The Trainer #78: How To "Read" A Wiring Diagram

#automotivewiring #schematic #blockdiagram #wiringdiagram #electricaltroubleshooting
“read·ing (ˈrēdiNG/) noun
1. the action or skill of reading written or printed matter silently or aloud.
"suggestions for further reading"
synonyms:
perusal, study, scan, scanning;
o
2. an interpretation.
"my reading of the situation"
synonyms:
interpretation, construal, understanding, explanation, analysis”
When you “read” a wiring diagram which definition of “reading” are you using?
To be successful in diagnosing any electrical concern, techs must be comfortable in three areas; electrical fundamentals (volts, ohms and amps and how they all interact as well as how a fundamental circuit operates), using electrical testing equipment (the basic functions of a Digital Multimeter is a start) and how to “read” a wiring diagram. This month, we’re going to focus on the last.
Reading a wiring diagram has often been compared to reading a street map. You have to follow the “roads” from one component to the next to see how everything is connected. And that is true - to a point.
It is even more important to use the diagram to understand what is happening in those wires and components as well as when it’s happening. For example, if the blue/white wire starts at a fuse and ends up at a relay, is that wire carrying power all the time or only when the key is turned on? Does that wire supply power to the control side of the relay or the component side? What causes the power to switch on or off - a manual switch request or is it a control module driven control?
Truly “reading” a wiring diagram will help you in your troubleshooting. Often, with this knowledge, you can isolate a very narrow section of the wiring to focus your actual testing on. And that’s what we’ll focus on in this month’s “The Trainer”.
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Due to factors beyond the control of Motor Age, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. Motor Age magazine (UBM Americas - Automotive Group) assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. Motor Age recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video. Due to factors beyond the control of Motor Age, no information contained in this video shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not Motor Age magazine or its corporate parent, UBM Americas.

[@coreydinsdale7258]

Clear and concise, very good. Excellent and understandable and to the point.

[@dgreathouse10]

Pete you are a awesome teacher. First time I have watched your KZbin videos God has definitely given you a gift to teach and wisdom to understand. Thank you for your willingness to do this.

[@peacepoet1947]

Tell me what you learned from the video. He asked if you had a problem with the positive side what would you look at? I would try putting the correct voltage to the first fan and then try putting the correct voltage to the second fan. If the fans work which I don't think would happen. I'd then go to check the thermostat for proper signal to EMC to regulate gas flowing into the cylinder.

[@larryscarry411]

Very good explanation of how the wiring diagram runs, and thank you for sharing your knowledge Pete!!

[@paradoxdea]

Love you man! You're the perfect example of an excellent teacher!! Bless you, sir!!

[@MotorAgeMagazine]

Thank you!

[@diagnosej.j.6833]

Thank you very much for the lesson, I watch all your issues with pleasure.

[@donovanlucibello379]

Pete, your videos are a gift

[@kevinjohnson9736]

This is an area that I need more skill in. Thanks for the video.

[@ReadyAsHeWillEverBe]

Hi Pete, thanks for taking the time to generate this video. Assuming I already know which fan is not working, my first step would be to find this fan on the schematic. Then the schematic can be my useful tool.

[@davidostring7627]

Thank you for sharing your knowledge about wiring diagrams. That's some good information to know.

[@salehal-saleh2158]

I swear I see to much people explain and I don't understand put in these video I learn too much thing you are great man

[@fieldsofomagh]

Really in depth assessment of the problem and all the relevant schematics explained. A replay is essential to grasp the concepts and skills necessary to make sure that the problem is tackled, correctly.

[@TheDisgruntledMechanic]

Thanks Pete! Great video.

[@mikeroyder886]

Thanks Pete

[@joaquinmariano6286]

It does help my little knowledge a lot. Thank you!

[@ramonm.1553]

Thanks for sharing your knowledge 🙏🏽👍🏽

[@davidm3210]

Very helpful introduction to w.d. - thank you, sir.

