Close this search box.

Another Look at Electrical System Diagnosis


Electrical system diagnostics is a subject that is virtually limitless in terms of the number of topics contained within it as well as the many nuances involved in each of those topics. This complexity is the reason why this subject is at the center of the content offerings of key industry trade publications, online webinars and in person education events related to vehicle service and repair. So, consistent with our goal of keeping you up to date on the latest resources around this tricky area of vehicle service, this month we’re pulling together some recently published resources to keep you up to speed.

The Trainer – Getting to the Root of Alternator and Battery Failures

At, Brandon Steckler has taken over The Trainer series from Pete Meier and he’s off to a great start. In this recent episode, Brandon addresses methodologies that technicians can deploy to get to the root cause of electrical system failures, specifically alternator and battery failures. He starts out with a really good explanation of the relationship between the alternator and battery, identifying how that relationship can change as the battery ages. He does a great job of explaining sulfation and its consequences for battery and system health.

He then jumps into his diagnostic process for assessing system health using a scope. He shows how he sets up his scope, what channels he is using (spoiler: just 2) and where he is placing his probes. Then, he shows a start event on a vehicle with a known good battery and alternator as his control (what it should look like). He goes on to address what it would look like if there were a fault in the system, both in terms of how the energy would distribute through the system and the consequences of such a fault on alternator health and longevity.

Like Pete before him, Brandon sets his focus on the entire system, stresses the importance of getting to the root cause of the problem vs simply resolving symptoms and highlights the value of voltage drop testing as a critical diagnostic tool. It is a great primer on this area of electrical system diagnosis.

Checklist for Dead Battery Diagnosis

This next resource is a great complement to Brandon Steckler’s video above. Where Brandon placed his focus on the specific relationship between the alternator and the battery, this article by Andrew Markel on provides general rules of thumb to keep in mind when troubleshooting a dead battery.

We won’t run through all the tips here, as it wouldn’t do them justice, but we would like to highlight a few of them. The first one is his Tip #2 – Look it Up. The reality of modern electrical system design is that the alternator is usually being controlled by a module, typically the ECM. This means that output is being precisely managed based on many factors (battery SoC, electrical demand in the vehicle, temperature, etc.), the result of which is that it is dynamic. This makes it very important to review service information and really understand how the system works. This way, you can avoid condemning a perfectly good alternator. The second one is Tip #6 – Temperature Matters. Most modern systems compensate for battery temperatures, as they should. As Markel notes, “If you run into a vehicle that is undercharging the battery, look for temperature-related data PIDs using your scan tool. When these sensors fail, they typically produce data that is way too hot or cold for the conditions.” Great advice.

System Overview – Things to Consider

As we continue to move out a wider view of system diagnosis, this is a great article by Russ Colket on In it, he runs through the various components of the electrical system and highlights things to keep in mind as well as his typical diagnostic routine related to each.

As you’d suspect, he starts with the battery, reviewing the testing methods available, starting with an electronic battery tester. While he makes a distinction between invasive load testing and electronic (conductance) testing, we would like to point out a difference between our SOLAR BA Series Digital Battery and System testers vs. his description of electronic/digital testers. He states, “Today’s battery testers… are conductance-type testers and do not load test the battery, instead, they test the internal resistance of the battery.SOLAR BA Series testers combine electronic “conductance” testing methodologies with micro-load technology, providing the best of both worlds. Adding micro-load technology to our testing process stabilizes the battery for a more accurate testing environment, better identifies lurking battery deficiencies and yields more decisive testing results with fewer “charge and retest results,” all of which are very helpful.

From the battery, he moves on to address the alternator, identifying potential failure modes and reminding the reader to make sure they fully understand the operational methodology of the alternator being diagnosed/serviced (common theme here). He cautions the reader to perform a proper diagnosis of the entire system before condemning any alternator. He then breaks down voltage regulator operation and dives into a case study involving a Ford F-150 with intermittent stalling concerns. Overall, the article provides a comprehensive overview of the electrical system and is a great building block for those wishing to improve their knowledge of this service area.

Checklist for No Alternator Output Diagnosis

This is almost a sister article to the Dead Battery Checklist article above. It even shares a few checkpoints with the other article. That said, this one, again by Andrew Markel on, is much more extensive and likely deserves a bookmark of its own, based on its different focus. Here, Markel runs down things to looks for when you encounter a no output or incorrect output alternator. Like the above, we won’t rehash the entire list here, but will highlight a few noteworthy checkpoints.

One key point is to “Put The Car To Bed Before Testing For Parasitic Draw.” This one is critical. Many modern vehicles can have significant current draws (>250mA) up to 15 minutes after turning them off and removing the key. When monitoring for parasitic draw or even when replacing a battery using a jump starter as a memory saver, it is critical to allow all modules to go to sleep before proceeding with your diagnostic or repair routine.

Another great point is to check for “Alternator Communication Lost.” We did a post a while back that focused on this very issue called “Is it a Communication Issue, Electrical Issue or Both?” The complexity of modern vehicle command and control architecture is such that a communication problem can make a component look like it is faulty. As Markel points out, “If communications were lost between the PCM and alternator, the PCM would turn on the battery light, but the regulator would still charge the alternator at about 13.6 volts.” Stopping the diagnosis at just a battery light would be wasteful, because the battery isn’t the problem and in most cases is performing properly. Deploying your scan tool and having a proper understanding of system function can help you avoid such pitfalls.

As always, we hope that these resources are helpful for you and that they identified ways to help you save time or save a headache when it comes to future electrical system diagnosis. In the past 12 years, we have published 20-25 similar articles providing resources related to electrical system diagnosis. These systems are always evolving and seem only to trend toward more and more complex operation, requiring today’s technician to invest time and effort to keep up using resources such as those above.

Do you have a few key diagnostic steps that weren’t covered in the above resources? Have you ever had a difficult to diagnose vehicle that, after much time and effort, you ultimately were able to get back on track? We (and our readership) would love to hear about it in the comments below.

Leave a Reply

Your email address will not be published. Required fields are marked *

Clore Story


Recharge alerts

Related News