How to Test Your Alternator Easily (Without a Voltmeter)

If on the other hand, nothing happens (and the engine continues to run just fine), then, the alternator is probably firing off enough power to run the system without direct help from the starter battery.

For older vehicles and classics without sensitive electronics, this is perhaps the oldest (and crudest) way to check if your alternator is bad.

However, on modern vehicles with sensitive electronics, testing an alternator in this manner isn’t only crude: it’s also inviting big trouble and a hefty repair bill (in addition to possibly, the cost of an alternator replacement).

So, how do you know if an alternator is bad, especially if you don’t have a volt meter nearby?

You’ll need a dedicated battery tester like the Foxwell BT705 or the Ancel BA101.

Things You’ll Need to Test Your Alternator Without a Voltmeter

To test an alternator without a voltmeter in the safest manner possible, you need:

1. A good battery tester (like the Foxwell BT705 or the Ancel BA101).
2. Patience.
3. Common sense.

Before I proceed, what are the (possible) signs of a bad alternator in the first place? What are the signs to look out for before proceeding on a test to positively isolate the alternator as the culprit and architect of your vehicle woes?

Signs of a Bad Alternator

Outlined below are the most common signs of a bad alternator. While this list is by no means either exhaustive or conclusive, these signs are usually the first ‘port of call’ before you isolate the alternator for testing in the first place.

1. Difficulty Starting the Car

There are numerous reasons why your vehicle may refuse to start. Curiously, an alternator that is already bad (or starting to go bad) is one of the major causes of such difficulty in the first place.

Your vehicle is designed in such a manner that it requires power from the battery to turn the starter, which turns a flywheel attached to the engine, giving the engine the roar of life that translates to the movement your vehicle gives you.

Unfortunately, with an alternator that has gone bad (or is going bad), this process is truncated as the battery doesn’t get enough (or any) charge from the alternator (the alternator, BTW, is responsible for recharging the starter battery).

With the starter battery weak (or completely flat), your car is at the mercy of strangers – who are kind enough to lend you their portable jumpers or starter batteries.

2. Dimming or Flickering Lights

If you have ever needed to drive on a lone country road during the night with the rains pouring down ferociously, you know how challenging it can be, especially if you’re unfamiliar with the route.

Now, add headlights that are dimming on their own, permanently dim (or flickering) and the horror story would be complete. This, without any shred of doubt or ambiguity, is what a failing alternator does.

This is why it happens: under normal working conditions, the alternator is responsible for running the electricals of the vehicle and recharging the starter battery. The starter battery, of course, chips in periodically, to augment, if/when the needs of the moment exceed what the alternator can safely provide at that time.

However, when the alternator fails – and the battery begins to get weak due to not receiving any charge- dim, dimming, and flickering lights are bound to occur – both from the headlights, interior lights, and the vehicle’s interior displays.

It’s a classic case of a bad alternator that needs to be checked especially when revs to the engine suddenly brighten the lights before having them go dim again.

3. A Lit Battery Warning Light on the Cluster

Most modern vehicles have a battery warning system that illuminates on the cluster when the system detects something wrong with the charging/battery system of the vehicle. This also covers the alternator’s failure to properly perform its battery charging duty.

Usually a red-shaped battery, this sign remains illuminated on the cluster till the issue is resolved.

If you notice that the battery warning sign of your vehicle has remained illuminated, then, there’s a great chance you have a challenge with the alternator – which is failing to charge the battery properly, leading to the illumination.

4. Unexpected Flat Starter Batteries

Starter batteries do a great job keeping their charge, even the basic lead-acid models, working under extreme temperatures of cold or heat.

However, when a starter battery that should be serving you functionally decides to go flat for no visible reason, then, you’re likely dealing with an alternator that has failed to keep up with the charging routine of the battery.

Remember, the starter battery is responsible for starting the vehicle and supporting the alternator when needed. For these actions, it needs to be recharged as soon as possible or it logically goes down.

A starter battery going flat unexpectedly would only mean that the alternator has failed to keep up with its battery recharge duties (as it should).

5. Malfunctioning Electrical Accessories

Electrical accessories malfunctioning in a vehicle is also one of the signs of an alternator that is either bad or going bad.

The clock, instrument cluster lights, and power windows all rely on the battery to do their thing and the battery, in turn, relies on the alternator.

In rare cases, these accessories may work, without the starter battery – assuming the engine is already running and the alternator is being driven.

However, when this isn’t the case and the alternator begins to get weak (or bad), these accessories will begin to work in a messed up fashion and in most cases, may outrightly malfunction or completely fail to work as they’re supposed to.

The alternator is the culprit when this happens (especially with the other signs above).

Done with the signs of a (possibly) bad alternator, how do you positively confirm that an alternator is bad without a multimeter (so that you don’t misdiagnose or worse, change a perfectly functional part?)

How to Test Your Alternator for Failure Without a Multimeter

As stated before now, to test an alternator without a multimeter (and get awesome results without compromising anything), you need a dedicated battery tester.

Now, any good battery tester will do. However, I’ve used and highly recommend the Ancel BA101 (if you’re looking to save on cost). However, if you want something more capable, consider the Foxwell BT705.

For this test, however, I’ll be using the Foxwell BT705. While the steps outlined below may slightly differ when using another battery tester, the logic, essentially, is the same – at least for battery testers that can also test alternators.

