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Sorting Out Intermittent Faults


Whether in trade publications, on iATN or other forums or at trade shows, it seems there are few challenges as vexing as intermittent faults. They present difficult diagnostic situations, often become a major time suck, result in a higher incidence of comebacks and can lead to customer dissatisfaction if not handled properly. In this article, we have pulled together several resources that we hope will help you as you tackle intermittent faults in your service bays.

Chasing Intermittents is Like Chasing Ghosts

We’ll start with an oldie but a goodie. In this article from Import Car, Larry Carley provides a great overview of the unique challenges caused by intermittent electrical problems. He invokes one of the most common mantras we have seen since we started writing these articles, which is that it is important to approach diagnostic dilemmas, especially intermittent electrical problems, with a clear plan, “The best approach to dealing with intermittent electrical faults is a logical approach that will help you find and fix the fault now.” He goes on to identify the tools and techniques that technicians can use to resolve intermittent electrical issues.

It Only Does It Sometimes

This article by George Arrants on brings an interesting perspective to the concept of intermittent faults, asking if they really are intermittent. “Are these problems truly intermittent or are the tools and equipment we’re using not able to keep up with the system or systems we’re trying to diagnose and repair?” He goes on to compare the speed with which vehicle systems are operating to the speed of many of the tools used to diagnose those systems, noting the disparity that exists in many cases.

The Diagnostic Ladder

Back to the mantra invoked in Larry Carley’s article referenced above – have a plan. Intermittent fault diagnosis is, in the end, just one branch in the overall tree of electrical system diagnosis. When it comes to electrical system diagnosis, virtually every article we have ever referenced emphasizes the need for a diagnostic game plan and how having such a plan, and executing it consistently, will save time and aggravation for technicians. In this article from Motor magazine, Sam Bell, discusses the need for a logical approach to diagnostics, noting that intermittent faults do increase the complexity of the challenge, but still need to be tackled within the framework of the diagnostic plan. He uses a crank/no start condition to show how a logical approach can help to avoid heading in the wrong direction. “Let’s look at the rest of that common crank/no-start diagnosis and see how it fits within this framework. Why was checking for spark a good first move? You can’t get a good spark without a good coil, power to it and a good trigger, so if you have that spark, you know for sure that your problem is elsewhere. That’s precisely the power of reasoning from the contrapositive.”

Electrical/Communication Error Red Flags Mechanical Failure

In another recent Motor magazine article that addresses diagnostic strategy, Dan Marinucci shows how a logical diagnostic process that started with stored DTCs and scoping signals from affected systems allowed the technician to use the scope data to identify the likely mechanical issue. He also identified how the shop in question was able to compare the scope capture from the problem vehicle to known good captures to point to the source of the issue. From there, the technician was able to verify an out-of-spec component, hypothesize the proper fix and verify that the fix resolved the original issue. Another good example of a logical process leading to an efficient, effective repair event.

A Critical Tool for Identifying Intermittent Fault Root Causes

From an equipment perspective, the common thread running through the above referenced resources is the Digital Storage Oscilloscope (DSO, or scope). In this article from, Bernie Thompson focuses on various circuit designs, noting where the scope would be connected to each particular circuit and what the expected output would be. For those less accustomed to deploying a scope for diagnostic purposes, he recommends using one daily, as repetition will allow improved mastery over its use and deployment, yielding significant benefits. As he states, “Once you start to see what the picture on your scope is telling you, you will be able to speak the language of electronics. This language will teach you many things and soon you will wonder how you ever repaired a vehicle without knowing this hidden language.”

Intermittent faults are a tricky business that can lead you down blind alleys and into diagnostic quagmires. The common theme from all of the authors of the above resources is the need for a diagnostic plan of attack and the usefulness of a logical approach. With such a framework in place, pinpointing the root cause of an intermittent fault can be done in a more effective and time efficient manner.

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