An alternator problem can manifest itself in many different ways. Most times the reason a car alternator doesn’t seem to be working is a result of poor electrical connections in the circuit or problems within the car alternator itself. So, how do you test a car alternator for problems? How can you tell if the alternator is the source of the issues you’re experiencing or if it’s from a bad circuit?
The good news is that testing a car alternator is quite easy. The important thing you need is a multimeter or voltmeter plus a couple of other things and you’re ready to diagnose the source of the problem, but you need to be careful: one mistake and you could possibly blow a fuse or worse, burn the onboard computer circuit. Here is the complete guide to testing a car alternator, but first, a peek at the alternator.
Any time our vehicle develops an electrical fault, be it a weak power supply, fried components, or blown fuses, the first things that come to mind are the alternator and battery. A car battery, like most other batteries, supplies a type of current known as direct current (DC). However, the alternator produces an alternating current (AC) hence the name ‘alternator’; this is similar to the power you get from the socket in your house, just that the voltage is smaller.
Both work hand-in-hand to power the different electrical systems in the vehicle. This includes the dashboard lights and meters, entertainment system, Radio, GPS, horn, headlights, and many more. The alternator generates energy from the mechanical movement of the engine and the battery stores this energy so it can be used to start the car, power the headlights, system clock, and other electrical loads when the engine isn’t running.
We’ve established how important of a role the alternator plays in a modern vehicle’s electrical system, but many times you might think the problem is from the alternator or battery, only to discover that, probably after spending money on new parts, that the fault lies within the electrical connections in the system. This could be a result of rusty connection terminals, load mismatch on the system, a blown fuse, or exposed wires.
Whatever the case may be, if the issue lies within the alternator, the battery, or the electrical system, then you can expect a reduction in the performance and functionality of some electrical systems and components. Here are some problems that may indicate that your battery or alternator is failing:
In most cars this light has the shape or symbol of a battery, in some other vehicles what you get is an ‘ALT’ for alternator or ‘GEN’ for generator. This is the vehicle’s way of telling you that it has detected a change in the health status of the electrical system. So check it out.
If you’ve noticed the luminosity your headlight and interior light rise and fall with the revving of the engine, then you might have more than an electrical system problem on your hand. This is because the electric components aren’t getting enough ‘juice’ to function properly, and in the case of headlights the changing brightness can be very dangerous especially at night or inclement weather when you need the headlight to operate at full brilliance for clearer visibility.
When you hear buzzing (static) noise from the speaker when playing music (not necessarily from a radio station), then you need to test your car alternator because this is a sign the diodes are in bad shape and are unable to clean up the electrical energy before allowing it into the system.
Other components that depend on electric power might not function well, for instance, the air conditioning unit blowing hot air, the starter might give off a slow cranking sound, or you might hear weird sounds coming from the engine compartment when the engine is running.
These and more are the reasons you need to test the condition of the alternator before moving on to troubleshoot other components. This helps ensure that the alternator is generating enough power to keep up with the electrical loads placed on the system because if it isn’t, or the charging voltage is low you might experience symptoms that make you think the battery is dead or the starter needs replacement.
A word of caution before we begin: Resist the temptation to disconnect the battery terminals when testing a car alternator. Though a healthy car alternator may keep the engine alive, it’s not a good way to test the alternator.
In older vehicle models (ones without onboard computers), you could disconnect the battery from the engine while it is running with little to no damage. Trying this on modern cars will definitely fry some electrical systems.
This is because in the absence of a car battery to act as a buffer (electrical load) the alternator can send as much as 90 volts to the electrical system depending on the engine rpm. This ‘simple’ test might end up costing you in hundreds, if not thousands of dollars when the smoke clears.
Here’s how to perform the test safely with the help of a voltmeter or multimeter.
To determine the state of your car alternator, you need to first measure how much voltage your battery is outputting when there’s no electrical load connected across the terminals.
Get the voltmeter or multimeter and connect it to the battery terminal. The red wire of the voltmeter has to be connected to the positive terminal of the battery, and the black wire connected to the negative terminal. You will not get an accurate reading if the car battery terminals are corroded, so make sure they are free of debris or dirt before you proceed.
The battery should record between 12.5-12.8 volts on the multimeter, if it falls below this, you need to get it fully charged using a battery boost charger. Now, replace the battery in the electrical system.
Now that you have a fully charged battery in the system, it’s time to test the charging system. Switch off all electrical loads (stereo, lights, GPS, etc.), start the engine and get someone to maintain it around the 1500 RPM mark.
Connect the voltmeter again to the battery terminals and record the reading. If you see the voltage increase by about 0.5-1.5 volts, then there’s a good chance that your alternator is ‘okay’ (you’ll need more than a voltmeter to detect shorted or open alternator diodes). So, if your load-free battery voltage was 12.7 volts, then you’ll see a volt reading between 13.2-14.2 volts.
If the battery voltage is higher than 14.5 volts, then the charging system is overcharging the battery and this is not good. Anything lower than the load-free battery reading indicates an issue in the charging system.
After testing the voltage of the battery and charging system without any electrical load, next up is testing the system again under full load.
Increase the engine RPM to 2000 and turn on all electrical loads. This includes the stereo, air conditioning, headlight, wipers, GPS, etc. Now reconnect the voltmeter to the battery terminal and record the voltage reading.
What we’re trying to get here is the system’s charging voltage under full load. If the charging system is working properly, you should get a reading that’s about 0.5-1.0 volts higher than the battery load-free voltage reading. If you get a voltage reading that’s lower than 12.8 volts or higher than 13.0 volts, then your car alternator might be faulty.
If the alternator has passed every other test, your final test is to check the diodes. To do this, you need to switch the multimeter to AC (we’ve been using DC for the previous tests). Choose the lowest possible scale available, then fire up the engine to 1200 RPM.
You should get an AC voltage reading of 0.00 volts if the diodes are working, but if you get anything like 0.01 volts, then your diodes are bad. This is what causes the buzzing noise in the stereo system. It means that the diode isn’t doing what it is supposed to do, which is to convert all AC to DC before supplying power to the electrical system.
This is not the most accurate way to test for a bad diode, but it helps you see where the problem is coming from.
One of the most accurate and practical ways to test the condition of a car alternator is by using a voltmeter or multimeter. If you don’t have one lying around you can evaluate the state of the alternator without a multimeter. Here’s how:
If you listen to the sound generated by your engine while in operation, you might hear a distinct clicking sound. Most times this sound is produced by a faulty alternator. The only problem with this method is that there are lots of moving parts in the engine that might give off such and sound and you need to have incredible hearing.
The engine supplies mechanical power to the alternator via the serpentine belt. Inspect this belt for cracks, tears, and weakness. If the integrity of the belt is not up to par, then it won’t be able to supply maximum power to the alternator.
You’ll need to perform this test at sundown or in a dark garage so you can pick up any little change in brightness. Power off all electrical loads except the headlight and take note of its brightness. Start the engine and notice what happens.
When you first turn on the ignition you’ll notice the headlight dim a bit then return to its original brilliance after a few seconds. If this is the case, then the car alternator is doing its job.
If you’re in doubt about how to test your car alternator, then you should consider getting the service of a qualified technician to inspect it for you. Though it’s easy to test the alternator and electrical system for a fault, there are some problems that will require a professional to fix.
