The importance of having a solid electrical diagnostic strategy

Jan. 30, 2018
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   

Editor's Note: This article was orginally published Jan. 30, 2018. Some of the information may no longer be relevant, so please use it at your discretion.

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.

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

Tooling

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 a voltmeter is a much better (and faster) choice.

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.

Resources

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.

  1. 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.
  2. 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.
  3. 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.

Strategy

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.

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

Electrical fault tips

If you would like more information on the basics of reading electrical diagrams, there are many very good articles available on this site. Once you understand how to read those diagrams, the next question becomes how to use them to speed up the diagnostic process. When looking at the diagram, you have to be sure not to get tunnel vision. In other words, make sure you remember the component(s) you are testing don't operate in a vacuum; they are likely connected in one or more ways to other components on the vehicle.

I like to start at the component itself, and then work outwards. I typically follow the positive side wiring on the diagram and find a junction point, connector or splice where there may be additional components supplied from the same voltage source. If you find any common positive side wiring like that, it's as simple as activating those components that share the wiring to see if they work or not. If they do work, then you know the problem is not further back in the circuit. If they don't work either, then you know the problem must be further back in the circuit.

This type of electrical diagram analysis isn't meant to replace pinpoint testing but rather to reduce the time spent doing that testing. A very simplified example would be: Wire "A" supplies wires "B," "C," and "D." A component connected to wire "C" is inoperative, but those connected to wires "B" and "D" still work. Given that fact, there is no reason to test wire "A" because a failure in that wire would have resulted in failure of all connected components.

In essence, you validated the integrity of wire "A" by using the diagram, and the other components that also receive power from wire "A." This type of diagram-based pre-testing work can significantly speed up your diagnosis.

Once you've narrowed down the parts of the circuit that will need to be tested as much as possible the next step is to perform a thorough visual inspection. This doesn't mean disassembling the entire vehicle, opening up all of the wire looms, etc. but you should perform an inspection on parts of the circuit that are readily accessible. Things you should be looking for include signs of physical damage, corrosion, previous wiring repairs and improper routing. Previous repairs should be inspected closely as it's not uncommon for a poor-quality wiring repair to fail again.

Of course, once you've narrowed down the parts of the circuit you need to test, you'll need to know what the normal readings should be at each point you'll be testing. This may seem simple to identify which wires should have battery voltage and which ones should have ground but remember to take into account the effects of switches, etc. Also remember it's best to test the circuit without disturbing it if at all possible. Any time you disconnect connectors for testing purposes you run the risk of covering up or temporarily fixing the problem you are chasing.

Even after you've done your testing and are fairly confident you've found the root cause of your electrical problem you still aren't quite done with your diagnostics. The last step in your diagnostics should be to attempt to bypass the problem area and confirm correct circuit operation which accomplishes two things. First, it lets you verify your diagnosis before beginning a permanent repair. Second, it allows you to actuate the rest of the circuit to ensure there aren't any other problems present that are being hidden by the failure you've already found.

Once you've identified the failure and repaired it you should do a final verification of the circuit. It's important to not only see if a component works, but you should also verify the integrity of the wiring to/from the component. That testing is best done using a voltage drop test on both the positive and negative side of the circuit while the component is operating. If the voltage-drop tests pass you can feel confident in your repair and not have to worry about a potential comeback due to an underlying problem that was missed.

DTC applications

This same process can also be used to help speed up the diagnosis of check engine light problems related to things like actuators or sensors. It's a similar process where you can use a wiring diagram to check for electrical commonalities in the components. If, for instance, you had a code for a single fuel injector circuit you could use the wiring diagram to verify which parts of that circuit are potentially shared with other injectors that are still working. Any parts of the circuit that are shared with other components which are still working don't need to be tested.

In the grand scheme of things, if you can explain and test the electrical integrity of a simple relay circuit you can test virtually any 12V circuit. The electrical tests on other components/circuits follow the same exact principles, but you do need to effectively interpret wiring diagrams to know what "normal" is for each test being performed.

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