[@victorlewis8970]

Thanks for all this information.

[@trentmozingo7176]

Great video! Thanks for taking the time to make and share these.

[@Blazer02LS]

Testing would depend on what tools you have on hand. Without a scan tool and only a test light and jumper wire as my tools, I'd pull the FC1 relay and connect my test light to a good ground, then verify power from the fuses to both the coil B4 and switching B6 contacts in the elec. center. Then test fan operation by jumping the B6 and C4 contacts. If the fan runs I know the fan, ground and power to it are good. If I show power on the B4 contact I know that the wiring back to power is good. Now I have narrowed the problem to either the control side wiring and driver transistor OR the relay I am holding. Testing the relay can be done by connecting power to the contacts on the relay that correspond to the B4 and B6 connections, then using my test light connected to ground I can touch the C6 contact to a ground point which will activate the relay (if the coil is good), and use the test light on the C4 contact on the relay, if it lights the relay is passing currant through the switching contacts. Next would be to find the PCM and the Dark Green wire. Disconnect the battery negative connection and then remove the PCM connector that the DK Green wire is in. Use the jumper wire to connect from battery positive to the C6 contact in the elect. center, connect the test light to battery negative and GENTLY touch the correct pin (B6 in this case ) in the connector shell. If it lights the wire is able to carry enough current that the relay will operate and the issue is a failed driver as the rest of the circuit tested good.

[@johnpalma7265]

Blazer02LS : Thanks

[@ThunderbirdRocket]

Thanks for sharing !!

[@Chaddz3]

what about the grounds for the PCM itself.. if the PCM isn't grounding properly, the driver may still be good and closed but the driver itself isn't connecting to ground so the relay never turns on. Also need to verify that the PCM is receiving the proper inputs (ac request signal or high engine temp signal) so that the PCM knows that the cooling fan is needed.. if the inputs aren't telling the PCM that the cooling fan is needed, it wont switch the driver to ground the fan relay and the fan wont turn on.. (Some systems will run one fan for engine temp only and the other for A/C only..)

[@joaquinmariano6286]

Thank you!

[@shawnhimel1969]

Another great teaching video Pete. Keep giving back. Your teaching process will make better TECHS if they are smart enough to watch your videos and obtain the KNOWLEDGE that you are giving back to them as well as teaching them the proper diagnostic process.🙏🙏

[@modousawaneh2005]

It's indeed a great tutorial video. Thanks a lot for sharing

[@georgeflensburg3087]

Thank you! You are awesome.

[@seshachary5580]

very educative. Thank you for the great teaching. regards

[@Jo-hannson]

thank you sir! very informative

[@sr5drift]

I just found this video but it has helped me understand automotive wiring and will help me with my project cars. Thank you for taking the time to dumb it down to help people understand this topic.

[@edwardlara1011]

Where do you get that he's dumbing it down? That's an insult for an instructor. He's not talking like he's talking to retards. He's talking and explaining at a normal pace. Don't be a dumbass and say stupid shit.

[@FilterYT]

Well done, thanks.

[@deqa]

This was definitely one of the BETTER videos on fundamentals for understanding wiring diagrams and applying them to the issue you have, figure out all the elements interacting with your circuit (where the load is, ground is, fuses, relays are, controls) and you can dissect the issue with the circuit through deduction of these elements. Another MotorAge video on this idea where he's like "go down and down the circuit with your multimeter, isolating that problem" also is incorporated here via use of the wiring diagram.

[@edwardlara1011]

This is electronics not chemistry or physics therefore they are called components not elements.

[@michaeldunagan7838]

My Answer: Check for Battery Voltage at the Cooling Fan. If not Battery Voltage at the Fan, then there is unwanted resistance on the Positive side of the circuit. Corrosion, damage, or partial shorting to ground, all whic may be a cause of the problem.

