Car alternators provide the electrical energy needed to run all of the electronics and power circuits of your vehicle. It also charges up the battery to full capacity ready for the next time your start the car.
The car alternator is a simple device made up of heavy-duty bearings, electrical copper windings, charging circuits, diodes, and a set of carbon brushes. Over time the bearings and or brushes can wear and fail to cause the charging circuit to not work properly.
In this post, we will go over how to test your car alternator both in the vehicle and out of the vehicle to see if it is outputting the correct voltage.
Table of Contents
How to test an alternator with a multimeter
With the vehicle turned off take your favorite multimeter and select DC voltage. Place the test probes onto the battery terminals and measure the battery voltage. It should read around 12 to 12.7 volts depending on the charge level of the battery.
Any battery voltage below 12v then you have a discharged battery and it should either be recharged or replaced if it is over 4 years old.
Once you have your base battery voltage start the vehicle and wait 30 seconds for the engine to idle smoothly. Take your multimeter and select DC voltage and measure across the battery terminals again.
Alternator output voltage
A good quality car alternator should output between 13.5v and 15v depending on the model and load on the alternator. Anything less than 13-volts and you will have a failed alternator. It should also be a smooth voltage and not fluctuate or jump between say 12v and 13.5 volts every few seconds.
As you can see the alternator output voltage on a 2003 Ford Focus is 14.34 volts. This is a good voltage output from the alternator. It will temporally dip down to 12v when a high load kicks in such as the air-conditioner. An inconsistent voltage usually means the carbon brushes are failing on the alternator.
Some smart alternators will actually lower the voltage/current to a fully charged battery.
Car onboard diagnostics.
Often when the vehicle alternator is failing you will get an engine, battery, or electrical check light on the dash. The car’s onboard diagnostics is testing the output voltage of the alternator and if it dips below a pre-defined voltage then the engine battery check light will come on.
You will need an OBD2 tester to further diagnose the problem as it could be one of 100+ general faults. I use a Bluetooth OBD2 tester for my Yamaha Motorcycles which also works on all my cars as well. A very handy piece of hardware that is very cheap.
How long can you drive with the alternator light on
This will be determined by a number of factors such as battery capacity size, battery charge state, and electrical load on the battery.
To get the best mileage if your alternator has just failed turn off all non-essential electronics. This means radio, air conditioning, and driving lights if it is daylight.
You should be able to drive around 50 miles with the alternator light on. this is with a full capacity battery. Older cars say 25 years+ old may run longer as they have fewer electronics in them such as no fuel injection.
The battery light is on but the alternator is charging problem.
This could be an indication the battery is not accepting the charge. If the battery voltage drops under 12v then the battery has failed or is depleted. I like to replace my vehicle batteries after 4 years of service life as they become unreliable after this.
If the battery light is on but the alternator is charging then the problem could still be at the alternator. If the alternator is over 190k miles old or 300,000km I would get it replaced as it is getting close to mean time before failure.
My Toyota Highlander alternator was replaced at 200,000 miles and was still going strong but preventative maintenance is the best.
How to test the alternator out of the car
There are a number of reasons you may need to test the alternator out of the car such as a reconditioned part, rebuilding the engine, scrapyard find, or just being curious.
Make sure to clamp the alternator to the bench securely.
Locate the positive terminal and clean up the bolt post with a fine bit of emery paper. Place the alternator in a vice and make sure it is clamped securely. Spin it by hand to make sure it moves freely. As the frame of the alternator is the negative ground clean up a spot to place the negative of the multimeter onto.
The only way to test an alternator charging correctly is to use another 12v car battery. Connect the car battery up to the alternator and get a small 12v globe and connect it to Earth and to the lamp terminal shown in the picture above. The lamp will turn off when the alternator is charging the battery. The lamp will turn on then the alternator slows down or stops.
Now find a socket the same size as the flywheel bolt and attach it to a battery drill. On my Toyota Highlander/Kluger 130A alternator I used a 10mm socket attached to a 1/4 inch drive extension piece. This is what we will use to spin up the alternator fast.
Now get someone to help you as you spin up the alternator. You should have the drill at its fastest speed to measure the voltage between the positive and negative of the housing. It should be between 13.5v and 15 volts. Make sure to use the fastest drill speed possible.
This is one of the best ways how to bench-test an alternator off the car. The video below explains it well.
How to test an alternator without a multimeter
There are a couple of bush mechanics ways to test if the alternator has failed without a multimeter.
Start the car and look at the battery voltage on the dash display if it has one. Then rev the engine to around 3000 RPM and turn on all the accessories such as air conditioning, radio, and lights. Check the battery voltage in the dash to see if it is the same, increases, or decreases in voltage.
If it decreases below 13v you have a faulty alternator and must have it replaced.
Another way is to use a cheap 3-48V circuit tester. These can check for 12v power and as it is digital it will give your the exact voltage as the alternator kicks in to charge the car battery. They are small enough to keep in the car glove box and come in handy to find electrical faults.
One end gets clamped onto the metal frame or negative battery terminal and the other can be used to detect voltage. I like that it has a sharp point so that you can push it into the insulation to access the copper wires underneath.
How to test an alternator with a screwdriver
The screwdriver test is a quick test to see if the alternator is making an internal magnetic field. This won’t test for the correct output but will only test that the internal windings are not shorted out or broken.
Find a long thick metal screwdriver and place it near the copper windings on the non-drive end of the alternator. It works better with a screwdriver that is magnetized.
With the vehicle off you will not feel any magnetic field or a pulling sensation on the screwdriver. With the car turned on and running do the same test but this time you should feel a slight magnetic force pull the screwdriver into the windings.
