How to Test a Car Alternator

Daven Hiskey 15
Pony for a mechanicToday I found out how to test a car alternator.

Equipment needed: multi-meter

With the engine on:

  • Step 1: Open your hood and so you have clear access to the car battery.  Locate your alternator and check to be sure the alternator belt isn’t loose.
  • Step 2: Turn your multi-meter to the 20 V setting.
  • Step 3: Start the engine.   At this point, check to make sure the alternator pulley / belt is spinning properly without slipping.
  • Step 4:  There are two ways to test the voltage coming from the alternator.  If you have easy access to the alternator without getting your body parts / clothing / etc tangled in the spinning pulley’s, then place the positive multi-meter probe (red) and touch it to the red terminal connector coming out of the alternator.  Now take the negative (black) multi-meter probe and touch it to some metal part of your car frame (like a bolt head nearby or even the negative terminal on the car battery).  Don’t ground to the alternator itself.  That would be potentially bad.  You should now be getting a reading on your multi-meter display.  If you don’t have good access to your alternator, then you can simple test the alternator by touching the positive multi-meter probe to the positive terminal on your battery and likewise the negative probe to the negative terminal on your battery.

Testing an AlternatorIf the alternator is working well, your multi-meter should read somewhere in the vicinity of 14 volts (typically 13.8-14.2).  If it is reading excessively higher than 14 volts (greater than 15 volts) it is possible that the voltage regulator on your alternator is faulty or going bad.  If it is reading lower than 13-14 volts, there are a number of possibilities as to the reason.  First, it is possible that your engine idle speed is too low for the alternator to put out sufficient voltage/power.   Try revving up the engine to 2000 RPM or higher and take a reading.  If the voltage is still too low, check to be sure all the connectors on your alternator are tight and that the alternator belt is not slipping and is spinning on the pulley correctly.  If it is still not putting out sufficient power, then the alternator’s voltage regulator could be bad or the alternator itself may need replaced.

A car alternator needs to be putting out at least 13-14 volts (ideally between 13.8 and 14.2 volts) to effectively charge a 12 volt car battery.  If the alternator is putting out too much voltage (15+ volts), it is likely your battery acid will boil over out of the battery.    When a battery is near fully charged the alternator will cease to put out sufficient voltage to charge the battery and the reading you will get from the battery probing method will simply be the voltage coming from the battery itself.  In this case, you can simply leave your lights on with the car off for ten or fifteen minutes to drain your battery a little bit.

It can also be helpful to test at the battery terminals with the engine off and then turn the car on and test again at the terminals.  If your battery is more or less fully charged, it should read at around 12-13 volts with the car off.  If you’ve run your car for a long time and while the car is running the voltage reading is in the 13-14 volt range, but then you shut the car off and the battery voltage instantly drops to well below 12 volts (10 volts or under; 9 volts or under in freezing weather conditions), it is likely that your battery needs maintenance or replaced.

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15 Comments »

  1. Corey Davis January 18, 2010 at 4:26 am - Reply

    you can also take a magnetized screw driver and touch the side of the alternator (with the car off obviously) and if it sticks its good, if it doesn’t its bad. my grandpa was a mechanic for years and he taught me that one

  2. Logan January 29, 2012 at 1:36 am - Reply

    @Corey Davis: Sorry, but with a magnetic screw driver, on any metal part will stick. That test is to be done as follows: With a non magnetic tip screwdriver, or any piece of metal, with the car running, touch the back of the alternator, and there should be magnetism.

    How this works: The alternator has an electromagnet in it, which is what “creates” the energy needed to charge the battery, therefore when the engine is running, your alternator is one big magnet and any piece of ferromagnetic metal should work (steel)

  3. ADAM February 20, 2012 at 10:46 am - Reply

    START THE VEHICLE, WITH THE MOTOR RUNNING PULL THE POSITIVE BATTERY CABLE OFF OF THE BATTERY. IF THE ENGINE STALLS YOU ARE RUNNING ON BATTERY POWER ONLY AND THE ALT. IS NOT CHARGING.

  4. Pat Stanford May 9, 2012 at 2:44 pm - Reply

    Years ago, I pulled the positive cable from a battery to “test” the alternator…It “spiked” and literally burned the headlights out, melted a wire from the alternator in two, and destroyed the radio…

  5. Justin September 2, 2012 at 4:09 pm - Reply

    Daven needs more electrical engineering classes because putting the other lead on the alternator housing will not ground it out. The housing is grounded to the engine which is grounded to the frame, therefor no differant than touching the lead to the grounded frame. Also potential energy does not get passed through one lead of the meter to the other. If that was the case you would get shocked if you stuck one lead in the power outlet in your home and touched the other lead. It don’t happen. Other than that good write up!!!

  6. looncraz September 12, 2012 at 7:45 am - Reply

    I think the point about not using the alternator for a ground was more directed at making sure no one chooses an unwise place since most alternators have exposed coils and such (like mine).

  7. mat May 28, 2013 at 11:41 pm - Reply

    My battery is reading high 11s but car will not start, replaced alternator six months ago can somebody help with goof advice.turned head lights on and voltage dropped.

  8. mike July 18, 2013 at 11:00 am - Reply

    mat i’m just a backyard mechanic but it sounds like a bad ground some where. check them all

  9. ian July 21, 2013 at 1:17 am - Reply

    i have a series 3 landrover that is not charging battery. have tried 2 brand new alternators,still no charge,have replaced every wire 1 by 1 to see if any are faulty and still no charge,,, any ideas ???

  10. Bob Mars September 15, 2013 at 2:10 pm - Reply

    First off, it is likely that most people reading this son know if their meter should be set on AC or DC to test their altenator and/or battery (DC). Second, when you try to read the voltage of a battery to see if it is good or bad, you need to read the voltage when the battery is under load, or you won’t get a true reading. This is probably the problem Mat is having. Finally, an altenator can work, but not put out the voltage it should, or needs to, so the “magnetic test” is not at all reliable if you want to know if you have an altenator that is going bad. Thank you.

  11. franklin October 18, 2013 at 10:28 am - Reply

    I replaced my 2002 opel astra 1.6 cd classic’s battery in june and as from last month have to jumpstar it everytime i need the car but when turning it of it dont want to start,Can it be the alt not chargin or someth else?Need advise urgently.

  12. pete October 27, 2013 at 11:00 am - Reply

    franklin- I replaced battery in my Jeep. Same starting problems as yours. See if there is good contact between the battery cables and the battery posts. Even if tightened all the way down, you might be able to still pull off one, or both cables from the post(s). If that is the case, pick up some battery post shims (they are round) from the auto supply store. They slip over the battery posts. Open clamps up so they have a gap and then re-install. A package of two costs about $2.50. Good place to start with your trouble shooting. Unfortunately, I am still working on the rest of the real problem (that’s why I am checking out this site), but the loose terminals are no longer part of it for sure!

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