[@rissanew858]

All wiring diagrams BMW from 1998 to 2022 are in this application android (Bimma EWD)

[@khannavarunobzvk]

Man, you guys are geniuses, this helped me a lot

[@MotorAgeMagazine]

Glad you found it helpful.

[@wackey3455]

Cool video

[@zenebehagos3537]

thanks more for your kind offer

[@sambitar8448]

DEAR SIR: THANKS A MILLION FOR SIMPLIFYING THE DIAGNOSTIC APPROACH BASED ON THE LOGICAL APPROACH EVER. YOU ARE THE FOUNDATION OF OUR JOBS. KEEP UP THE THE EXCELLENT WORK EVER.

[@ParabellumX]

First, I would first check that fuse. Then, if it was still good, I would then proceed to checking that ground connection at that splice. If that was good as well, I would next test the resistance of that cooling fan, and compare it to the resistance of the other Cooling Fan, #2. After all, Occam's Razor. "When there's multiple solutions to a problem, the simplest answer tends to be the correct one."

[@GripFreak]

Great video! Slight correction though. At 16:11 you state that there's a diode in parallel within the relay. The symbol indicates it's a resistor in parallel and not a diode.

[@selwynadams9592]

I would check for power at the relay coil and then for switching at the computer. By the way the fan relay has a resistor in it.

[@NewLevelAuto]

The coolant fan circuit grounds at g117 in GM S.I 🙃, Bosch diagrams g110 and Mitchell g110 , Alldata g117 😂 I sure do love the video Pete . Great job

[@NewLevelAuto]

Would you mind if I do a sort of follow up using this video as a reference? Reasons why one source of information is never enough?

[@MotorAgeMagazine]

Thanks! I think the ground designations are dependent on the source. Mitchell numbers from front to rear on theirs, as do the others, but the numerical designations may be different - as you pointed out.

[@MotorAgeMagazine]

Not a problem! We are all trying to do the same thing - help techs.

[@rusco321]

New Level Auto Get back to work Keith. Those 20 vehicles aren’t gonna fix themselves.

[@MotorAgeMagazine]

LOL!

[@youtube.comdarlingbrank9290]

Desde bogota,, gracias

[@frankdavidson9675]

if i may comment i am 88 a retired repairman built my first meter from a mail order heath kit in 1965 iwas analog only back then ---- just looking at what you are showing i fill it just a little bit to much to start with by that i mean instead of an auto with lots a wires in a drawing its a confusing a golf an electric is a good example of what i am trying to say vary simple 1 wire at time all color coded - the charge circuit is is under left side seat is a good start point for a no run tbl. a 36 volt bat arrangement the you have a lead that goesto the dash to a switch from that switch it gos to throttle which is under the floor from it gos to the fwd rev trans then o the control solenoid that applys power #^ volts to drive the wheels a basic ohm meter will work great on this type tbl and give confidence to a rookie repairman i hope this helps some one a simple cheap meter is all that need to learn on too many options on high $$$ meters young guys are lost

[@thereckchanic4709]

I'd check the fuse on the positive side.

[@196clint]

Hello, Pertaining to the question if the problem were on the power side for the a/c fan issue and not on ground side, I would check circuits from the top down to the path to load (fan),and relay module or switch since it is a negative temperature coefficient issue i believe it would be a relay problem wire connector loose or burned wire, would use a ohmmeter to test resistance in current, and check specific wires for shortages opens or faulty loose connections using voltmeter since it supported by variable resistant capabilities.

[@caduceus33]

Why didn't you put a link to the wiring diagram? That would have been a great help!