1. Connect the Battery Tester (And Test the Starter Battery)

Testing your alternator with the Foxwell BT705 is simple, but you must begin with the starter battery first.

The logic is that the starter battery is part of the ‘trinity’ making up the charging system – the other two being the starter and the alternator.

After hooking the clamps, the process is pretty straightforward: the battery tester guides you with onscreen instructions such as entering the battery value, figuring out which part of the battery terminals the clamps are hooked, etc.

Once completed, you’ll get a summary of your battery’s condition and a recommendation regarding what to do (this remains one of the reasons why I actively prefer a battery tester to a multimeter when testing a vehicle’s charging system. The tester gives a summary that every vehicle owner will understand, from Grandma to the ASE-certified technician).

I have found this feature helpful in preparing for a battery replacement. Remember, a starter battery could be almost gone but will still crank a vehicle only to disappoint in a time and place you least expect.

2. Testing the Starter

The next logical sequence, if you’re interested in testing an alternator you suspect could be bad and are using the Foxwell BT705, is to press enter after the battery test summary has been relayed.

You’re then asked to start the engine. The tester takes notes of the volts and time it takes for the starter to fully crank the motor and advises you on its condition (even as the metrics are displayed for your consumption).

As with after the battery test, you’re given the option of pressing ‘enter’ if you’re interested in the alternator output, after the starter results have deployed.

Since this is your interest, pressing enter and following the prompts should be what you should do, right away.

3. Using the Foxwell BT705 to Test the Alternator

Upon pressing ‘enter’, the battery tester takes you through a series of tests to test the alternator with loads on and off; at idle and when the engine is revved.

The grand summary tells you if there’s anything wrong with the alternator (or not). Following this summary closely is the alternator output both loaded and unloaded, all in volts.

This, effectively, wraps up the alternator testing without a multimeter. At this point, you’re cleared if the alternator is the cause of your motor woes or if you need to look elsewhere.

But, why does using a dedicated battery tester beat using a multimeter (or voltmeter) to test an alternator?

Why a Dedicated Battery Tester Beats a Multimeter in Testing an Alternator

There has been a plethora of arguments online about why anyone should consider using a dedicated battery tester to test an alternator as opposed to using a simple ‘versatile’ multimeter.

When I shared the video of this article on YouTube (linked to below), the first comment I got was to this effect. The commenter didn’t see the logic of avoiding a multimeter and going the battery tester route, to test an alternator.

The comment was particularly nasty – but in my usual fashion, I didn’t delete it: I pointed the commenter to the relevant part of the video that addressed their concern.

Characteristically, they never replied.

So, if you’re reading here and have the same concern, allow me to allay your fears. This is why a dedicated battery tester is much better than a multimeter when it comes to alternator testing.

1. Not Everyone Understands a Multimeter (Or Knows How to Use One)

This goes without saying – but often, it’s missed. For instance, what volts should a multimeter output when working great? When is it NOT working as it should? During idle? Under load?

If you don’t know these values, having a multimeter is plain useless – and won’t exactly help you isolate a bad alternator.

It’s worse when you don’t know where the multimeter probs go. If you’re thinking: ‘should I touch the probes on the battery or the alternator itself, then, great chances are that you should be looking at a battery tester – and not a multimeter.

If this is you, I highly recommend the Ancel BA101 – it’s small, cute, and inexpensive. Don’t let the reasonable price tag fool you though: it does what it says it will do and is as accurate as a (good) multimeter (I checked and compared).

2. A Multimeter Is General; An Automotive Battery Tester Is ‘Niche’…

Multimeters are what they are: general, ‘multi’ tools that test things in the house, on the car, and just about any place where wires and voltage reside.

That said, a multimeter is a great thing to have around – but it doesn’t take away a battery tester’s ‘niche’ power and value.

A battery tester, on the other hand, is focused on testing ONLY the charging system of an automobile. With this setup, it does a thorough job on a vehicle and gives you smart summaries it was designed and programmed for – something most multimeters are incapable of.

What does this mean? Grandma can easily use a battery tester and come to the same conclusions as an ASE-certified automotive engineer.

A multimeter? Good luck with that!

3. Most Multimeters CAN’T Test What a Basic Battery Tester Effortlessly Does

Finally, most multimeters are designed in a basic and cheap manner – and focus on what most users need: testing AC/DC voltage and current.

This is fine, if what you’re looking out for is simply that. If you’re concerned about resistance and the ton of other values a battery tester plays with, most multimeters can’t/won’t help.

And, the ones that can help are priced around the same level as a good battery tester.

Why waste money on what you don’t need while actively neglecting what you desperately need (to test your alternator)?

Wrapping Up

The fastest (and easiest) way to know that your alternator is bad is to disconnect the battery cable(s) from the battery terminal(s) when the vehicle is running.

Unfortunately, on most modern vehicles, doing this is actively asking for trouble; at the end of the day, you won’t just have a suspected bad alternator: you’d likely (also) have a completely messed up electrical system in the vehicle.

This is why, a multimeter is generally recommended. Unfortunately, not everyone has a multimeter – or knows how to properly use one.

The next logical thing? A dedicated battery tester. The Foxwell NT705 is what I recommend (for its versatility) and the Ancel BA101 (for its ease of use and affordability).

With any of the above, you won’t simply get better value than a traditional multimeter, you’ll also be able to smartly test your starter and starter battery – eliminating 90% of the chance of being stranded on the road…

…ever again.

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