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The importance of having a solid electrical diagnostic strategy

Tuesday, January 30, 2018 - 09:00
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If you don't know what you should see on a voltmeter — before you hook it up to the circuit — there's no point in taking the meter out of your toolbox   

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All too often it seems technicians begin diagnostics without a clear plan of attack. That often results in them randomly scrolling through data on a scan tool without really knowing what they are looking for or connecting a voltage meter (or worse, a test light) to circuits without knowing what they should see if it's operating correctly. This article will focus on the importance of having a solid electrical diagnostic strategy that includes the use of wiring diagrams and service information. I'm a firm believer that most electrical problems, even the seemingly complex ones, can be solved by any technician that has a solid understanding of how to diagnosis a relay circuit as long as they also know how to leverage the available service information and diagrams.  

This article will include information on:   

  • Diagnostic tools needed for electrical diagnosis 
  • Resources available to help with electrical diagnosis 
  • Overview of building a diagnostic strategy 
  • Using electrical diagrams to speed up diagnosis 
  • Using wiring diagrams to assist with check engine light diagnostics 
Meter with detachable remote display


When thinking about electrical diagnostics a few tools typically come to mind immediately. Most of the time the first two tools that people will mention are test lights and voltmeters. There are actually many different tools that fall under what most people simply group as test lights and the differences can be pretty important depending on what circuits are being tested. The three most common types I've seen referred to as "test lights" are incandescent, LED, and logic probes. Some very old commercially available incandescent test lights (or a homemade one) may actually draw enough current to damage computerized circuits, though newer ones typically draw less than 10mA to help prevent that. Of course, that low current draw makes the test light very limited in its ability to provide diagnostic information. In essence, all three of the types of test lights listed above aren't good for much more than identifying if a pin/wire is connected to a ground or to a power source. So, while many technicians tend to grab this tool first, I hope this article will help convince you that a voltmeter is a much better (and faster) choice. 

Relay circuit testing represents the skills needed for virtually all 12V system testing

Another tool more commonly being used in electrical diagnosis is an oscilloscope. This is about the complete polar extreme from a test light from a complexity and capability standpoint. There are many different types of oscilloscopes available but the most commonly used today for automotive diagnostics are digital storage oscilloscopes (DSO), or graphing multimeters (GMM). DSOs and GMMs are very similar, but GMMs may miss some glitches that a DSO wouldn't due to the sampling rate difference. This very powerful tool definitely has its place in the tool box of an advanced diagnostic technician, but it's far from being needed to diagnose most electrical problems. In fact, I've even seen some technicians that skip a voltmeter and grab a DSO for problems they could likely have diagnosed faster without the DSO. Understanding the circuit, expected signals, and potential failure modes will help determine where using a DSO is most appropriate. 

The digital volt meter (DVOM) is by far the tool that should be used most often during electrical diagnostics. DVOMs have evolved over the years and are currently available with a wide range of options. With the increased number of hybrid and electric vehicles it's important to be sure the meter is rated appropriately. The general recommendation related to the safety for use on hybrid and electric vehicles is to ensure the meter is rated at a minimum of a Category III 1000V, and also to be sure the rating of the leads being used are at or above the rating of the meter. You don't have to spend a fortune to get a good quality DVOM. Entry-level meters with all of the basic functions needed can be found for as little as $50, mid-level meters with higher quality leads, etc. will run close to $150, while a very high end commercial quality meter will run in the range of $250+. Personally, I've had a PDI Meter (Precision Diagnostics Instruments) DVOM in my toolbox since I was in tech school (nearly 20 years ago now) and it has operated flawlessly. Over the years I did add a Fluke 233 with remote display for those instances when I wanted to measure a reading without using super long test leads. Since then other models have come on the market with Bluetooth capability that allow you to use a phone as your remote display (such as the FLIR DM91 or Fluke 3000 FC). Regardless of which brand or model of meter you purchase the most important thing is to become familiar with its settings, capabilities and use. 

If 6V are used by (dropped across) the resistor, do you know what each of the other meters should read?
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