Diagnostic Corner: How to Identify a Failing Alternator


You’ve just left the office.  It’s a cool evening and you’re strolling across the parking lot, ready to hop in your car and head home.  The parking lot is nearly empty, as everyone has headed home for the weekend.  You open the door, sit down in the driver’s seat, turn the key, and….





Is your battery dead?  If so, is it due to age or has your alternator failed?  Is there a loose battery cable?  Maybe just a bad relay? 



Any of these issues can be the culprit, but when you know how to diagnose an automotive issue, you can save plenty of money and time.  Let’s look at how you can tell that the ALTERNATOR is to blame…


What Does an Alternator Mean to your Vehicle?


The vehicle’s alternator essentially charges the battery while the vehicle is in operation.  The average car battery can turn the engine over a couple of times or get you down the road a bit, but the moment the energy is drained, you’ll see your lights dim.  Once this happens, you can expect to be on the side of the road in moments.


Diagnostic Corner How to Identify a Failing Alternator


When the alternator is functioning properly, your battery will be replenished consistently, ensuring that you always have enough power for the next “start”.


How to Identify a Failing Alternator


As we mentioned above, anytime you are driving and begin seeing the dashboard lights “dim”, you can usually assume that you have a faulty alternator.  But, what about when you try to start the vehicle and get absolutely nothing from turning the key?   There are a few tricks that you can try to test your alternator/battery combination to see which component is really at fault.  However, you will need to get a jumpstart to at least have a running vehicle during these tests:


(1)  Use the Gauge


These days, alternator gauges are becoming quite common, as our vehicles become increasingly “electric”.  If you have such a gauge, put a bit of strain on the electrical system by turning on the headlights, radio, or air conditioner.  If you see the volt/amp output DROP, then something is amiss, and your alternator isn’t getting the job done.


(2)  Listen


Far too often, we get caught up in the technical aspects of diagnostics.  In fact, I have a good friend who is often referred to as the “Car Whisperer” in social circles.  Why?  Because he listens to an engine run, then diagnoses the issue with incredible accuracy.  For our purposes, here, turn on the vehicle, and listen to the alternator.  If you hear odd sounds (such as a squeal), then you could have a damaged bearing.  And, a damaged bearing means it’s time for a new alternator!


(3)  Rev It Up!


When you have the vehicle running, turn on the headlights.   Now, as you rev the engine, does anything change?  If the brightness of your headlights remains constant, then you’re getting a solid charge.  However, if they dim or flicker, you aren’t getting the necessary charge from the alternator, and it’s time for a replacement.


Repair Prognosis


Diagnosis is one of the most difficult aspects of auto repair.  Because finding the necessary information to facilitate a part’s replacement is simple, many get hung up on whether or not they are actually replacing the culprit of the issue, and end up taking their vehicle to a professional.  While we would never say that this is a poor decision, as we’re mechanics ourselves, it can also be difficult to afford such a repair these days.


Alternators are quite simple to fix, provided that you have a few tools and about an hour of free time.  In most cases, the repair involves loosening the belt pulley and removing the belt, disconnecting a couple of wiring harnesses, and removing a few bolts.  From here, the repair is a simply reversal of the removal process, and you’ll have a new alternator before you know it!


1 comment

  1. I really cannot imagine being stuck in that scenario mentioned at the start of the blog. It is necessary for everyone to check the failing alternator. Also, I do agree with your blog that the alternator is very essential for the vehicle. An alternator charges the battery and keeps the vehicle running on the road. You have shared some really great tips for checking it. Thanks for sharing this information through your blog.

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