Be careful as there are a lot of moving parts and belts around this area.
How Do I Know If I Have A Variable Voltage (Smart) Alternator?
Variable Voltage Alternators are now becoming a standard issue in most modern vehicles. These only run or output a large voltage over 13v when the power demand becomes large. Another name for a smart alternator is called Computer Controlled Alternators.
Variable voltage alternators reduce the load put onto the engine by only operating when required to charge the battery or a large load is applied like the high beam headlights, electric heaters, and the sound system. They help to reduce the emissions of vehicles to pass strict laws in many countries.
Smart alternators require battery sensing technology to determine the load coming from the battery. This is done by a battery sensor which is found on one of the battery terminals often it is on the battery negative connection point.
To test a smart alternator start the car and turn off all of the electronics such as light, radio, and heaters. Use a multimeter to test the battery voltage. It should be under 13v if the battery is full or close to 100% charged.
Now turn on all of the electrical systems of the vehicle such as headlights, sound system, heaters, and internal lights, and test the battery again. The output of the alternator should now be over 13.5v with a properly functioning smart alternator.
You can also remove the small-signal wire from the negative terminal. This will trick the vehicle computer that the battery needs charging to always output over 13v to 14v.
Alternator Frequently asked questions.
Why do car alternators fail?
As a car alternator uses carbon brushes these will wear out over time. Alternators also have metal bearings that are under a side load due to the serpentine belt. These can fail if the belt tension is too tight.
The picture above shows the carbon brushes from a 2005 Toyota Highlander/Kluger alternator. These only had about 1mm of carbon brush left before they would have failed. The car had been driven for 300,000km on the original alternator.
They are easy to replace just remove the back black plastic cover and remove the 3 screws holding the carbon brush block in place.
The alternator electrical circuit has a full-wave bridge rectifier inside to convert the AC voltage to a DC voltage. Sometimes these diodes fail but it is rare.
Can a car run without an alternator?
Yes but not for very long. Often part of the alternator circuit is a range of belts and pullies. Sometimes these belts run other car items such as water pumps, air conditioning, and power steering. If you are fully removing the alternator and wanting to start the car make sure these other items or not going to be affected.
As mentioned earlier you may only get around 50 miles of car run miles before the vehicle will run out of battery power due to not having a working alternator.
Can you jumpstart a car with a bad alternator?
Yes, you can jump-start a car with a bad alternator but if the car won’t start with its own battery then it won’t travel very far because the battery is already depleted. You may only get a few extra miles before the voltage in the battery is so low the fuel pump fails along with the fuel pump injectors or the onboard engine management circuit.
I can recommend any of these wireless jump starters for your vehicles.
Do motorcycles have alternators?
Yes, they do but the alternator of a motorcycle is more commonly called the “stator.” It is often located on the opposite side of the clutch plates and includes a stator and rectifier/regulator circuits.
Testing of the motorcycle stator will be the same procedure as the car alternator battery voltage test. Any voltage between 13.5v and 15v with the motorcycle engine running means the starter is good.
Alternator replacement cost
Depending on the make and model of the vehicle a new alternator replacement cost will be between $400-$1000 for just the part. Mechanical labor costs will usually be another $200 on top due to a few hours of labor.
You can save this labor cost by changing out the alternator yourself as this is not a hard job. Just make sure to set the tension correctly of the alternator belt otherwise it will squeal and or slip.
I also like to replace the alternator belt also called the serpentine belt if I replace the alternator.
Serpentine belt noise
Sometimes when the serpentine belt gets old and worn it can start to slip. This causes a squeal that may or not go away as the belt heats up. This indicates either an over-tightened or slack serpentine belt.
With the car engine off use your fingers to check if there are any cracks in the alternator belt. If you find lots of cracks in the ribs or there are chunks missing it is time to replace the serpentine belt.
If it is slipping due to just being slightly worn you can tighten the belt slightly so the belt noise will go away. Most adjustments will be done via the alternator adjustment screw check your workshop manual for the exact instructions for the model of your car.
Alternator belt tension trick
To set the correct tension on a serpentine belt that has a manual adjustment screw tighten the belt until you can only turn it 90deg on the longest belt run.
Just use your fingers to turn the belt from horizontal to vertical. The quick video below explains it well. It’s a handy tip.
Conclusion
Alternators are a slow wear part on your vehicle and should be replaced according to the service schedule. I have seen alternators reach over 250,000 miles with no problem with older model Fords, Toyotas, and GMC cars.
Sometimes you can just replace the brushes and get another 200k miles out of the alternator. Although you risk the bearings failing over time.
I like to replace alternators every 200k miles just to be on the safe side. Nothing worse than breaking down in the middle of nowhere because you have run out of battery power due to a faulty alternator.
Most cars may take 20 years to get to these high milages where the alternators need replacing. But if you are pulling a large current out of your battery like running spotlights or car fridges the alternator life may be less.
A word of warning please don’t buy the cheap eBay alternators from China as many don’t last as long as proper aftermarket OEM parts. In my research, over the years I have found that the alternator carbon brushes are much shorter and the bearings smaller. There is a reason why they are so cheap and will fail just over 100,000 miles.
A bad alternator will not charge the battery and is one of our top 16 reasons a car turns over but won’t start.
I am a qualified Industrial Electrician for the past 20 years and I love to share my knowledge on home repair and maintenance jobs.
I love fast toys like Motorcycles, Cars, Jetskis, Boats, and Computers so writing about them is easy. Working on them is fun.
To keeps costs down I do all my own mods, repairs, and servicing. These skills I want to share with everyone. DIY is a skill everyone can learn.
