You might be the only person on KZfaq that actually explains the right way to test a relay. Even after 8 years, in my searches I haven't seen one.

I just can't understand why this guy hasn't gotten more subs than he has. He's a mine of information, and he always conveys his diagnosis in such a clear manner - really makes you want to listen. You can tell by his voice how enthusiastic he is about his work. Let's just hope that those 81K are the cream of the crop.

how i found you is a mystery, this is such a great explanation on how one relay can be "shared" for control multiple devices,e.g headlights and control room,thank you, this saves me several electrical books

Matt you would not believe how much your relay videos have helped me! I now have total confidence to perform all these tests which I would not have dared to go near before. Thank you so much - once again you have lifted the fog. I would have to rate this relay series among your very best.

This is by far, the best video I've seen that explain how relays actually work. Plus, great instructions...BRAVO!!! This guy would be an amazing instructor.

Thanks a million for the videos!! I love the process of eliminating variables when troubleshooting. Keep up the awesome work, look forward to the next video.

wow! thanks so much! That is the clearest explanation I have ever seen. I knew nothing about relays before this. - I didn't even know what they were for. I also loved the video explaining how a Fuel injector regulator worked, too. Same clarity. You are a very gifted teacher.

Great vid Matt. Thanks so much. Your wiring diagrams are easier to read than a Haynes manual.

Excellent demo. Great communicater. Wish I had a master communicater like you as my teacher during my collage days, when I did my engineering subjects. I learnt these lessons, then, the hard way!! Matt, thanks a lot for your lucid explanation . It's simply amazing. I hope to see more such useful videos from you.

Thank you so much, I am working on a diesel Blizzard and now I am confident how to check the glow plug relay. Your video was long, but it was excellent and I watched it to the end.Cheers.

Excellent video on basic testing Matt. Great work. Regards,

Man your really smart. You know your stuff. All your videos are great from your Fuel Trims to MAP sensor and everything in between. You teach way better that my ASE booklets. Thanks for all your videos!

Really enjoy the way you explain things :) great videos

Really good video, thanks for sharing!

Matt, you may not be a professional mechanic. So, I will just let you know from where I stand. “You Sir, are an expert mechanic!” And, an awesome Auto Tech. Instructor. Keep on doing what you are doing and I certainly appreciate it. Thanks for your help and hard work.

You’re top of the line, just awesome...Matt 👏🏻👏🏻👏🏻. I became a do it yourselfer for so many reasons. I’ve come across so much BS from “professional mechanics” that just common sense makes me doubt in so many ways...it could be very confusing at times. A good one to diagnose would be a CCRM to a 1996 Ford Windstar...it looks a bit overwhelming...🤪

Most comprehensive. Thank You

Really, your vedios is completely informative. Thanks you.

Very Very Very informative. Thanks

great explanation matt thank you .

you are very intellectual professor. I enjoy watching your videos at all times and thanks for sharing your know how or knowledge to people like me..

Thank You well done!

Thank u sir ..its is very educated.i really had a very hard time to understand relay but u just made my life very easy. thanks

A very well organized and presented video. Thanks.

Matt. You are absolutely Awesome. I love watching your channel.

YOU are a legend, i’ve learnt so much. Everything on my motorcycle works, other than the high beam after I had a low side crash, this has given me enough knowledge to narrow it down. Its strange as there is not 12v going to the light socket (tried the bulb and it works) but there is 12v leaving the high beam switch on the left hand switch gear on the handle bar when its pressed. Frustrating but I’m sure I will get there

Thanks brother for the amazing Teaching !.

If it clicks you must acquit.... another great breakdown of the nitty-gritty

Thanks for making this!!!

Also, your video about fuel trims was very well done! When i was in automotive school I never really got it, but after your video it all makes sense!

nice work

Great work.

Glad i found u on youtube! Thank u

Very informative! Thank you!

Found this page today… awesome content btw 🙌

SWAPTRONICS ... LOVE IT... AME FROM THE DEPARTMENT OF REDUNDANCY DEPARTMENT HEH HEH THANKS FOR SAYING WHAT YOU DO ABOUT ELECTRICITY - MY KID IS AFRAID AS YOU ARE AND ALL SO SAYING THAT HELPS HIM AND OTHERS REALIZE ... ONCE YOU KNOW IT, IT'S EASY

First thing I noticed was the original description assumed the relay is normally open. That might confuse beginners at first but this channel isn't for beginners right? :P I'm early on in part 1. Hope you cover the difference between a 4-pin and 5-pin relay, normally closed and normally open. There are a ton of different configurations. That's why they print diagrams on them so you'll know exactly which configuration you're dealing with. So far this is a great lesson for how to test a relay without any diagram (paper or screen printed on the relay). When it comes to relays that's the most important skill to have and you seem to have a good grasp of it.

