The problem with alternators isn’t that they fail, but that you don’t know when they fail, and you really only find out when your car won’t start.
By using these tips on how to test an alternator, you can catch a bad battery earlier on and replace it without being concerned about getting stranded on a roadside or dealing with excessive battery replacement costs.
Here are five ways to test an alternator for a bad battery so you can avoid an inconvenient situation and save money in the process.
5 Ways To Test An Alternator
1. Check The Current Draw
The first way to test an alternator is to check the current draw.
You do this by using a multimeter and checking the voltage at the output terminal of the alternator against its voltage rating.
If there’s a discrepancy in the current draw, it could be because of a bad battery or other issues with your car’s electrical system.
2. Check The Voltage Output
To check the output of your alternator, use a voltmeter, then place the black lead on the negative terminal of your battery and place the red lead on the positive terminal.
If you have an alternator that is charging correctly, your voltage should be at least 13 volts with the engine running and less than 12 volts when it’s off.
This can on time be checked by using a multimeter or voltmeter in conjunction with the black and red leads.
If there is no form of energy flowing from the terminals when tested, then you will need to replace your battery or alternator.
3. Check The Resistance
A quick way to test an alternator is by checking the resistance. You will need a multimeter, which can be found at any auto store for under $20, and two alligator clips.
Connect one alligator clip to one side of the positive battery terminal and another alligator clip to the other side of the negative battery terminal.
The multimeter should read anywhere from 0-1 ohms if it’s working properly, but if it reads anything else, there may be something wrong with your alternator.
4. Check The Continuity
To check the continuity, you’ll need a set of jumper cables and a multimeter. To begin with, confirm that the battery is fully charged and that the car is turned off.
Place one end of the jumper cable on the positive terminal of the dead battery and place one end on the positive terminal of a working battery.
Next, connect your other ends to the two corresponding terminals on your multimeter.
5. Check The diodes
Check the diodes. You will need to replace them if they are burnt out. The battery is still under warranty, so you may want to contact the manufacturer and see if they can replace it for free.
If the batteries are less than a year old, take them back to where you bought them and ask for a replacement or refund.
Lastly, try replacing your alternator with another from an auto parts store in your area. However, the best way to test an alternator is by disconnecting the power wire from your car’s battery terminals and testing with a voltage tester or voltmeter.
Also read: 15 Signs Of A Bad Alternator
Effects Of a Non-Tested Alternator
When your alternator is not working correctly, it can significantly affect your vehicle. The battery is the power supply for all of the electrical components in your car.
Without a fully-functioning alternator, the battery will die very quickly and prevent you from starting your vehicle, which means that you’ll need to get a new one pretty soon. A list of some of the negative effects of an untested alternator is provided below:
- You’ll notice that your headlights are no longer as bright.
- Your air conditioning and heater will not work as well.
- Your power windows and locks will malfunction.
- Your battery could die completely.
- The engine may suddenly shut off, preventing you from driving your vehicle to a repair shop for help.
- An expert technician should check the alternator periodically at the first sign of trouble.
- Other parts of the system will likely fail when an alternator fails because they’re relying on power from the same source.
- Replacing an alternator might be necessary if there’s still any doubt about its condition.
How to Test an Alternator Without a Voltmeter
If you’re not sure whether the alternator is working, there are a few ways to test it. One way is to have a friend start the car and rev the engine to 2000 RPM while you watch the battery voltage.
If your voltage dips below 13.6 volts when they rev the engine, then your alternator isn’t doing its job.
Another quick way to check is with a clamp-on amp meter; if the alternator is putting out amps (around 10 amps), it’s likely that the battery needs to be charged.
And of course, one of the most basic tests is that the lights stay on after turning off the engine. You can also jump-start your vehicle or use jumper cables to test this theory.
How to Check Alternator Output
There are five ways to check the output of an alternator. The first is to simply turn on the car and see if the battery charges up. If it does, then you know that your alternator is working properly.
Another way would be to turn on the headlights or any other accessory that draws power from the battery and plug in a voltmeter or multimeter. If the voltage reading is higher than 13.0 volts, then you know that your alternator is working correctly.
How to Check if The Alternator is Bad or Battery
- Check the battery voltage with a voltmeter. If the voltage is low, skip step 3.
- Disconnect the battery and test the alternator by grounding one of its terminals to bare metal on the engine block with a jumper wire. Continue this process for each terminal and note if there is any change in voltage.
- Connect the discharged battery to the alternator and repeat steps 2-3 until you identify which terminal(s) are giving low readings. These are connected to the bad component.
