Solving Automotive Electrical Problems
Basic Automotive Electricity
Electricity is the flow of electrons within a conductor. You can get these moving electrons to do things for you in automotive systems, such as starting your engine, charging your phone, controlling vehicle emissions, and opening your windows. Electrical systems are everywhere in modern automobiles. Having a basic understanding of how electricity works can go a long way toward helping you repair automotive electrical systems.
The first thing to talk about is the flow of electricity. For the purposes of this article, we’re going to say that electricity flows from positive to negative. Yes, there are other theories on electron flow, but to keep things simple we’re going to stick with electrons flowing from positive to negative in this article.
To get this electricity to flow and go were we want it to, we use conductors. Conductors are materials that allow electrons to flow freely. Resistors are materials that inhibit electron flow. We use resistors to insulate our conductors so we keep the electricity flowing where we want it to go. Electricity is funny like that; if you let it, it will find the shortest route to ground, which might happen before you want it to. Insulators help keep this from happening.
Then there are semiconductors: transistors, resistors, and diodes, just to name a few. We use these to help control things in automotive electrical systems. Semiconductors can allow electron flow sometimes but not others. They can block electron flow in one direction, but allow it to flow in the other direction. They can also be used to change the resistance of a circuit based on temperature. Semiconductors are used extensively in computers and control systems. As a result, you run into a lot of semiconductors in automotive applications.
Another big component of an electrical circuit is the load. The load is what does the work in an electrical circuit. The load can be a lot of things. It can be a motor, a heating element, a bulb, or any number of other electrical components that do work. The load and the resistance in the circuit dictates the amount of current flow in the circuit. We’ll talk more about that in a minute.
Aside from the wires that carry electricity and the wiring insulation that keeps the electricity on the path we want it to take, we also need the ability to control a circuit. We often do this with switches. Switches come in all shapes and sizes. They can be switches that we activate, or they can be activated automatically when certain conditions are met; they can also turn things off at a given time.
This is a very basic overview of what you’ll find in automotive electrical systems. Now let’s talk about how we label and measure electricity in order to diagnose electrical faults.
Voltage, Amps, Resistance, and Ohm’s Law
We can’t do much work with electricity if we don’t know how to measure it. As I said, electricity is not something we can necessarily see and put our hands on. Because of that, some very smart people came up with ways of measuring and using electricity. Georg Ohm was one of those smart people. He came up with what is now called Ohm’s law. Ohm’s law is written like this:
I = Current Flow or Amps
V = Voltage
R = Resistance
Basically, what this formula says is that amps, voltage, and resistance are interrelated. If you know two of the three in the equation, you can figure out the value of the missing one. If you know the volts and the resistance, you can divide the volts by the resistance and find out how many amps are flowing through the circuit. It’s a handy formula that can give you a better understanding of how electricity really works.
Voltage
Voltage (V) is the potential energy of electricity. Voltage can be present even if there is no current flow. For example, your battery can have 12 volts in it even if it’s not being used. That 12 volts is still there, waiting to be used, and is the battery’s potential. Think of battery voltage like you think of a full tank of gas; it can get you somewhere, but not until you start the engine and begin to use it.
One really cool thing about voltage is that in an electrical circuit, all of the voltage the electrical circuit started with is used up when traveling through the circuit. For example, if you start with 12 volts at the battery positive and you run it through a circuit, by the time it goes through the load and reaches the negative battery post to complete the circuit, it’s down to zero volts. Why is this cool? Because it allows us to do one of the best tests in electrical diagnostics: the voltage drop.
Voltage Drop Testing
Voltage drop testing is probably the best way to find an electrical fault. Because the voltage drops as it travels through the circuit, we can tell where it’s being used simply by measuring the voltage at different points in the circuit. Think of a car that leaves the positive battery post with just enough gas to work the load and get to the negative battery post. By the time the car reaches the negative battery post, it runs out of gas and that’s it. That’s how voltage works; it starts at its highest level at the positive battery terminal and ends up at zero when it reaches the negative battery terminal.
Let’s say that somewhere in the circuit, we take a detour and go down a different road and decide not to take the road (circuit) all the way back to battery negative. We call this a short circuit. When this happens, we often blow fuses and sometimes melt or burn up parts. This is because of the unrestricted amp flow that occurs during a short circuit; we’ll talk about that in the next section.
