As automotive technology has changed, so has the technology of diagnostic tools and processes. When I started in this business in 1966, a good volt/ohm meter and test light pretty much rounded out the arsenal for electrical test equipment. Several years later came the digital volt ohmmeter, then labscopes. Finding the temperature of something required a mechanical thermometer; years later, along came the non-contact infrared thermometers. Today we have the use of infrared thermal imaging.
In the March 2016 issue of Motor Age, I wrote an article on this same subject — “A picture is worth 1,000 words.” This month I would like to carry on this thought with a little more in-depth study of how thermal imaging works and a case study on an electrical system problem. Let’s take a few minutes and explore how this works.
The right way to take a temperature
I think most of us are familiar with the handheld non-contact pyrometer, or thermometer. Those tools can save a lot of time and money when it comes to testing the temperature of automotive parts. Have you ever considered the knowledge needed to use the power of this tool? There is more to its use than just pointing and shooting. To make this sort of tool take accurate temperature readings, the technician must also have some knowledge of the system they are working on to understand the story the temperature readings are telling.
When using the non-contact thermometer, the tool is looking at the infrared radiation that is radiated from the object. That sounds simple, but what is infrared radiation? The dictionary defines this as light waves in the 8 to 15 micron wavelength. These are light waves that are invisible to the human eye, but can be seen by sensors in a non-contact thermometer or an infrared camera.
All objects emit infrared radiation as a function of their temperature. This means all objects emit infrared radiation. Infrared energy is generated by the vibration and rotation of atoms and molecules. The higher the temperature of an object, the more the motion and hence the more infrared energy is emitted. This is the energy detected by infrared cameras. The cameras do not see temperatures. They detect thermal radiation.
Why do you need to know this? Different surface colors radiate the light waves differently and the sensors in the infrared device will not report the proper temperature. As an example, a shiny surface, such as bright aluminum, will radiate the heat differently than a flat black surface. Why does this matter? In some cases, it does not matter, especially if the technician is only using the temperature readings to compare the temperature of two different things that are made from the same material and have the same surface color and texture. Now if the technician is trying to get an accurate temperature reading, then the surfaces must be a dark color, preferably a flat black. An easy way to accomplish this is to keep an aerosol can of flat black paint in your tool box and just give the shiny surfaces a quick shot of the black paint.
Since the handheld infrared tool we have been talking about only takes its temperature reading from one small spot, the information gathered is quite limited, much like using a volt meter to watch the voltage output from a generator, or to watch the voltage signature of a working fuel injector. The infrared camera uses this same technology, except it takes its temperature readings from several thousand little spots and plots this information on a screen so the human eye can see the results.
• A spot pyrometer just gives you a number; a thermal imaging camera generates an image.
• A spot pyrometer reads the temperature of one single spot; a thermal imaging camera gives you temperature readings for each pixel of the entire thermal image.
• Because of advanced optics, thermal imaging cameras can also resolve temperatures from a longer distance. This allows you to inspect large areas very quickly.
With this background of thermal imaging, we need to apply the tool to a diagnostic process. If we are working on an electrical system, one very important thing to keep in mind is current flow produces heat. Any time current flows through a component or conductor, there is some amount of heat generated. Will a thermal image of a circuit or component tell us how much current is flowing in the circuit? No, it will not, but it will tell us if there is unwanted resistance in the circuit, or if the circuit is actually flowing current.
A real-world example
This afternoon, a new customer brought in his 1999 Ford Expedition. The vehicle is a high-mileage vehicle with 388,190 miles on the odometer. The vehicle is powered with the 5.4L engine, with its power running through an automatic transmission.
The vehicle owner says the vehicle had been sitting for three months and when he went to get the vehicle, it started right up and ran good. The vehicle was driven about 100 miles and parked overnight; the next morning, the engine would not crank. A trickle charger had been hooked to the battery and left over night. The next morning the engine would not crank. At that point, a 70-amp charger was hooked up and the engine started with no problem.
After being parked for two days, the engine again would not crank. The owner says the headlights were nice and bright, so he thinks there is a problem with the starter solenoid that is mounted to the firewall. Three years ago, he had this same problem and a starter solenoid fixed it. When the vehicle was brought to my shop, the engine cranked and started just fine.
Before we embark on this diagnostic journey, we need to have a direction. Would you start looking for a defective part, such as a starter relay? Maybe pull and tug on the battery cables and associated wiring? How about suspecting a defective battery, or a poor connection at a battery terminal? Since the art of diagnostics is to get the problem to come to you, I want to start with something simple.