- Reconnect the bad component to find out which wire it connects to (positive or negative).
- Disconnect the wiring at that connection and reconnect it using a new piece of wire to complete the circuit.
- Test again to see if it fixed your problem, then go back through steps 1-5 until you have found and fixed all your problems.
How to Test Alternator With Screwdriver
Start the car and rev the engine to 2000 RPM. If the alternator is working correctly, the battery light will go out.
The other way to test is to use a screwdriver and touch it to the two wires on top of the alternator, and if it sparks, then there is a problem with the battery and not the alternator.
The third way to check for a bad alternator is to take off the belts from the pulleys so that they are exposed, and you can see them turn as you rev the engine up.
If you see any change in what they’re doing, then you need an alternator. Because if they dim when you rev up the engine, then it’s time for an alternator.
FAQ
Can I test my alternator without removing it?
Yes, you can test your alternator without removing it. Once that is done, check the battery terminal voltage. Alternators are designed to be tested with a voltmeter while they’re still in place on the vehicle.
To do this, disconnect the battery’s positive cable and put an ammeter between it and the ground, and then use the voltmeter to measure the voltage between the alternator’s terminal wires.
You should read around 14 volts if everything is working properly.
How do you check an alternator with a screwdriver?
You can make use of a screwdriver to check whether the alternator produces a magnetic field. This can be done by moving the screwdriver in circles around the alternator’s brushes. If there is a magnetic field, it should repel the screwdriver and produce movement.
How powerful the battery is and how much voltage it has affects the magnetic field’s strength.
The same procedure can be used for the drive belt with an oscilloscope or voltmeter at different points on its surface. However, this could cause damage if not executed properly.
Which battery cable do I disconnect to test the alternator?
If you are not sure which battery cable to disconnect, the negative battery terminal is most likely the one to disconnect.
Disconnect the negative terminal and then wait 20-30 minutes before reconnecting it.
If your car starts and stays running without sputtering or stalling, then you know that your alternator is bad. You will need to replace your alternator with a new one in order for it to be repaired.
If your car does not start when you disconnect the negative battery terminal, then there is an issue with the cables or terminals on the car itself.
If your battery has been draining even after the replacement of a faulty alternator, then you have a problem with some other device draining power from your battery.
Can you ruin an alternator by jumping a car?
Reversing the polarity of the clamp connections can cause instant damage to the alternator, and they will still need a new battery.
In order to jump-start a car with an alternator issue, connect the battery clamps in reverse on both vehicles.
First, connect one end of your jumper cables to the positive terminal on your car’s battery. Next, connect one end of your jumper cables to the positive terminal on the other vehicle’s battery.
Next, take the second cable from the first set of jumper cables and attach it to the negative terminal on your car’s battery.
Lastly, attach one end of the second set of jumper cables to a metal surface like a bolt or screw at least 12 inches away from any part of your engine, then connect the last cable from this set to a metal surface like a bolt or screw at least 12 inches away from any part of the car with an alternator issue.
Does the negative battery cable connect to the alternator?
Nope, it only connects to the battery terminal. Connecting the negative battery terminal to the alternator can lead to irreparable damage to the alternator and render the car undrivable.
Alternators need only to be connected to a large ground, like the engine block or transmission case on a manual-transmission car.
There is no need to disconnect a battery cable; the red wire will always connect to a battery terminal unless there is an issue with the charging system that necessitates testing while the engine is running.
You may want to detach one of the positive wires from its appropriate clamp and place an ammeter in between these connections so you can see if your alternator’s output voltage exceeds your battery’s voltage under load.
Can you spray wd40 on the alternator?
Wd-40 is a good cleaner and can help dissolve some of the gunk on your alternator. However, if you spray wd-40 on your alternator, then it will not be able to test for a bad battery. This is because spraying wd-40 will cover up the voltage reading from your battery.
If you still want to give it a try, be sure to use only a small amount of wd-40 and do not spray it directly onto the connections that connect to your battery.
Does negative or positive go on first?
The first step to testing an alternator for a bad battery is to make sure that the battery cables are correctly connected. You’ll need to disconnect the negative (-) cable first and then the positive (+) cable.
Next, turn on the ignition and look for any sparking coming from the alternator. If you see any sparks, then you know that your battery may be bad and needs replacing.
Also Read: How to Solve Car Steering Wheel Vibration
Conclusion
Testing the alternator is a good way to see if you have a bad battery or not. There are numerous methods for testing an alternator, and some are more complicated than others, but with the above list of five easy ways, it should be no problem to find out if your battery is bad and get on the road again quickly.