Or, let’s say there is an obstruction in the road (circuit) and we use more gas getting through the obstruction on our way back to battery negative. We’ll still make it back to battery negative, but since we used so much fuel (voltage) getting through the resistance and the load, there’s hardly any fuel left to do any work. This is what happens when we have increased resistance in a circuit. If that’s the case with, say, a window motor, the window motor will still work; it will just work very slowly. Why? Because of the increased resistance we had to go through to get back to battery negative. We ended up using almost all our fuel (voltage) overcoming the resistance in the circuit. Less fuel (voltage) means there isn’t enough to run the window motor at full capacity.
If we did a voltage drop test following the path of the circuit, we could see where the fuel (voltage) was being used. We should see the biggest drop across the load of the circuit, which, in the case of a window motor, is the window motor. While the circuit is operational, we can check the voltage before and after the motor. Voltage drops need to be done on a live circuit; if they’re not active, you won’t get any readings. Remember that voltage is electricity’s potential energy; if it’s not being used, it’s just potential. Before the motor, we should see something close to the 12V we started with. We won’t see the full 12V because we had to use some of the gas (voltage) to get through the wires, and probably switches, going to the motor itself. These things give us a little resistance that we need to account for when checking the voltage before the motor. After the motor, however, we should see some pretty low voltage, because we used all our gas (voltage) to run the motor.
This works for every electrical circuit according to Ohm’s law. Once you know the circuit, you know where the gas (voltage) is going and you can measure it at different points in the circuit to get a good idea of how the circuit is operating.
One last example that might give you some practical insight: Say you have an electrical connector with some corrosion in it. You can’t see the corrosion, but you suspect it’s there. You can take a voltage reading before and after the connector to check the voltage drop across the connector. You should see close to zero volts. If you see a higher voltage than expected, you’ve found increased resistance in the circuit.
Voltage drop testing is your best friend when it comes to diagnosing automotive circuits. Learn how to do it, and you’ll be an electrical wiz in no time.
20 thoughts on “Solving Automotive Electrical Problems”
Hans Veen
January 12, 2024 at 6:15 pm
mein honda civic 1999, EK/EJ nach Zylinderkopfdichtung weches wolle nicht starten.alle kontrol lampen brennen. verteiler ausgebaut Zündspule + modul gewechselt. läuft. habe motor abgestellt, wegen akühlwarren war noch nicht im motor, dann wollte er nie mehr starten 3 wochen suchen. verbindung vom Zündschloss zum Verteiler, kommt kein strom an. nicht zu finden. suche schaltplah. eventuel ein versteckter relaey. sicherheits schalter gefunden. perfekt. brauche dringens einen rat. Hans Ibiza Spain
Dan Kafun
June 2, 2023 at 4:03 pm
Eric… having an issue with my 2004 Element.. electric windows sporadically won’t work..I believe it has to do with the wire loom in the driver side door.. Do you have any videos on how to work/ troubleshoot the wire loom where it goes through the rubber conduit from the door to the Door frame..??
Thanks
Dan
Paul
January 10, 2023 at 10:58 am
Good morning. Im a ASE cert mechanic that was givin a 1994 honda accord ex 4 cyl. i put a quick used engine. I thought it would be a good idea to let my daughter use it as a car to learn how to drive in. I drove it for 2 months as a back up car and all of the sudden……. crank, no start. THIS IS WHAT I NOTICED. I turn the key to the on position…….. no fuel pump prime noise, check engine on, and d4 will flash. ALL INTERMITTINLY, you could sit and wait……. and sometimes click, all the sudden check engine light goes out, fuel pump primes and runs like nothing ever happen. MY QUESTION IS. is there a wiring diagram you may have or can get for me to start checking for powers, grounds and ignition voltages for the computer, ignition/fuel pump relay, ignition switch. Sorry to bother you. Im the kind of guy that wants to keep old cars going. Thank you in advance. Asking from Modesto, Ca
Vince
July 11, 2022 at 9:39 pm
Hi Eric just watched a video of yours for the Odyssey, I have 2006 Odyssey I put a new alternator , it’s not charging, wish you were near me you are the person I would trust. I changed alternator fuse 70/120 checked all fuses with voltmeter on ohms,. I really can’t figure this out , paid extra money for new , Remy , called Remy customer service state it’s possible the regulator inside alternator is defective. I’m not a mechanic, diy took me 3 hrs to do this,I definitely installed correct.it starts then battery is drained as I check w volt meter. If you have time can you please let me know what it could be so a least I can go from there thanks
Phil B
May 3, 2022 at 2:52 pm
my problem, 2015 elantra, the left turn signal lites , when turned on , blink at twice the rate as the right ones , left is abnormal condition, now if left turn signal on , and headlights turned on , the left turn lites both go off. any help here ???
