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Electrical System Diagnostics: Start with the Building Blocks of the System


Modern starting and charging systems are complex and can be very difficult to diagnose, especially when the diagnosis involves chasing down an intermittent problem. Often though, one of the best ways to manage complex challenges is to start with the basics to ensure that the source of the problem isn’t something related to a basic system building block. In the case of the vehicle’s electrical system, the basic building blocks are the battery, battery cables, starter and alternator. With this month’s article, we present a series of resource pieces that address those building blocks and how issues with each could disrupt vehicle function.


As we have pointed out previously, batteries have changed more in the last twelve years than the previous 40. It really helps to stay updated on the latest developments and revisit the key service considerations of each battery you are likely to encounter in your service operation. This article, by Michael Smyth on is a great primer on today’s batteries and the key things to remember when working with and on them. He focuses specifically on AGM and Lithium batteries, how their construction is different from traditional designs, where they are used and how they should be serviced.

  We especially like his list of AGM best practices:

– Assume that every battery you service/replace is an AGM

– Use smart chargers whenever servicing an AGM battery low on a charge

– Never downgrade the vehicle to a flooded battery if the OEM equipped it with an AGM

– Always wear the personal protective equipment (PPE) when working on or around batteries

Pro-Logix PL2320 20A Battery Charger

We would add that you should always use an AGM-optimized charger when servicing AGM batteries (whether they have low charge or not), since proper charging of AGM batteries requires precise control of output voltage, without which there is risk of damage to the internal plate structure. Similarly, when servicing LiFePO4 Lithium vehicle batteries, it is critical that your charging equipment has a dedicated routine for that battery type, since lithium batteries are even more sensitive to voltage than AGM batteries.


This article is a great primer on the two battery types that are experiencing the greatest growth in terms of deployment as vehicle starting batteries in today’s carpark and well worth a read.


Another key building block of the electrical system is the cable array linking the various components. As noted in this video from, cables are an often overlooked component, but they can have an outsized impact on vehicle performance. As Andrew Markel from Babcox notes, compromised connections and poor cable integrity, both of which can result in excessive voltage drop, can cause numerous driveability issues that could cause a technician major headaches and potentially lead to the replacement of unnecessary parts. This quick video provides a great overview of the issue.

Battery and starter - Like Two sides of a coin

In this article, written from the point of view of the parts store and appearing on, Brendan Baker does a great job of explaining the relationship between the starter and the battery.
As he states, 

“The battery and starter are inextricably linked, and sometimes what seems like a problem with one is really an issue with the other.”

He then goes into a suggested diagnostic routine to determine whether the root cause of an electrical system problem stems from a bad battery, a bad starter or is from another cause. It is a good basic review and a worthwhile read.

Parasitic Drain

This is a topic we have covered many times on our blog. Judging by how often it is addressed in the trade publications, parasitic drains (or draws) continue to be an area where there’s a knowledge gap among many technicians and shops. This is understandable, since it is such a complex subject and varies so drastically from one vehicle model to another.

In this detailed article from, Jeff Taylor takes a deep dive into the topic, providing a suggested diagnostic path to follow to find and correct these elusive issues. He first defines parasitic draw and covers the basics that should be addressed prior to digging in, including a thorough customer debrief on how the issue has materialized, confirming that the proper battery is installed in the vehicle, testing the battery for health and capacity and identifying any aftermarket accessories installed on the vehicle.

He then discusses what is an acceptable level of parasitic draw, providing specific examples by make to emphasize how it can vary from one to the next. From there, he walks the reader through his diagnostic strategy or path. It is a very thorough and detailed process that combines the use of scan tools, road testing, voltage drop testing, fuse removal and more. As he states, 

“Typically, identifying a parasitic draw takes time, a detailed approach, and patience from the tech and the customer…. There isn’t a magic bullet or special tool that will quickly and easily point a technician at the source of a parasitic draw.”

If you commonly run into vehicles requiring excessive parasitic drain diagnostics, it is a great read.

Alternator Ripple

There are dozens of great technical articles related to alternator diagnosis and repair. Like the starter, it can be very tricky to detangle an electrical problem related to the alternator and distinguishing it from the battery. But one obvious area where the alternator can wreak havoc on the electrical system and cause a wide range of communications, driveability and other problems is when the alternator is outputting excessive ripple.

This article, by Brian Sexton on, provides a quick overview of alternator ripple: what it is, where it originates, how much is too much and how to quickly and easily spot it when excessive ripple is present. He explains why vehicle network communications are so sensitive to excessive ripple: 

“Today’s vehicles are heavily dependent upon computers and networks to perform even basic functions such as rolling the windows down or unlocking the doors. These modules operate on a lower voltage signal, typically ~5V DC. These low voltage systems are much more vulnerable to excessive ripple voltage produced by a defective alternator or rectifier.”

It is a quick, concise look at this issue and a great reference article to read now and bookmark for future use.

We hope that these resources are useful to you in the ongoing battle known as vehicle electrical system diagnosis. These articles reinforce the need for patience and a solid game plan, combined with good vehicle information and the right tools, if your goal is to make these repairs as efficiently and painlessly as possible.


Have you had a tricky electrical system repair recently? We love to hear about it and how you got it resolved in the comments below.

2 Responses

  1. I recently had to replace a battery on a F150 KingRanch, 2018 model with 28k mileage. When I was given the cost I almost passed out! $754.00. The info I was given by the dealer was very weak and felt as if I had been con’ed. The preview of the article got my attention and I would like to read it in detail so I can better understand what is changing in the industry.
    Joseph Hicks. Woodstock Georgia

    1. Joseph – Thanks for your comment. $754 – Yikes! Good luck with your research. Hope it works out well for you. Thanks again, Jim from Clore Automotive

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