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The importance of having a solid electrical diagnostic strategy

Tuesday, January 30, 2018 - 08:00
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Once you have your basic diagnostic tools in hand, you'll need to know what to do with them. In addition to understanding the basics of electricity, you'll need access to reliable service information. For most electrical related diagnostics there are three main items you will need access to. First, and most important, are wiring diagrams. These come in a variety of formats today depending on the age and manufacturer of the vehicle, as well as the information source you are using. Many of the service information providers utilize the OEM diagrams and package them for you to purchase, others may actually re-draw the diagrams so all diagrams appear in a similar format regardless of the OEM. Either way, you absolutely must have access to diagrams. The second thing you should have access to are the OEM diagnostic flow charts. While I typically refer to these flow charts as a tool for those that don't understand how a system operates, they do at times contain specifications related to normal resistance, normal voltage ranges, etc. that aren't found anywhere else in the service information. Lastly, if you are dealing with an electrical diagnosis that involves a trouble code, you'll need access to the code set parameters. If you aren't familiar with code set parameters, these are the basic set of rules the determine when a code sets (voltage drops below "x" for "x" number of seconds, etc.). Understanding code set parameters will help you understand what "normal" operation is for a circuit. 

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If two loads with a similar resistance are wired in series do you know what each of the meters should read?

Once you have your diagnostic tool(s) and have access to service information, you'll need a diagnostic strategy (plan). I define a diagnostic strategy or plan as a series of steps used to locate the source of the problem. A diagnostic strategy involves: 

  • Gathering information 
  • Duplicating the problem 
  • Defining when the problem occurs 
  • Checking for diagnostic trouble codes 
  • Researching information related to the problem 
  • Performing a thorough visual inspection 
  • Performing pin-pointed diagnostics 
  • Confirming the diagnosis before repair 
  • Verification of the repair    

Information gathering should come from the primary driver of the vehicle if at all possible. It's extremely important to ensure you are getting as much information as possible about when the problem started, any conditions that make the problem worse or better, any additional complaints, and any history related to other recent repairs. 

Once you have as much information as possible you need to duplicate the problem. If a problem can't be duplicated a repair should not be attempted for numerous reasons. The bottom line is if you can't duplicate a problem there is no way to accurately diagnose it or to validate the repair when it is completed. Once you are able to duplicate the problem you should define when the problem occurs. For instance, if the complaint is a blown fuse make a note of what has to occur for the fuse to blow as that information will likely help you in determining the root cause of the problem.  

If only one of the motors is not working, the common wire cannot be at fault.

The next steps will be to check for diagnostic trouble codes and research the problem. This should involve checking for any related Technical Service Bulletins (TSBs) based on codes and/or symptoms, reviewing wiring diagrams, etc. Of these the absolute most important for electrical diagnostics is reviewing the wiring diagrams for the affected components/circuits. Since you've already identified which component(s) aren't working the diagrams should be used to identify: 

  • Power source 
  • Ground source 
  • Commonalities if more than one component or system are affected 
  • Fuses 
  • Connectors 
  • Ground locations 
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