Steven Ortiz
October 22, 2021 at 2:45 am
Tail lights on when battery gets connected
Felix Gonzalez
September 24, 2021 at 3:07 pm
I have a 08 accord and it has the vsa abs and brake light on and the dtc code DTC 121-11 I would like to know what to do because I have herd of a system update I called the honda dealer where I live and they don’t know what to do
Peter B
November 8, 2022 at 1:30 am
I have a 2013 CRV and a very similar problem. The VSA light comes on when in reverse and doing a turn. The code that comes up is 111-11 and here also the local Honda dealer does not know what to do.
george rodriguez
April 19, 2021 at 6:48 am
inhave a ford 2005 superduty 6.0 the 10 milli wire that goes on the positived side sparks
Randy
February 15, 2021 at 12:52 am
I have a 2010 dodge charge
When it’s in cruise control it starts to shuts off after a while the engine willt shut off.
The
Ross Melvin
October 28, 2022 at 10:43 am
HELLO…MY 30 FT. MOTORHOME HAS TOTALLY SHUT DOWN ELECTRICALLY. TURN ONTHE IGNITION AND NOTHING.. Was working fine till I parked it to winterize it..2 weeks later turned the key and nothing.Im wondering if it has lost it’s Ground….the vehicle is a 1997-98 GM Majestic Flyer ..
Remi
February 11, 2021 at 3:12 pm
Hey I have a problem I have a Honda Civic coupe 2005 I have change the speed sensors que the meter will go crazy and it shift hard a want to know what it could be because I already change the speed sensors
Shane Doxtator
February 11, 2021 at 2:52 pm
I have a 2007 Chevy Impala and i believe the issues i am having are stemmed from one. I was told it was a ignition problem don’t lock doors or it goes into security mode and you have to unplug battery until it resets. After i have drove it for a couple days i have realized that when the car isnt reading what gear it is in all the cluster gauges shut off and the doors start locking and unlocking, if i shut car off without seeing what gear it is in it goes into security mode. I also noticed whenthe cluster gauge goes out there is a noise by the fuse panel inside car
Areg
October 19, 2020 at 11:38 am
I have an 2012 civic ex. I have only 1 working key and want to buy 1 more and program it to the car… My question is what is the proper part number for my car key and how to program it to my car?
JERRY T.
August 15, 2020 at 12:39 am
I got this 2008 Chevrolet Aveo 5 4 door, can’t seem to find the power door lock relay for it the locks will trigger without hitting the key pad on my keys for it & it has drained batteries mis- firing. checked on all data but either I misnamed it or something
Nolan Ancel
August 12, 2020 at 2:57 pm
I have a problem with my 2009 Mitsubishi lancer SE… I am starting to lose hope. I did a parasitic draw test and took out 2 fuses that were pulling .84A. Now my car draws .07A or around 70mA. But my battery still dies… I don’t know what to do… I really love this car but for now it isn’t really reliable…
Scott
October 5, 2020 at 7:18 pm
Eric: finally a mechanic who understands cars.
Question: got a 2005 Chevy Express 3500, 6.0l, 4L80tranny.
Problem: when idling, vehicle shakes, kinda like a misfire. Their is no code.
I had a new engine installed, but new engine does same as old engine.
No mechanic has been able to guide me to a fix.
Any thoughts? 864-325-9939.
Juan Pendon
May 29, 2020 at 1:15 am
I got a problem on my dodge caliber 2008 sxt. Horn not working. I can’t find the horn relay. Need your help.
Jonathan Vasquez
May 9, 2020 at 7:51 am
Hi Eric, thank you for helping people like me fixing my own car. Have a question? My 1st and 2nd ignition coil is misfiring. They are both new with new spark plug. So i’m guessing the wire is broken is maybe the Computer module is messed up. Any tips or advice what to look for and how to fix it. Appreciate and thank you so much.
Ken Kastelic
May 1, 2020 at 2:51 am
Hi Eric, Love your videos. I have an Acura Tl type S 2007 (92k mls) and love the car. Its just i gave it to my wife and it just not her type of car. Maybe a little too sporty for her. I just watched you video on replacing the timing belt and water pump before i sell it. My problem is that i do have aftermarket wheels on it and the “TPMS” light is always popping up. How do i fix that issue? I bought new sensors and had someone install them but it still is happening. I want to sell it, but I want it to be a great car for the next buyer. Thank you so much for what you do. It is really appreciated.