[@Chaddz3]

How I would diagnose the fan issue. #1. Swap relay from known working fan to bad fan (does the fan run if so, bad relay, if not go to step 2.) #2 place a jumper between bat source terminal on relay connector to load side of relay connector (does fan run, if yes all wiring for the load side (power and ground are good and the problem is on the control side. If fan doesnt run. using a test light to ground check for power on bat source terminal at relay... if good.. connect test light to bat positive and check continuity from fan motor to ground.) Once I verified the load side of the relay is good and it turns out to be a control side issue.. connect a test light to control side power terminal and touch test light to ground.. does light come on, if yes.. Go to step 3 if not. correct open circuit between battery and relay control power. #3 with test light connected to battery positive.. probe control side ground and command fan on with scan tool. if light lights, verify with scan tool that inputs for PCM to turn fan on are receiving a signal.. (diagnose and repair those as needed). if the light does not light when commanding fan on with scan tool, verify signal at PCM connection.. if still no light.. Check the grounds for the PCM itself, if PCM grounds are good, faulty PCM.

[@mechanicshaficug9456]

I would check the power to the fan motor connector and if no power I would check fuse and relay ,plus temp sensor conector to ensure correct ground

[@How2Wrench]

Very nice work. New fan and sub...

[@MotorAgeMagazine]

Shane Conley Thanks!

[@How2Wrench]

I've been teaching Motorcycle Mechanics for the last 12 years and am now the corporate trainer at Suzuki in the Motorcycle and Marine Divisions. Check out my work if you'd like. I'm sure yours will inspire me and I have already shared your website with some friends in the auto field. I can't believe we never heard of this at the college. Then again not all folks see the value in cross-training and networking together. Keep Wrenching, Shane Conley

[@frankdavidson9675]

a lawn mower is a good easy wire diagram to practice with no diodes and res to wory about just relays and wire most wire s color coded switchs show a lletter a b c etc very detail but simple t tess you where each wire go from a to B and the color its ver practice so get you digram for your mower somare listed in maint.manual on line you can print you a copy if you have a printer

[@MD-173]

A link to the chevy lumina wiring diagram would've been great.

[@k50159]

Thanks so much!

[@youtube.comdarlingbrank9290]

Gracias,,,lo sigo aunque mi inglés es poco,,,,pero ahí voy aprendiendo incluso inglés!!! Gracias

[@MotorAgeMagazine]

¡Gracias! Me gustaría poder ofrecer más traducciones al español, pero es difícil de hacer. ¡Me alegra que puedas seguirnos!

[@SurferJoe46]

A lot of this guessing about IF the ground is at fault - could be quickly checked with a cheap HF $9 logic probe. If there's a ground at the other end of any wire, any circuit ---> it'll turn green. It's good to learn theory ... and I'm all for it ... but open this class up to better diagnostic tools too.

[@solomonakpojovwo2607]

Is it possible for an ecu to supply power and ground to the control side of a relay in a circuit? Not this particular circuit though

[@Rick-tb4so]

At what point do you check the fan ??

[@JD-526]

Where do I get this wiring diagram. To learn I need to practice.

[@bobkassmeyer6122]

Pete, I would, on the nonworking fan, check for voltage at the cooling fan first with a volt meter and if there were voltage present I might try with a head light bulb to load the circuit. If there is voltage I would replace the fan motor. We did already check the ground with a voltage drop right? If there was no voltage I would then check for voltage at both relay terminals 30 and 85. If voltage is present I would check voltage at terminal 86, if there were voltage there I would request control with a scan tool and look for the voltage to drop when the driver is toggled on. If there is no drop there I would try grounding the relay at terminal 86 with incandescent test light, if the fan ran I would check for positive potential at pin B2 or B2 depending on which fan was not operating by back probing the connector, if potential were there I would again try grounding with my test light, if it starts I would check pin fitment of the connector and if OK I would replace the PCM. I did have one question though, you said that on the control side of the relay there is a diode wire in parallel with the coil, is that not a resistor shown in the diagram?

[@MotorAgeMagazine]

rwbishop already beat you to the resistor in the relay! I'm so used to seeng diodes I jumped right past it! A high value resistor performs a similar function to that of a diode by absorbing the high voltage spikes created by the collapsed magnetic field. The disadvantage of a resistor is that it allows a small current to flow in normal operation of the relay (unlike a diode) and is not quite as effective as a diode in suppressing voltage spikes.

