Automotive electrical troubleshooting will always be a challenge for auto repair shops. And the biggest challenge technicians face today is the increasing complexity of vehicles. By understanding the system we’re working on and utilizing as many resources as possible, we will find success. Here we offer advice on checking electrical systems, from batteries to cables to charging systems and more.
As a mobile tech, when I get called by a shop to diagnose or troubleshoot a problem vehicle, I never really know what I will be up against. All I do know is that I will have to be on the top of my game.
My job description isn’t any different than yours. I have been tasked to look at a problem vehicle and determine the quickest and most direct way to finding a solution to the problem. It doesn’t matter if it’s an electrical problem, mechanical problem or just a plain old noise or vibration problem. We as technicians are expected to be able to fix anything... right? Well that’s where the fun part comes in.
Electrical problems in the automobile have been around since they were introduced a hundred or so years ago. Believe it or not, a lot of the issues from years back are still here today and will be here for many years to come. Vehicles of today are more complex, have more components and a lot more wiring. Outside of that they are essentially the same.
Now before you think that I have lost a marble or two, let’s take a look at what we still have. We still have the 12 volt battery. That is the heart of the vehicle’s electrical system. That’s where it all begins (positive side) and ends (negative side). Think of it just like a doctor would. If your heart isn’t healthy, your body won’t work correctly. The same holds true with the battery. If a circuit in the vehicle requires 12 volts to operate correctly and the battery is only producing 10 volts, that circuit will be under-performing and will suffer a failure at one point or another.
One tool that is a must have is the DVOM or digital volt ohm meter (see Figure 1). On today’s vehicles I personally don’t recommend using a test light, as the accuracy just isn’t there. A test light will power up in some cases at fewer than 6 volts. How are you going to know if your voltage is dropping and just how much is that voltage drop?
Here is a simple battery check that is a quick way to determine what the state of charge is on a battery by doing a quick and easy test. For clarification on the test light issue I checked this battery using a test light first and then I used a DVOM (see Figures 2 and 3). See the difference? I have no idea what the test light is telling me other than the fact that it did light up. Kind of useless information, I would say. Now this test is by no means a complete battery SOH (state of health) check, but by performing this test first it will give you a good idea on whether or not this battery needs to be charged. I always isolate the battery away from the rest of the vehicle so I am not getting any interference from anything else on the vehicle. I place both leads across the battery. I want to see at least 12.5 to 12.8 volts. If it’s less than that then a battery charge should be done first before any testing is attempted. Also now would be a good time to do a more thorough test on that battery.
Voltage drop test
Once I know that my battery is in good shape, my next test is a voltage drop test. This test will measure the voltage in a circuit from point A to point B. When doing a voltage drop test you are basically looking for voltage that has leaked or left the circuit, so to speak.
Traveling from the battery to the vehicle are the battery cables. These are important because they need to deliver as much voltage to the vehicle as possible. A loss of voltage can occur because of a loose connection, corrosion inside of the battery cable and even a frayed or lack of copper windings in the cable itself.
The first step is to test the positive battery cable. I set my DVOM scale to 500 mv because I don’t want to see any voltage over 200 mv. 500 mv is a half a volt. Your DVOM might have a different scale and that’s all right. The important thing is that the voltage setting should be low. You can always adjust your scale on your DVOM that best suits you.
Place your positive lead on the positive post of the battery and the negative lead to the other end of the positive cable. The reading should be close to 0.00 mv. Now put your DVOM in record mode and crank the vehicle. You always want to test a circuit while it’s working. Cranking the vehicle puts a load on the circuit and it tests the integrity of that component while looking for any stray voltage that has left the circuit.
On the starter circuit, if your reading is under 200 mv, then you are fine. Now take your leads from your DVOM and place one at the negative terminal of the battery and the other at your ground connection of the battery cable. Again crank the vehicle and look at your reading. It should be very similar to your positive side reading.
So let’s say your positive reading was 100 mv and your negative reading showed 850 mv. That’s telling you that you are losing about three quarters of 1 volt on a 12 volt system. That’s enough to possibly show some symptoms while cranking or even not allowing the battery to properly charge itself.
Make sure the circuit you are testing has a good clean connection at both ends. It’s always good to do a visual test here. You can’t see the internal corrosion that has built up inside that cable, but if you are sure that your connections at both ends are good then it’s more than likely that the cable itself is bad. This test is essential to do if you want to properly test the rest of the electrical system. Remember, it all starts at the heart.
The charging system
After you have tested the battery and the connections leading to and from the battery, the next test involves the charging system. The battery needs to maintain a state of charge to properly operate all the various circuits on the vehicle effectively and accurately.
By this I mean that the control modules, switches, sensors and all components need to maintain a certain voltage to be able to operate properly.
If the battery voltage isn’t correct for the circuits being operated, then issues arise, such as control modules shutting down, circuits overheating and circuit relays sticking or maybe not working at all.
Today’s newer cars rely on the proper voltage to run the numerous different circuits. Modern charging systems often rely on the vehicle’s engine control module (ECM) to control the rate of charge. The ECM acts like a voltage regulator in that with various driving conditions and different loads that are placed on the system, it will control the proper voltage that the battery needs to maintain all the electrical requirements needed to keep the vehicle operating safe and sound.
