How to Test an Alternator With a Multimeter?

This blog post will provide you with instructions on how to test an alternator using a multimeter. If your car battery is not charging, then the first thing you should do is check your battery and starter voltage. This can be done by attaching one lead from the multimeter to each terminal on the battery. 

The next step would be to measure the voltage drop across various parts of your electrical system until it’s found where there is a problem for you to fix it.

When you have to test an alternator on a car, it can be not easy. This is especially true for beginners who are unsure which wires go where and what each wire does. 

Fortunately, this guide will show you how to use a multimeter to test your alternator if you’re unsure of the process.

Signs of a Weak Alternator

Before you test your alternator, here are some signs that it might be failing:

  • Your battery light is illuminated on the dashboard. 
  • The engine cranks slowly or won’t crank at all. 
  • Battery, headlights, and interior lights seem weak. 
  • The alternator makes rumbling or squeaking sounds; You’ve tried to jump-start the car unsuccessfully.

Testing Alternator

You may be wondering why your alternator isn’t charging the battery. If you suspect that it might not be related to issues with an alternator itself, then make sure to check for other problems such as bad wiring connections or a faulty PCM (or power control module). Alternators aren’t infallible, and there are many reasons they can fail!

If you have a voltmeter, it can be used to check for faults in the connections of your engine when it is running. 

To perform this test correctly and get accurate results from the readings on your meter, make sure that there are no resistances in any connection by touching both positive and negative leads across each one while testing voltage drop with an electrical probe. Ideally, if everything checks out, the reading should stay at zero or less than 0.1 volts (ideally). 

If not careful enough, though, during these tests and throughout other maintenance procedures like hooking up fuel lines- leaks may occur, causing gas fumes directly into areas where they could ignite first thing leading to explosions without warning!

Poor ground connections can result in low charging output, which will lead to your alternator failing prematurely. Make sure that you are looking for voltage drops at the positive and negative battery cable connections and BAT+ power connection of the new alternator. 

Voltage drop on a negative side may cause overcharging, while voltage drop on a positive side may cause undercharging, so be mindful of these factors when replacing an old part with a newer one.

If the old alternator passes the tests, you’ve missed something. It’s time to get out your voltmeter and check for voltage drop in the battery and charging circuit. 

When a new alternator is installed, make sure that before returning it to your customer (and they drive off), plug-in their car using a battery charger so that all of its electrical systems are fully charged up again first – including both engine computers as well those onboard diagnostics which could be giving false readings if not correctly reset after an alternator replacement.

Testing Alternator Using Multimeter

With the engine on:

  • Step 1: If you open your hood, make sure you have clear access to the car battery. The alternator belt may be loose; locate it and check if there is any slack or fraying in the belts.
  • Step 2: You will need to turn your multimeter to the 20 V setting.
  • Step 3: Start your engine. The alternator should be spinning smoothly without slipping at this point. If it is, you are good to go!
  • Step 4:  There are two ways to test the voltage coming from your alternator. One way is by placing the positive multimeter probe (red), touching it to a red terminal connector on top of an alternator, and then taking the negative, black, multimeter probe and grounding it somewhere else in close proximity like near a bolt head or even with another clump of metal nearby that isn’t part of your car’s frame. The other option is not as risky, but you need easy access without getting tangled up in spinning pulleys; place both probes against each side at once because this will give you twice as much information – which might be better if there was some discrepancy between them previously unknown during one reading session. You should now be getting a reading on your multimeter display. Suppose you don’t have good access to your alternator. In that case, you can test the alternator by touching the positive multimeter probe to the positive terminal on your battery and likewise the negative probe to the negative terminal on your battery.

When using a multimeter to test an alternator, the output tone of voice should be Helpful. This is because once you have established whether your alternator is working well or not by reading any voltage at all (14 volts) and if it’s over 15 volts, then that means there might be something wrong with the regulator on top. At the same time, anything under 13 will require some more testing for other possibilities as to why this may happen, such as engine idle speed being too low.

If the voltage is still too low, check to ensure all of your connectors are tight and that the alternator belt doesn’t slip. The problem can also lie in a faulty regulator or an underperforming alternator itself – it’s time for replacements!

The battery should read 12 volts or higher to charge. Suppose the alternator is putting out too much voltage. In that case, your acid will likely boil over and damage the car’s wiring or irreparably harm both engines of a diesel engine vehicle that uses two batteries. 

When you are charging an electrical system with more than one battery (most vehicles have 2), it can help if they each receive fresh juice at different times from opposite directions so as not to overwhelm either bank; this way, they’ll be equally charged when their time comes up again for use.

It is helpful to test at the battery terminals with the engine off and then turn on your car. If you’re fully charged, it should read about 12-13 volts when your vehicle has been turned off for a long time. But if the voltage drops below 10 or 9 in cold weather (depending), the chances are high that there’s maintenance needed or replacement needs to happen soon!

If your battery isn’t charging correctly, it may be time to enlist the help of a professional mechanic. The alternator is one part that could cause this issue, and if you have any other warning lights or notice issues with electrical systems in your car, bringing them to a pro can make all the difference.

About Blake Sutton

Blake has worked as an electrician for over 10 years, receiving his Journeyman Electrician license in 1998. Looking to take his professional electrical career further, in 2008 he received his Bachelor of Science in Electrical Engineering (BSEE) from the University of Texas in Austin. Blake now works full time as an electrical engineer, specializing in power systems.