[@SEDHACARREPAIR]

I would do all the diagnosis at the fuse box where the relay goes Give direct power to see if the fan runs to make sure that circuit is ok and check if pcm is sending ground( if the driver is ok), relay as well Fuse obviously will be my first bet Not one but all is another bet All learnt on Ac compressor check video from you Applying here Thanks Teacher

[@edwardlara1011]

First and foremost, I would do a scan to assure proper PCM inputs. Assuming there's no DTC from the scan, I would start circuit testing beginning at the bum load, fuse box, power distribution center, and fan relay, ohm respective conductor leads from power distribution center to PCM connector pins for continuity. This is my condensed version for troubleshooting a bum coolant fan.

[@harveyferrolino5562]

Fuse side

[@kckuc310]

Could you not jump the relays to eliminate half the circuit?

[@MotorAgeMagazine]

If you mean bridging the pins that supply power to the fans, then yes - that would prove out that that side of the circuit was working properly.

[@POP-ir5jx]

since this is a secondary fan circuit, I would first determine the conditions that cause the fan to activate? Perhaps the engine is not reaching a high temperature?

[@solomonakpojovwo2607]

I wil backprobe the blue wire first to carry out a voltage drop test. And secondly check the fuse or the wire that is connected to the fuse on the relay location to see if I have power there. Third,I will if the pcm is doing its job by checking the control side of the relay. Then i should get a way out to solve the problem

[@rwbishop]

Curious... whats a 'diode wire' as mentioned starting at 16:09? I saw a resistor in parallel with the relay coil... but no diode/s.

[@MotorAgeMagazine]

Great catch! You're right, it is a resistor. I'm so used to seeng diodes I jumped right past it! A high value resistor performs a similar function to that of a diode by absorbing the high voltage spikes created by the collapsed magnetic field. The disadvantage of a resistor is that it allows a small current to flow in normal operation of the relay (unlike a diode) and is not quite as effective as a diode in suppressing voltage spikes. Thanks for bringing this important distinction to everyone's attention!

[@garyvanremortel5218]

The use of a resistor alone for flyback suppression is frowned upon due to the conflict between making the resistor large (to avoid excessive steady-state dissipation when the coil is being driven) and making the resistor small (to limit the back-EMF voltage in a meaningful way). A more practical non-diode-based snubber network uses a capacitor in series with the resistor -- this allows a small resistor (say on the order of 30-100 ohms) to be used without dissipating any power in the resistor under steady-state conditions.

[@rehannazgulkhan8020]

Thank you sir, suggest me and teache

[@douglastaylor2141]

I'm putting a new fuse,relay box in my 2002 Mitsubishi eclipse GS with a 2.4l engine manual 5 speed the fuse box was cut completely out.nowI have the problem of figuring out what wire gos to what

[@marshallzeek4650]

Well assuming I know the ground side is good, and my white wire to fan relay 2 also good. I would go straight to dark blue wire at the control side of fan relay 2 and check for continuity to pcm. If good, then I know the driver is bad.

[@POP-ir5jx]

Not to nit pick but B4 is a resistor and not a diode.

[@rain0aldwaib]

the question is how do i get the wiring diagrams for different manufacturers your diagram seems a high quality diagram

[@ramonm.1553]

All data, but you gotta pay for that I think

[@arnoldabadilla3291]

could you please do a video on ford mustang (any model) from 2005 to present?