What a great teacher....

thank you!

Thx Matt! FN brilliant!

small point regarding to the relay my be the internal switch in NC normally close which means that without applying the voltage for the coil you also will have continuity not all the relay is NO (normally open)

The pins are either numbered or lettered on the case. In the schematic you showed, there was a diode across the coil. There are a lot of relays that have this diode built into the case. Like a tire full of air, a relay coil full of magnetism has stored energy. Then the magnet field collapses it makes voltage in opposite polarity because the magnetic field is moving in the opposite direction from when it was created by the source voltage. So much voltage that it can damage transistors that might be in its circuit. So we place a diode across the coil in such a way that it shunts the voltage during this brief time when the magnetic field is collapsing. The relay will work without the diode, but if you put voltage across the coil in the opposite polarity from the way is is designed, the diode will short and burn out because of too much current. A lot of time the ground side if the relay goes to the computer and connects to a collector of a transistor. When the transistor is turned on by the computer, then the transistor acts like a switch to ground energizing the relay. This is why there is a diode installed to protect the transistor.

Right Matt.... the same electrical engineers that designed the Delayed Accessory Relay to be placed right into the inside of the smart junction box on my 2005 Ford 3.0L. Impossible to repair if it burns or sticks opened or closed. Everything else can be replaced. Fuses, 30 Amp breaker, etc. The things that run the NECESSARY components on my vehicle. The one thing that's only for CONVENIENCE is built right into the Smart Junction Box. If this Relay goes on my car I'd need a new Smart Junction Box. Because I NEED to be able to run my windows, radio, moonroof, and have my interior lights on for 5 minutes after I turn off the engine. Unbelievable.

you remind me of one of my teachers in automotive school! you're awesome! Keep it up!

This cat is an allstar, right on bro'!

The way you wired that relay, when the relay coil is activated, that relay will create a short circuit. Dude, you are funny as hell.

KNOWLEDGEABLE Schrodingers box Tutorial great video brilliant thank you for sharing what you know with us. Take care and have a great day Schrodingers Box From Nick Ayivor from London England UK 🇬🇧

Thanks

ok thanks for that matt!!

Very good videos

Awesome tutorial. How anyone can thumbs down it is inconceivable! Going to watch part two now. Just liked and subbed.

Just finding your videos and you are pretty detailed and correct in what you say. But your teaching skills stand out the most. Turning over the circuit tester cuz it was scary and complex looking lmaooo I'm dead 😂 so conscientious of your audience. Edit. Sp

One evening my headlights never turn off even if the headlights stalk switch is turned off. I had no option but to disconnect the battery wait until next day and connected back and took it to the repair shop. Now I understand the reason, good to know . Had I known this before, would have tried replacing the relay on my own and saved few bucks.

Thanks again.

Hi Matt, could activate the subtitles in your video please. another great video, thanks for your work and dedication =)

Thanks very much, the video is very much appreciated. I have found the hard way how important to understand the car electricity actually is, which brings me to my question: How do I begin to study the car electric system without having to go to school? I think I'm a bit to old for that! I do appreciate all the help you may be able to give me to understand the why, how, and the what happens in order to diagnose any problem. Or at least get close to the problems at hand. Thanks very much.

Great diagrams. @sshats lol v funny and insightful. Thks

Love it .. keep it up 👍

At 10:20 I was very intimidated. I nearly turned off the video. Thank you for turning it over.

The best teacher

Although he says that electricity is not his strongest suit, he seems to cover every aspect of electrical auto diagnosis in his videos. Imagine his depth of knowledge of his stronger suits.

'Elektrictrickery...' 'THE Dark Art' of automotive diagnostictrickery!!

Superbbbb

Matt, Congratulations, You took the plunge! ( Electrical Diagnostics ). Suggestion, you should use a test light when applying voltage to the relay not a jumper wire as you suggested. The test light will limit the flow of current should you be on the load side of the relay. Most incandescent test lights will limit current flow to around 200 ma or less and most coils in the relay only need around 100 ma to energize them. I know this was a simple circuit ( I suspect you picked the horn to limit damage should you mess up! ( LOL ) but most circuits ( window, heater control circuits, etc. ) do run thru the PCM or some other computer module on most newer vehicles these days. Using a jumper wire is just way to risky! When using a jumper there is no restricting of current flow and you could instantly fry your PCM!

