Charging System Diagnostic Resources

Photo by Jason Young

This month, we return to alternator/charging system diagnosis, a topic that seems to get more difficult with each successive model year. We have addressed this topic in several articles over the last few years, including this guest article from our friend and diagnostic expert Albin Moore. It provides a great starting point for this topic.

Since it has been a few years since that article, we thought we’d refresh this topic with some new resources published in the last six months or so. The different resources referenced below cover much of the same ground in terms of diagnostic strategy and things to keep in mind when venturing into this repair area (essentially, “things are getting more complicated – see our first resource below”), but we thought it would be useful to pull together a wide array of voices and approaches, assuming that each reader might connect to one author’s approach more than the others or that the specific vehicles or examples of some articles would be more relevant than others for each reader. Without further ado, let’s jump in.

Things are Getting More Complicated

Our first referenced resource,
an article by Andrew Markl on, gives us a good overview and introduction to this topic, providing examples of why testing charging system performance today is much more complicated than it was twenty years ago. He offers a key piece of advice that we hear echoed throughout the literature on this topic: “Take time to look at the service information so that you don’t overlook critical problems and to prevent a misdiagnosis.”

He also introduces the concept that alternator output is now typically controlled by a module, such as the PCM, which means that it delivers charging on demand, resulting in the fact that alternator output of 13.0V could be perfectly normal for a given situation on a given vehicle, while output of 16.0V could also be normal for that same vehicle. In other words, the old guideline of “if output is between X and Y, it is OK” is over. It is a great starting point on this topic. 

Why Volts Are Smart, But Amps Are Smarter

Our next resource, an article by Gary Goms on, delves in great detail into the topic of smart charging systems, citing a diagnostic misstep he made on the first smart charging system he ever serviced. He goes on to discuss the difference between voltage based and amperage based charging systems. After quickly covering the operating theory behind voltage based systems, he covers amperage based systems in detail, which is the meat of this article.

He identifies the basic operating assumption of amp based systems: “In short, the diagnostics of an amp-based charging system depend very much upon the PCM seeing voltage potentials and amperage flows at the battery that correspond to current operating conditions. If volts and amps don’t meet specified parameters, the PCM will turn on the red ‘BAT’ warning light, regardless of what the battery voltage might be.” It is a great article that provides plenty of theory with a solid dose of specific details, examples and suggested methodologies.

Simplify and Isolate

Our next resource, by Dan Marinucci on, takes a little different approach with a guiding principle of attempting to simplify what we have already established can be a complicated diagnostic area. As he notes, if you have a charging problem, it could be caused by any number of system components, including the battery, the alternator itself, the PCM (isolate module) or a wiring problem.

For him, a key step is to isolate the alternator and verify its proper output. He addresses potential objections up front. “Yes, you must access the alternator and connect something to it in order to isolate it. But you probably were heading toward that suspect alternator anyway. And once you make the proper hookup, this approach completely separates the alternator from the computer and its wiring. Now if that alternator still doesn’t charge normally, you’re confident that you must replace it and retest.”

He uses GM and Chrysler alternator configurations as his example systems and identifies his isolation methodologies and the test procedure he uses to verify proper alternator operation. He also adds that caution is required when pursuing this strategy. “Perhaps the most pertinent precaution for this discussion is a reminder that bypassing the ECM full-fields the alternator. That means the alternator operates uncontrolled on a vehicle laden with electronics, so exercise appropriate caution.”

This is an article with a lot of detail that provides a different way of approaching charging system diagnosis and one worth a review. 

Quick Approach to Testing Output

We’ll wrap with a more generalist article on electrical troubleshooting by Edwin Hazzard published last week on One entire section of the article is centered on charging system diagnosis. He reviews how a technician, armed with a DVOM, amp clamp and the specs for the vehicle being serviced can perform basic diagnosis and testing to confirm charging system performance. It is likely that, if this basic testing were to cause the technician to suspect a charging problem, the tactics deployed in the above articles would probably be used to pinpoint the source of the problem, but this article is a good complement to the other resources referenced and also another good starting point for those not as familiar with diagnosing these systems. 

Perhaps more than you ever wanted to see about alternators in one sitting, but a lot of good guidance in the above articles. Armed with these strategies, the necessary vehicle data, the relevant schematics and a few tools, technicians can make sense of even the most challenging charging system diagnosis. Have you ever faced a particularly difficult charging system error? How did you overcome it?














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