[@jeffreywilsonrubikscubeonb2428]

I would not start my diagnostic process with just a work order that said "customer states one fan is not coming when it should". Most customers don't know when one fan should come on or not, let alone two! I would give my service writer a gentle but firm reeducation on how to do their job properly and ask them to get me a DETAILED complaint. While I was waiting I would bring in another vehicle with a properly written complaint...diagnose it and have it fixed before the service writer probably even made contact with the customer about the fan issue. If they didn't get a proper complaint, they probably didn't get a phone number either. I would probably fix 3 or 4 more cars before the fan customer came back to pick up their car expecting it to be fixed.... then i would speak to them myself and find out that they were really having an issue with the blower motor fan. Then I would pull out the the rats nest from the blower motor and suggest they watch KZbin next time.... 😉😂😎 #iworkintherealworld lol

[@asifsheikh6791]

Sir please diagram site

[@babblingidiot7903]

is there a website that allows DIY at home have access to schematics, wiring diagrams of any vehicles?

[@MotorAgeMagazine]

Sorry for the late reply. Try www.eautorepair.net/

[@jesselyons2002]

I just need to see you do this in real life. Im a better visual learner.

[@grindhard4312]

Yeah me too

[@reynoldfloresluna8896]

Excelente ke precio tiene la información diagramas p ecus pint out inmovilizadores módulostransmision electrónica sensores actuadores p autos asiáticos europeos chinos etc precio

[@adon2424]

that "diode wire" is actually a resistor symbol.... perhaps the diagram is incorrect... oops! i see that the discrepancy has been addressed....

[@SEDHACARREPAIR]

That way i dont even have to reach the no room connectors as well

[@rmatrix4777]

"Did you just see a re-e-a-l bri-ight light?" "I see the truth of it!" "Dr. Livingston, I presume?!" "Go west, young man." "Pork chops and applesauce." "Frankly, my dear, I ..." --nevermind (don't ask, idk)

[@rmatrix4777]

Hey, wow, glad I found you (again). Back for a refresher. Thx, Boss!

[@user-pg3ev5kz2p]

Схема понятная, переведите на русский пожалуйста, если не сложно

[@renegosek859]

check motor2 b side. ?reading.

[@nickayivor8432]

SUBSTANTIAL Sir Pete Meier Motor Age Thanks 👍 Sir Pete Meier Motor Age From Nick Ayivor from London England UK 🇬🇧

[@mikepetrus9501]

if one motor work the other one don t not wiring or switch it the motor

[@HAMILTONPROVIDEO]

Diagrams need to be larger

[@ohmslaw6856]

Check fuse lol

[@dennismusgrove3824]

To Many commercials, I trying to learn and keep losing my train of thought ???? because of all the commercials , guess i will move on to another video .........

[@MotorAgeMagazine]

Sorry about that. A few of our videos have the "during" selection and I'm trying to remove them as I find out about them. Don't give up on me yet! 🙂

[@bryandavis341]

Its funny you say to not learn from youtube guys and your video is posted on youtube! You have to take anything and apply it yourself ,no matter the source, and see if the information is correct

[@MotorAgeMagazine]

Bryan, we encourage our audience to make sure the source of their information is valid - no matter where it comes from. There are a number of great automotive channels on YT but for every one that is teaching correct processes and procedures, there are several that are teaching hack techniques that can cause more harm than good.

[@bryandavis341]

Motor Age You should say that instead of discrediting all KZbin sources

[@MotorAgeMagazine]

Regular viewers of our channel know we support, even promote, YT channels that contain great content. Check our home channel page for a list of some of them. But, it has been our experience (and judging by many of our readers' comments, their's to!) that most contain material that can be seriously misleading. It's not just professionals that view these videos - it's consumers and DIYers that end up in your shop telling you how to fix their car, based solely on what their Google search or KZbin "guru" said.

[@bryandavis341]

Motor Age YOU said what you said. I do watch your videos, I work in this field EVERY SINGLE DAY, ASE certified, degree in automotive technology, and I still watch KZbin sometimes. Don't try to make me out as a idiot. You said you can get in trouble watching KZbin sources, when you should have said that some are good and some not so good.

[@MotorAgeMagazine]

Bryan Davis Nobody is trying to make you out as an idiot, and if it came across that way, I certainly apologize.

[@kenswitzer4133]

Kill the music