With any electrical problem like this, I want to use a systematic diagnostic approach. This will always start by qualifying the battery. It doesn’t matter if the battery is old or new; always qualify the battery condition before any other testing is done. Since the battery is the power source for the starting system and if the battery is not up to its task, all other testing can be wasted time. In this case, the battery was new and clean and when load tested at 50 percent of its cold cranking amperage (CCA), the voltage did not drop below 9.9 volts.
With that hurdle crossed, the next thing to confirm is if there is any parasitic draw. All doors were locked and the ignition key taken out of the ignition switch. With my current probe hooked around the negative battery cable, I found a current draw of .537 amps (537 milliamps) (Figure 1). At this point, I want to leave the vehicle alone to see if the current flow decreases as the different electronic modules go to sleep.
Tech tip: To find the allowable parasitic draw for a battery, divide the battery reserve capacity by four to determine the amount of parasitic draw in milliamps the battery will sustain. In other words, if the battery reserve capacity is 100 minutes, 100 divided by 4 = 25. The battery will support a maximum 25 milliamp draw or .025 amps. This is an important piece of information to keep under your hat, since most vehicle manufacturers do not publish a maximum value for parasitic draw.
The vehicle was left alone for about an hour and when I went back, the inline amp meter was still showing a current draw of .537 amps. The next step in the diagnostic process is to find which circuit is drawing the current. Over the years, many different ways have been used to find this problem, of them: using a test light in series with a battery cable, pulling fuses and waiting, and using a voltmeter to check the amount of voltage drop across the fuses. Some of these methods are very accurate, some are not, but they all take time, which is a very valuable commodity for the diagnostic technician.
My first grab for a diagnostic tool to perform this task was my cell phone with the infrared camera attached. Since the underhood fuse box is the easiest to access I took a look there first. Figure 2 shows the underhood fuse box. This box has a few small fuses, but most of the fuses are large fuses that feed other sub-circuits, which are fused in the central junction box that resides under the left side of the dash.
The next stop was inside the cab to take a look at the central junction box. Figure 3 shows what I saw. There are two relays and one fuse that are warm from current flowing through them. I say they are warm, but they are not warm to the touch. The thermal image shows them as warm, since they are warmer than their surroundings. To determine what these components are, a quick look at MotoLogic service information gave me the picture in Figure 4. The battery saver relay is activated, as is the accessory delay relay. By comparing the thermal photo to the fuse diagram, I can see fuse 15 is also flowing current. By using the fuse description chart in Figure 5, we know fuse 15 powers the GEM (general electronic module) that is sandwiched on the back of the fuse panel. This chart also shows the amperage rating of each fuse. This chart is very helpful after someone has either removed fuses from the fuse panel, or has installed fuses of the wrong amperage rating.
Now we know where the current is going; the next step is to pull the fuse 15 and monitor the current flow with the amp meter that is hooked in-line with the negative battery cable. I am sort of old school when it comes to monitoring very low current flow on problems like this. I like to see both digital numbers and a graph of the activity on the circuit. Yes, I know this is a little overkill, but I also don’t like doing the job the second time because something happened when I wasn’t looking. To solve this problem, I use a low-amp probe and scope along with the amp meter in my vantage pro. The tools were hooked up, fuse 15 was removed from the under dash fuse panel, and within a few minutes the current flow dropped to .211 amps (211 milliamps) (Figure 6).
This current flow is still too much, about 10 times more than the current flow allowed for this battery. By waiting a few more minutes, the current flow dropped to .010 amps (10 milliamps) (Figure 7). I have also been capturing the data on my labscope as seen in Figure 8. On the scope capture I noticed it took close to 20 minutes for all modules to go to sleep. Yes, this scope capture is redundant, but by using the scope, it is easy to step away from the broken vehicle and be productive on something else while modules are getting ready for their naptime. The scope leaves a visual recording of what happened in the circuit and is very accurate.
Now I have narrowed the problem down to the GEM module. By using a thermal image of the fuse boxes, it is very quick to find where the current is flowing. This does not discount the need for using an ammeter or voltmeter in the diagnostic process, it is just another great tool in the electrical technician’s toolbox.
Thermal imaging is useful for more than electrical problem analysis. All automotive systems, whether electrical, mechanical or hydraulic, all generate heat when they operate. Knowing how the system works and applying that knowledge to thermal imaging is a great way to make problem analysis quicker and more accurate.