Electrical diagnostics: going beyond the Volt Meter

Feb. 1, 2016
The basics of electrical theory haven’t changed over the years, but that doesn’t mean your electrical diagnostic process shouldn’t be changing. This article will focus on how a thermal imaging camera can be used to supplement your electrical diagnostic process.

The basics of electrical theory haven’t changed over the years, but that doesn’t mean your electrical diagnostic process shouldn’t be changing. This article won’t talk about oscilloscopes or other more traditional tools discussed when looking beyond the Volt Meter. Instead, this article will focus on how a thermal imaging camera can be used to supplement your electrical diagnostic process.

Thermal imaging cameras have been coming down in price significantly over the last several years. With the entry level price point of true thermal imaging devices starting around $350 and full featured units starting around $700, thermal imaging cameras are now a much more viable option to consider adding to your tool box. If you’ve ever wished you could just see where the electrical problem was this may be your wish come true. This article will focus on the following:

·      How thermal imaging can visually show you the location of electrical problems

·      Differences between IR thermometers, visual IR thermometers, and thermal imaging cameras

·      Features to look for in a thermal imaging camera

·      Principles of thermal imaging including; thermal emissivity, thermal reflectivity, and thermal transparency

Thermal Imaging Camera vs IR Thermometer

While I’ll do my best to relay as much information as possible in this article if you are looking for more depth on this subject I’d highly recommend checking out the Thermal Imaging Training Center’s website at MotorAge.com/infraredtraining. They have some great free online training available as well as an in-depth comparison of temperature measurement tools.

To understand how thermal imaging can visually show you the location of an electrical problem we have to discuss basic electrical principles. Whenever I’m talking about the foundation of electrical knowledge I’m always reminded of a great OEM training instructor I had years ago.  He started his electrical training classes by telling technicians that electricity is simple, it can only do two things. Those two things he said were to make things hot, and make magnetic fields.  While that may be somewhat overly simplified it does cover a lot of the electrical applications in the automobile. The one that lets thermal imaging help with diagnostics is the fact that electricity makes heat.  Any time a circuit has electrical current flowing through it heat is generated. How much heat the circuit generates is dependent on the amount of current flowing in the circuit. 

Simulated high resistance light circuit with 2.25V Drop in 6 foot harness
High resistance section of harness shown with thermal image (C2 Camera with MSX)
High resistance section of harness shown with thermal image (TG165 - no MSX)

So what types of electrical problems could this technology help you find? Because we are relying on heat to help locate the problem the circuit has to be able to operate. That means this is not going to help you find the location of a blown fuse, and completely open circuit, or any failure that would completely prevent current from flowing in the circuit. It can however help you find problems like circuits with excessive resistance, open leg(s) of a parallel circuit, or a parasitic draw to name just a few.

Probably the most likely application for using thermal imaging to find automotive electrical problems is a circuit with excessive resistance. If you are involved with electrical diagnostics you’re likely already familiar with the term voltage drop. In a properly operating circuit such as a headlight, power window motor, etc. nearly all of the available voltage should drop (source voltage in, near 0V out) across the electrical load (bulb, motor, etc.). If you have a circuit with excessive resistance it will prevent the electrical load from operating properly because it doesn’t have enough voltage available for the load to use. This is because the excessive resistance is now using a portion of the available voltage. Using a volt meter you can check the positive and negative (ground) portion of the circuit to determine which one is dropping voltage due to the excessive resistance. Once you determine which half of the circuit is causing the problem however, you still have to locate the actual location of the fault. You could start taking apart the wire harness hoping to find a visually damaged wire or loose connection, you could bypass that entire part of the circuit, or if you have a thermal imaging camera you could simply look for the part of the circuit that is generating excessive heat. Because resistance in a circuit generates heat, the portion of the circuit with excessive resistance will be hotter than the rest of the circuit. This concept isn’t limited to just wire harnesses either, think of applications such as hybrid vehicle batteries, electrical junction blocks, etc.

Fuse block with no current draw (C2 Camera with MSX)
Warmer fuse on the left indicates current draw (C2 Camera with MSX)

In a parallel circuit (such as a rear window defroster) with one or more open legs the principles will be slightly different. In this case you’d be looking for the portion of the circuit that is cooler than the rest. Because the open leg(s) can’t flow any current they won’t be generating any heat.  Again, this is just one example circuit. This could also apply to things like seat heaters, etc. depending on how they are wired. 

Finally, let’s look at parasitic draws. Remember, all electrical current flow generates heat. That means when a vehicle is shut down the electrical system as a whole should start to get cooler because current flow has been stopped. Any circuit still flowing electricity when the entire vehicle should be shut down will either not cool down at all, or will cool down less than those circuits that are fully shut down. Of course, since parasitic draws tend be small even when they are causing problems, the amount of heat being generated will be minimal. That means this must be handled slightly differently. Leaving the vehicle overnight and allowing the temperature to stabilize in the rest of the vehicle can help make that low current draw stand out as a warmer spot in a thermal image.  

Right about now you may be thinking “Why can’t I just use my infrared (IR) thermometer?” or “What’s the difference between the visual IR thermometer I saw and a thermal imaging camera?” The bottom line is that while both IR thermometers and visual IR thermometers have their purpose they are much more limited than a thermal imaging camera. For the purpose of this article I’ll just focus on the two major limitations that both of these IR thermometer options have. The first limitation is related to something known as the Spot Size Ratio (SSR). SSR refers to how large of an area the temperature measurement is looking at based on how far away from the object the measurement tool is. Typically the SSR on the lowest cost IR thermometers is around 6:1. That means if the thermometer is 6 inches away from the object being measured the area being measured by the thermal sensor is 1 inch in diameter. Many of the IR thermometers have a laser guide, or even possibly two. It’s important to understand that while the laser helps you see where you’re aiming the measurement is still limited by the SSR.