Amazing video. I have always had a hard time understanding relays. Watching this video made me feel stupid hahaha I've always been able to test fuses. All the wiring but once I got the relay it had me confused but you made it look so easy that even Scotty kilmer could do it... Actually he would probably put in a new one and try to tell you that it's a general maintenance item. And with 2 grunts it's out and in. Presto. You are almost done... Still doesn't work but we can rule out the relay now. Right? Hahaha

thank you for this video. I learned a lot from your clear explanation. It also provokes a question. At approx 23:50 in the video you did somthing where you might want to warn your viewers. This is where you send 12 volts into the ground side of the relay (horn) circuit. This is done to prove what side of the circuit the horn is situated (I believe you did this because your were certain the horn was located). However would it be appropriate to warn the viewers of the risk of injecting voltage on the wrong side of the load circuit? It seems to me that it would not be a good idea to guess and inject 12 volts on the wrong side of load circuit. Please let me know if you agree or disagree. Thanks.

Matt, thx alot! But I've been more of a motorcycle backyard mech. so cars can b a challenge (I just bot an AMG Mercedes!) So what I've heard so far on your horn circuit instruction is the power (red) goes through the relay and ends at the horn (which beeps) to ground. If that's correct then would headlights r tail lights b on the same type of circuit?? If that's so then wouldn't all that circuitry put a great drain on an alternator esp. at night? Or take a cable put it on the + side of a battery and ground it. What happens? Sparks and great heat. Like u said in the beginning, 'electricity can b daunting'!

Hi Matt..just came across your channel....thanks for sharing....One ? if you would might know.... the air/fuel ratio on a normally asperated car vrs. turbo charging that car....does the ratio change with added forced air.

Great channel, love the theory. I had a crank no start engine on my 2001 focus so I viewed this video for some inspirationation thinking. BEHOLD I had a bad fuel pump relay!

can the relay load side ever be on the negitive wire of the device they are powering up or are they always spliced into the positive side?for example instead on a typical 4 pin with relay removed then seeing usual power/ground on control then power/ground on load side could it be power/ground on control then ground/ground on load as its spliced into the ground side instead?

Hi, I enjoy watching your videos which are very relatable. Ihave learned a thing or two from them. I'd like to point out that starting on 23:25, your concept of where the horn is, could be a little confusing. My take would be that the horn IS the point where the ground and the hot meet. Both sides of the load circuit in the relay are the "hot", just a switched hot. The actual "ground" side of the load circuit is between the grounded end and the "negative" side of the horn. Keep up the good work !

you do know that you were lucky at 24:00? Your test guessed the horn to be on the ground side but you had no guarantee that it was on the ground side. All of your tests done prior to that had not determined the location. If you were wrong, you would have created a nice short to the ground when you went to blow the horn. Fortunately, your PowerProbe3 has the circuit breaker in it, so you would have been spared from any real damage but personally I think that was not the right thing to do if you are in a teaching mode!

Thank you! Excellent explanation of relay basics. Please continue the good work because you are helping people that you will never meet and that makes the world a better place. No reply necessary.

Thanks for another great video Matt! Just a little feedback: I think you assumed all relays are Normally Open. If that had been a Normally Closed relay then you would have continuity on both sides and probably set you off direction... Another good thing to do when checking relays is to measure the resistance of the coil. Most cars I've seen, have more than one relay of the same type and maybe a "relative"value between them can help you know if the relay in question is good in the case that you don't have the specs for it.

And also maybe one of the reasons to use the relay is the PCM which it can't give a high current to all systems in the car it only provides the signal to let the high current activate the component

Awesome video. I love watching these because I'm an aspiring mechanic. But I have a question. How could I test these circuits and know I won't fry my pcm?

The power side switch is in the relay. The control switch is outside the relay. The horn is a load.

Great video. When you were powering the relay up, how come it didn't matter that you connected the negative alligator clip first & the positive 2nd?

