ways to see if you're getting different results. This will help determine if your alternator needs repair or replacement.

The alternator is a crucial component of your vehicle because it will keep the battery charged and in top condition. If you don't get your alternator checked regularly, you could be headed towards a very expensive situation.

This article will show you how to test your alternator with the battery connected.

Is it safe to test an alternator with the battery disconnected?

Yes, it is perfectly safe to test your alternator with the battery disconnected. However, if you are doing this simple task at home then we highly recommend that you get a qualified mechanic to perform the inspection for you.

If left unchecked, the problem could become more serious and cost you more money in troubleshooting and paying for repairs.

If your alternator is not working, you could be headed for trouble. What you need to do is get it checked right away so that any repair or replacement can be done quickly and easily.

How to Test An Alternator With A Multimeter?

If you are the type of person who likes to work on your vehicle, then you know how important it is to have a multimeter.

A multimeter can help diagnose problems with all sorts of electrical systems in a car, from the alternator and charging system to the fuse box. 

How do you know if your alternator needs to be replaced? You can test it with a multimeter. Below are some tips:

Step one: make sure your battery has enough charge to start the engine by using a modern trickle charger when needed. The black wire should be connected to one terminal and red wires should connect with the other end.

Step two: make sure all accessories in the vehicle are switched off before performing this test for safety reasons. After completing these preliminary steps make sure that each connection is secure by twisting them together tightly so they do not come apart easily.

Step three: do a voltage check which should be around 12.6v. When you start the engine of your car, it's important to make sure that there is enough voltage. The gauge should read somewhere between 14.2 and 14.7 volts for optimal performance.

Step four: turn on your car's electrical accessories, including the radio and lights. Check the voltage again to make sure it stays above 13 volts; if not then you need a new battery!

Step five: the reading should show 13volts that indicates the alternator is charging the battery after switching off the accessories and the engine.

What is a "Full-field Test"?

A full-field test is a method of testing the alternator while having it under load. It involves using a circuit that will draw around 70amps.

This is sufficient to test the alternator when everything is being charged properly, including the battery and electrical components in the car's instrument panel and seats.

If you have an alternator that requires the field terminal to be connected to the positive battery terminal, make sure you find out which type of alternators are installed.

If it produces a large voltage increase when full-fielded then there may be other problems with your regulator and its wiring connections - or even wrong polarity in some cases.

Your Guide to Test The Alternator Without Multimeter?

To get a reliable reading, you must first inspect the screwdriver for rust and clean it off. Any metal near electrical components can create false readings so make sure that your hands are insulated with gloves and be careful when using tools around these systems.

You can find the alternator under the hood. You will need your screwdriver; just pick one with a tip as thin as possible. With caution securely grasp hold of the bolt holding down its pulley wheel.

Turn on the car's ignition, for the dashboard lights to be illuminated. But don't start the vehicle just yet! Use a screwdriver to touch one of those pulley bolts and you'll feel an unusually strong magnetic pull from it - even when you're standing quite far away.

Alternators are responsible for powering electrical systems in cars. If you don't feel the strong magnetic pull, either your alternator is faulty or your regulator needs to be replaced.

When to take your alternator to an auto parts store?

Inspect your alternator carefully if it does not have a strong magnetic pull. If there is no pull you might need to install a new one.

If you see that the pulley wheel is rusty, or the belt looks frayed, broken, loose, or worn-out then get them replaced immediately before they cause problems.

Some auto parts stores (AutoZone) offer a free checking service where customers bring in their cars' alternators and batteries so that mechanics can check them out over there.

How do they (AutoZone) test the alternator?

Auto zone offers a bench test for your alternator. Their test rig will load-test the charge output of your car battery to ensure that you get enough power from it while driving down the road.

The old item for a refund - on offer?

When you bring your old item in for replacement, AutoZone will refund the core deposit amount. This is usually around $40 which means it’s worth getting this service if only to avoid wasting that much money and effort on a new battery!

How accurate is the alternator test?

The AutoZone test is a quick and easy way to know if your alternator will last for the day or not. Though these tests are not indicative of how much longer they will work, they can give you an idea of what's going on at that moment in time.

What are the signs that the alternator is failing?

The alternator may suffer from a shorter life than expected. This is most often due to old age, but it can also be shortened by an underpowered battery or the presence of other faulty components in your vehicles such as belts and hoses.

How to spot the sound of a bad alternator?

If you hear a screeching noise, it could be that the bearings have failed. On the other hand, if there is a whining sound coming from your furnace or air conditioner then perhaps it's their regulator that has gone wrong.

How much is it to fix an alternator?

With the high failure rate of car alternators, owners need to know how much they cost and where you can go to find one.

Replacement prices range from $100-$300+, depending on what kind of vehicle you have. Some might be easy swaps- others will need additional parts removed to replace your faulty part.

How to spot a potential problem with the alternator?

A sign of a faulty alternator can be seen when those interior lights start to flicker randomly while driving at night; this flickering indicates there may be something wrong with how it's producing energy internally so look out for any signs like these next time you go into town.


The alternator is a critical component of the car. The battery will die and you won't be able to start the engine if this item is missing.

Hopefully, this short guide on testing has helped diagnose problems so that you can get back on the road as soon as possible.

About the author, Phil Borges

Phil Borges is a battery aficionado. He's written extensively about batteries, and he loves nothing more than discussing the latest innovations in the industry. He has a deep understanding of how batteries work, and he's always on the lookout for new ways to improve their performance.