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Alternators, Starters and PCMs, Oh My!


Diagnosing starting and charging problems has never been easy and, in fact, seems to be getting more difficult every day. As these systems increase in complexity, getting to the root problem of a customer concern takes an increasing amount of time, requires greater knowledge of system design and demands a higher level of creativity and determination. In this month’s article, we have gathered several resources related to resolving charging system problems and no-start complaints.

Larry Carley Takes on Alternators and Starters

In recent issues of Counterman, Larry Carley published two articles related to alternator failures: one that explains why today’s alternators are so “stressed out” and one on how to determine if an alternator or starter is functioning properly or is inoperable. In the first article, he does a good job of identifying how vehicle design has changed in ways that put increasing demand on alternators. In the second, he reminds readers that lack of proper charging current doesn’t necessarily mean the alternator is bad. Taken together, these two articles make for a great refresher on alternator design, system performance and alternator/starter diagnosis:

Alternator1If there’s one product category that causes more headaches and returns than any other, it’s alternators and starters (alternators primarily!). These rotating electrical components are a frequently replaced item on many older vehicles – even some newer ones. Starters are lasting much longer these days, thanks to quick-starting fuel-injected engines, but alternators…
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Charging problems can have many different causes, with the alternator being only one of many possibilities. Yet because the alternator is the heart of the charging system, many people jump to the mistaken conclusion that the alternator must be…
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Digging Deeper into Charging System Diagnosis

In this article from Import Car, Gary Goms breaks down Toyota charging system design, identifying the common failure modes that cause charging errors and dead batteries. He covers a lot of ground, with the result that this article is a great primer on developing a diagnostic strategy for all makes, not just Toyotas. He runs through all the potential culprits: a weak battery, faulty diodes or sticky carbon brushes in the alternator, loose or defective alternator wiring, belt wear, a poor connection at the fuse box and more. Definitely bookmark material:

BA9G actionAccording to my experience, I estimate that replacing the alternator solves 95% of all charging system failures. If that’s true, what happens in the remaining 5% of charging system failures that results in customer comebacks? To explore charging system comebacks in greater detail, let’s consider… Read the rest of the article.

Voltage Drop Testing to the Rescue

One of the common issues identified by Gary Goms in the above article is loose or defective connections between the alternator and battery and between the alternator and voltage regulator. His recommended diagnostic method to pinpoint these issues is voltage drop testing. In this article, one of a series of articles he has written on the topic at, Pete Meier provides a nice rundown on this method, it’s many uses and things to keep in mind when using it:

ma0516-tc1-copyI invite you to search the term “voltage drop” on the Motor Age website and YouTube page for a ton of information that can help you fully understand this valuable testing method. The idea behind the test is simple enough – applied voltage will “drop” across… Read the rest of the article.

Battery and Charging System Impacts on PCM Longevity

One more by Gary Goms. In this article in Underhood Service, he looks at the relationship between the vehicle battery, charging system and PCM. Specifically, he draws a connection between shorted or otherwise damaged batteries and failed PCMs, using three Jeep failures as examples. Here’s a great quote from the article: “All of these PCM failures were likely caused by exposure to high voltage in the electrical system. Keep in mind that a battery acts as a capacitor that absorbs amperage and buffers voltage spikes. Without a battery in circuit, the vehicle’s electrical system is completely exposed to voltage spikes and high charging voltage. While most modern PCMs are protected from high-voltage spikes, none can withstand prolonged exposure to high charging voltages.” Great insight into how the system is so interdependent that, if one element within the system is faulty (battery), it can have knock-on effects on other system components:

pcm-overvoltage-1_jeepDuring the past nine months, I’ve had three instances of powertrain control module (PCM) failures on Jeep Wrangler vehicles that were evidently caused by an over-voltage condition. The first case involved an owner who requested that a local shop replace the fuel pump because his ’98 Wrangler was slow-starting in the morning. The shop found the pump pressure was marginal and replaced it. Read the rest of the article.

Battery Cable Reset to Remedy Software Errors

Lastly, we’ll wrap with a Tech Tip from Import Car. In this article, they highlight the software errors that can result from a low battery or interrupted reprogramming routine. As a remedy, they suggest attempting a battery cable reset to resolve these software errors:

Installing Battery with WrenchMany automotive systems today, including those in Acura models, rely on software. Should that software develop errors, which can happen from something like a low battery or when a learn procedure is interrupted, it can cause all sorts of…. Read the rest of the article.

These articles reinforce some of the main themes of the past four years of our blog when it comes to electrical system diagnosis: understanding the system is the key to efficient repair and diagnosis, the electrical system is interconnected and understanding how each component interacts with the system and each other is critical, and voltage drop testing can be your best friend in sorting out these issues. As systems continue to grow in complexity, these themes will only become more central to effective and efficient diagnosis and repair.

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