Thank you for spreading the knowledge! Working on improving my 2%er skill set using my fluke 88V

not to be picky but voltage does not move it is applied. also current flows from negative to positive. conventional theory has been replaced by the electron theory. this does not really matter with most electrical diagnosis, but does at the electronic level. fine job on explaining relay theory and operation.

i have an 2002 audi s4 wont start after smoking in the coolant container what relay can i change

have a question matt, trying to get a good grasp on electrical here.... These two vids you clearly show measuring voltage and resistance in circuits which as I understand is safe regarding 12 volts....when would it be beneficial to measure current in a circuit...and for what purpose? ?..as I have read a little that current is dangerous to measure and most dvom's only read max 10A... Thanks in advance! !

Mat you are correct my dads van fan stopped working I checked relay just by the click but I decided to replace it and fan started working again i was confused i thought the click meant its working

You narrowed it down to either an an open on positive side or a bad switch (control side). What if the input was controlled by the BCM? Is this possible? How would proceed to troubleshoot in that case?

Great video, I’ve watched your channel for a couple of years now and i also subscribed to the SBQM channel. Just out of curiosity, when applying 12V on the Load ground side, If you were not certain that the horn was on the ground side and it happened to be on the power side, were you not concerned with applying voltage directly to a ground, or you knew for sure it was in the ground side.

Hello... I have a question. Why am I getting 3.6 volts on the control negative, does that mean a bad ground. It's on the fuel pump circuit...thx

Can you do something on inverters & how to choose them on the white board of knowlege? I blew up many fuses trying to power my laptop in my car cause I dont understand why or I dont understand peak powerer, start up power, should I be looking at amps or watts? does higher volts mean more power? Theres info but nobody breaks it down like you... how long can I run my car battery on a laptop.. should I recharge my car battery... I have no idea with these questions but would love to be armed with the knowledge

A 12 volt test light and the proper knowledge of how the circuit works. can tell ya alot

What do you call INTELLECTUAL AND PRODIGY and SUBSTANTIAL add one more KNOWLEDGEABLE the answer is Schrodingers Box My teacher Thank you very much helpful information video tutorial outstanding video 👍 👌 Take care and have a great day INTELLECTUAL Schrodingers Box From Nick Ayivor from London England UK 🇬🇧

Your channel has changed the life I live in my garage. Really. I just wish you lived closer so we could drink beers and talk shop. Plus, I can't figure out why my Golf runs rich. Watched all your fuel trim videos and diagnosing and I must be missing something. @_@

Noob here, are you using resistance option on the multimeter? I thought it could be done in a live circuit. Thank you

At 17:30 when you press the horn button you are on DCV not continuity. You do get a good 12v signal but that's not exactly a continuity test that's a voltage test. If you would have had the meter setup on continuity the meter should have beeped when you pressed the horn. That's a much easier test when you are in the cabin far away from the meter as the audible alert would let you know you are getting the desired result.

I've posted some additional resources and other tech tips on the Comments below Video Part 2.

I have a 2005 gmc envoy. I have a starting issue.if the car sets. Like all night. It starts every day. I can start in the morning and drive fine. When I stop. And park , and go do what Iam doing. I come back and no start. Crank only. Then I have to let it sit again for about 3 hrs. My pwr/ trn relay get hot. Also I can’t turn the key completely to off position. I know it could be many possibilities.

At ~23:17, where you talk about giving 12 volts to ground to test the horn, you say the load (the horn) is most likely on the ground side. But what if you lacked that sort of prior knowledge and guessed wrong (i.e. what if the load were on the power side), would you be putting 12 volts straight to ground, creating a short-circuit?

If the switch in relay clicks and has continuity is possible it still can't carry a load?

Matt, to fully diagnose relay issues wouldn't you have to test both the control and the load side under actual normal operating load and not just run a voltage and/or continuity test? A length of relay (or other) circuitry that reads proper continuity could still show improper functioning (a large voltage drop) under load, no?

I'm a "KZfaq Certified Mechanic" from watching you're videos. I might even be better than some "real" mechanics. :-) LOL

At 16:50 you say you're looking for continuity in the control side. You will never see continuity there across these two nodes, so not sure what you're looking for. When you push the horn switch, you still don't have continuity on the control side, you have merely closed the ground circuit so you then have a voltage potential across the control connections of the relay and hence you see 12VDC. Good video though.

If you want to make life easier for yourself and don't have any real scan tools, you should invest in a PowerProbe 3 and The Hook. I specialize in computer systems and electrical diagnosis. I noticed you guessing a few times or not checking (Maybe for the sake of the video), but I have used scan tool/scopes that cost more than my car and my $400 powerprobe setup never fails me.