The second limitation of the IR thermometers is that they typically only have one temperature sensor. That means the measurement displayed will be an average of the temperature within that 1 inch diameter area being measured. When trying to measure the temperature of a wire, a small fuse, etc. that simply won’t provide an accurate enough measurement.

Warmer fuse on the left indicates current draw (TG165 Camera - no MSX)
Warmer fuse on the left indicates current draw (i7 Camera - no MSX)

If you step up to a more advanced visual IR thermometer things do get a little better, however exactly how much better can be a little hard to determine given the published specifications. One of the more well-known manufacturers of visual IR thermometers currently has two models available. One of those units has a 6:1 SSR, and the other one has a slightly improved 9:1 SSR. What makes it somewhat difficult to compare this unit to traditional IR thermometers and thermal imaging cameras is the lack of information provided about the temperature measurement sensor being used. The published specifications provide pixel ratings for the VISUAL camera, but not for the thermal measurement array being used. Upon further research, I was able to find some information published by the Thermal Imaging Training Center that discussed thermal sensors typically used in visual IR thermometers. According to their data, visual IR thermometers typically use a “two dimensional 15x15 array” which would correlate to 225 pixels. Their test results showed that this configuration required the tool to be within 1.5 inches of the object being measured to get an accurate reading on a ¼ inch conductor. Obviously that isn’t very practical for most automotive electrical applications.

Let’s take a look at how this all relates to thermal imaging cameras and also discuss the big question about how much you’d have to spend to get one. I’ve been using FLIR brand thermal imaging products for about 5 years now so I’m most familiar with their product line. Currently their lowest cost unit with a true thermal imaging sensor is the TG165. Current pricing for that unit from online retailers is right around $350. The TG165 has a sensor array with an 80 x 60 resolution, which correlates to 4800 pixels. To put it in perspective that means every time you take a measurement with the TG165, or any other thermal imaging camera with similar specifications, it is the equivalent of using 4800 IR thermometers or about 21 visual IR thermometers (4800 pixels/225 pixels). That increased resolution definitely makes it much easier to see temperature variations of small components from a reasonable distance. The TG165 doesn’t however use the thermal imaging sensor to take the actual temperature reading. While this is a true thermal imaging camera, the thermal sensor isn’t calibrated. Because of that the TG165 relies on an IR thermometer with a 24:1 spot size ratio for the numeric temperature that is displayed. The lowest cost FLIR model with a calibrated sensor that I’m aware of currently is the C2, which is selling online for around $700 followed by the E4, which is around $1,000.

If you start to research thermal imaging cameras online you may be overwhelmed by the number of models available, the wide price range, various features available, etc. To help narrow your search here are a few things that I’d consider focusing on:

·      Resolution of the thermal imaging sensor (not the visual camera). Bigger is better, but may not be necessary. For most automotive applications anything at or above the 80 x 60 of the TG165 will likely suffice.

·      Temperature measurement range. Lower cost cameras typically can’t measure as high of temperatures. For automotive electrical diagnostics this typically isn’t an issue but if you want to measure catalyst temperatures you’ll likely need a higher cost camera.

·      Visual display capabilities. Some thermal cameras now have the ability to combine a digital photo and the thermal image. This makes it much easier to see edge definition of components and even read labels. FLIR calls their version of this “MSX,” which is included in the C2 and E4 models mentioned above, but isn’t on the TG165.

·      Image portability and file format. If you want to be able to include these images as part of a repair order be sure to get a camera that allows you to export the image in a useable format. You may also want to consider the ability to further analyze the image using specialized (and in many cases free) software from the camera manufacturer. This software also typically allows you to add notes to the image and generate reports. As they say, a picture is worth a thousand words, so when selling the electrical diagnosis and repair to the customer this could be your best sales tool.

Seat heater circuit (TG165 Camera - no MSX)
Seat heater circuit (C2 Camera - with MSX)

If you decide to make the leap and get a thermal imaging camera make sure you also take the time to attend some training on the topic (see the free resource listed at the beginning of this article). A basic understanding of thermal imaging principles is critical to avoid potentially misinterpreting the data. At a minimum, I’d recommend looking for information on thermal emissivity (ability of an object to give off heat energy), thermal reflectivity (ability of an object to reflect heat from other sources), and thermal transparency (ability of thermal energy to pass through an object without significant loss). Understanding those aspects of thermal energy will help ensure you get accurate results and will help you explain any anomalies that may show up during your testing.

Remember that while this article focused on the use of thermal imaging as an electrical diagnostic tool, there are numerous other potential applications in our industry. My experience has been that once technicians get a thermal imaging camera in their hands they come up with all sorts of potential uses for it. Also remember to keep your eye out for new tools coming up in the future. With the rapid changes in technology we’re likely to continue to see great new options. Just this past fall FLIR released a new electrical clamp meter with an integrated thermal imaging sensor which was geared toward industrial applications (model CM174). Who knows what might come out that could be geared more toward our industry in the coming years as more technicians embrace the technology?

About the Author

Jeff Minter

Jeff is currently serving as the service director for a group of dealerships in the heavy duty vehicle industry. Previously he was the training director for a company specializing in hybrid/electric vehicles providing both training and technical support for technicians nationwide. Jeff also has approximately 8 years of experience as an automotive instructor at the technical college level and is still a technician at heart. He is an ASE certified Master/L1/L3/F1 technician with OEM training from numerous manufacturers. 

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