To test for a proper charging system, you will need a DVOM, an amp clamp and the spec for the vehicle that you are working on. A quick way, and by no means the most accurate way, to do a charging system test is to place the test leads of your DVOM across the battery like you did with the SOH check I mentioned earlier, and record your reading. With the test leads still hooked to your battery, start the engine and record your reading. If the alternator is showing a higher voltage reading than the SOH reading, then you could assume that the system is at least attempting to charge the battery.
I like to see at least 4 to 5 tenths of a voltage increase over your SOH check.
Next, while the vehicle is still running, turn on as many high load accessories as you can (i.e. headlamps, windshield wipers, blower motors, etc.) By doing this you are placing a load on the alternator by basically making it work harder.
Now depending on the battery’s state of charge and its condition, your voltage reading may go up a little more or it may not. That is controlled by the voltage regulation which is either in the alternator itself or the ECM. Keep in mind: I said this is just a quick test but not a true test. Now, if you have an amperage clamp, or amp clamp as they are commonly called, then that would be a better test to perform as you can actually measure the amperage output of the alternator and compare it to the spec of what it should be.
Some amp clamps can be attached to your DVOM and others are a standalone unit with the DVOM. I would recommend an amp clamp with a jaw that is big enough that can fit around a battery cable and can measure a high enough amperage. Using the same amp clamp that you would use to test an electric fuel pump or a battery, parasitic draw would not work too well.
Now set your amp clamp around either the positive battery cable or the positive cable, with the large one going to the back of the alternator. NOTE: Some manufacturers place the alternators in spots where the power feed to the alternator is just not accessible, let alone with an amp clamp attached. That’s why I like to keep it simple and go to the positive battery cable (see Figure 4).
Now turn on the DVOM or amp clamp. Set the tool to the proper scale. Make sure that the amp clamp auto zero procedure has been performed, as you don’t want inaccurate readings to show up and mislead you during your test. Start the engine and look at your reading. If for some reason there is a minus in front of your reading, it’s because the amp clamp is positioned in the wrong direction.
Remove the clamp, re-zero the clamp and turn the clamp in the other direction. Most amp clamps have a directional arrow stamped on them. For example, a 2015 Chevrolet Colorado LT 2WD with a 3.6L V6 calls for a 150 amp alternator (see Figure 5). When testing a vehicle, the alternator amperage size is used as a base of which the vehicle tolerances might run in. The alternator generally won’t put out that amperage but in a depleted condition where the battery isn’t charged fully and other electrical components are demanding higher loads that aren’t working to their full capacity, it could output close to that.
Like before, based on the load requirements at that particular time, your amperage reading will vary. I like to see an amperage reading that at least shows a good measurable amount, meaning that the meter is displaying an amperage reading that is within spec for the vehicle you’re working on. Some high-end electrical testers will have all the necessary specs for the vehicle that you are testing. Those units are great for testing the complete electrical system on the vehicle.
Driver’s seat diagnostics
Our last quick electrical testing tip is an easy one to perform and you can do it from the driver’s seat. I call this test driver’s seat diagnostics. Whenever there is a vehicle with a driveability problem that comes into your shop, one of your first instinctive tests would be to hook up a scan tool to see if there are any diagnostic trouble codes or something that looks amiss.
One of the first things I like to look are the data pids that listed. In that list are some data pids that the control module is monitoring that are voltage pids. Some will display battery voltage, others will show ignition voltage (see Figure 6 ).
Look closely at those different voltage pids and make sure that none of them are reading out of the norm. Some could be showing too low of a voltage and others could be showing excessive or voltage that’s too high.
If your charging system is working correctly but your control module is showing a completely different reading, you could quite possibly have a wiring problem for that circuit or maybe there is a problem with the control module itself.
If the module isn’t receiving the correct voltage on the power circuit, for example, then the module might not be able to control the vehicle’s correct programming strategy.
You also need to look at other modules, as well, since it’s possible that the problem with the vehicle’s driveability issue is actually coming from another module and not the one you thought it was.
Now that we have covered a couple of simple ways to test electrical circuits, there are many more in-depth tests that can be performed, but due to the fact that I can’t cover them all in this article, look for the rest of them in a future issue of ASP.
When I go to a shop I try to make my diagnostic routine as simple as I can. I try to incorporate a mini training class with the technician who was working on the vehicle and if time permits, show them that electrical troubleshooting isn’t as hard as some techs make it out to be. Yes, vehicles are becoming more complex as technology changes. That is the biggest challenge we as technicians face today.
By understanding the system you’re working on and utilizing as many resources as possible, I believe that you will find success. However, automotive electrical troubleshooting will always be a challenge. ■
Edwin Hazzard owns South East Mobile Tech in Charleston, S.C., which is a mobile diagnostic and programming service providing technical service to many automotive and body repair shops. He has 35 years experience in the automotive industry. He currently is an automotive trainer, a board member of TST (Technician Service Training), a member of the MDG (Mobile Diagnostics Group), a member of the Professional Tool and Equipment advisory board for Pten magazine, a committee member of Nastaf, and is a beta tester for multiple tool makers.
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