MUST WATCH - Can a Test Light Damage Your Cars PCM ? 

Hey Do it yourselfers, sorry for not uploading videos for a while, I've been super busy doing projects around the house and with my election campaign. Will be uploading soon and updating you all. cheers

Your videos are to the point & thorough, without rushing to complete a/some thought. Not many engineers, electricians, etc. can explain as well as you do. Thank you & please keep up with the good work.

Is everything ok? Haven't seen any new videos from you in a while I hope everything is ok. Your videos are great hope to see one soon.

Well stated! I'm willing to bet that the vast majority of misadventures happened because of more glaring ignorance, such as accidentally jumping the high amp side of a removed relay into the computer ground relay pin for the control wire. In fact I think I remember ScannerDanner telling a story where he did just that, killing the PCM. I've seen guys apply 12 V with a power probe straight into a communication circuit, relying on the internal resistance inside the PCM to come through. Not wise, even if they do get away with it. Yup, test lights are incredibly good at limiting current, and you get a bonus light to tell when significant current is flowing. Also your remarks about current are spot on, power is what fries connections, and power is the product of resistance and the _square_ of current (voltage drops out of that equation). The test light is critical insurance, especially if you use a low amp light. Many guys use an LED test light when testing computer circuits, or even, as you say, the human body. All that said, I don't know how the op amp devices inside the PCM are designed to react to voltage the circuit is not designed for. There's something very appealing about not sending anything to the PCM that it doesn't see every day.

The gynecologist joke was gold. LMAO I'm stealing that one. Bless you for trying to squeeze a 40 hour basic electricity class into a 12 minute video. Important thing to remember is that Volts are pushed, Amps are drawn. Example: a 12 volt battery in a 6 volt system will fry anything that gets turned on regardless of amps drawn but you can run a small 12V test light with power from a 12V, 800 CCA (Cold Cranking Amps) battery without worry. AND, electricity, regardless of voltage, amperage, or if AC or DC always follows the least resistive, path to ground. BUT, the higher the Voltage (not amperage), the more resistance it can overcome. It's high voltage (like spark plug wires) that will let you know you really screwed up, quickly. Happy New Year!

Hey man, great videos. I’m a new subscriber and inexperienced on working on cars. Besides a Haynes catalog, old guys at work, and a few other RU-vid car guys, you have been a big help with your knowledge and videos. Simple, clear, and informative. Top 4 RU-vid car mechanics that I follow. Excited to see what you put out next. Keep up the good work brotha. 🤘🏽

I learn alot from ur video's. Could not stop laughing though, when u said " oh my god, im dead".

Remember folks he is NOT an Electrical Gynecological Engineer, but he does play one on RU-vid.😉 Great video.

"These electrical engineers that design this stuff"... Jerkoff hand motion.... Priceless!

A couple of observations: 1. "It's the current that does the damage, not the voltage". While mostly true, in practice it is the voltage that causes the current in the first place. (and it is voltage that kills electronics ) 2. Electron flow (from - to +) while technically correct, it should be ignored as it just causes confusion. Convential Current (from + to -) is what everybody else uses. 3. I am an electrical engineer, and I am confused after watching this video. You should have just lead with "Yes, a test light could (in theory) damage a PCM, the engineers have built lots of protection circuitry around the inputs to prevent this happening".

Never stop pretending

Some output circuits controlled by the PCM are low amperage circuits. Some test lights flow much higher amperage than the one in your video. That could be a problem...

Can a test light damage a PCM? Yes. Here's why. The test light in this video takes 270mA at 12V. That's after the filament has heated, glowed and it's resistance has increased. Roughly 44 Ohms. Let's say that when cold the resistance is only about 11 Ohms. So initially over 1 amp could flow for a fraction of a second. Now what's important here is the circuit design of the PCM. A lot of the time, they have protection diodes so that the input cannot go more than about 0.6V above the 5V reference. BUT If the loading of the 5V reference is less than your test light can supply, you will cause the voltage of the 5V reference to be pulled above 5V and cause internal damage. Initially tool suppliers sold LED test lights (10-20mA) as PCM-safe but in practical terms a lower current (100mA or less) incandescent test light is safe. The one in this video COULD cause damage, dependent on PCM design.

I love this guy 🤣🤣🤣🤣 Very good video, bro. Can't leave that out.

i love the sense of humor in all your videos

Who needs college when there's guys like this on RU-vid 😂😂😂

In extremely simple terms, as I was taught in my Broadcast Electronics class, volts are the pushing of those electrons through a circuit and current is the flow of electrons through a circuit and it can be measured in voltage and amperage.

Classic! Thanks for sharing your knowledge and the humor is a great touch!

Enjoyed you video...while we had them. Wish you well. Hope to see you farther down the line.

10:43 hand move, lol

The resistance of the test light bulb functions as a shunt or ballast, thus only allowing about 7/10 of an amp of current. When you use your body resistance your doing the same but with a lower current value that you can calculate using ohms law. E = I x R E= Electromotive force, and it's unit of measure is volts, and it represents electrical pressure. I = Intensity, and it's unit of measure is amperage, and it represents flow of free electrons of a conductor. R = Resistance, is what impedes flow of free electrons... E = I x R or in it's easy to understand form V = A x R. or Volts = Amps x Resistance. Or Amperage = Volts ÷ Resistance Or Resistance = Volts ÷ Amperage. Free Electrons are the electrons that are on the conductors (element) last orbit of electrons, also known as valance electrons. The more Free electrons an element has, the better electrical conductor it is. Mercury is the best electrical conductor, but it can't be drawn into a wire. Next in line is silver, but cost prohibitive. Copper is the 3rd best and best choice when it comes to cost. Gold is 4th in the list but also cost prohibited, but it used when its resistance to corrosion are more important than cost. eg: electroplating connections of high end stereo equipment. Gold is used in cel phones and satellites... It is the free electrons that in a conductor that is moving, this movement of electrons is measured in amps. Their is a misconception that amps causes heat, it's due to that it's used as guide due to its relationship to heat, as long voltage remains consistent. The truth is that it is the product of volts and amps ( Volts x Amps = watts ) watts is power, watts causes heat, wonder why hair dryer are rated in watts and not amps. Amps is equal in the entire circuit, but the fuse is installed on the postive (Hot side of the circuit) side not the negative side. eg: simple circuit, you have a 12.6V and your load us a head light that draws 3 amps, the whole circuit is going to draw 3 amps, be uses amps is equal throughout the entire circuit, and you can measure this with a DVOM on the postive side and the negative side of the circuit. And the result will be the same. But is you measure the heat on the postive wire vs the negative wire the positive wire will have a higher temperature value then the negative wire. If you measure the voltage drop from the battery positive to the negative end of the head light, then save this valve. Then measure the voltage drop from the negative side of the head light to the negative side of the battery, and also, save this [near zero voltage, in milivolts (mV)]. Then you apply the watts formula [ W = V x A ]. The resultant is the positive side of the (hot side) a high value of watts when compared to the negative side of the circuit, this is why most ECM, BCM, TCM and other computers use ground side switching. I used the term most becuase I have found that Chrysler uses in some of their circuits positive side switching through transistors like un eg: EGR valves and in lighting, In lighting if a similar bulb is used but the watts value is higher than specks, it usually results in smoking the BCM. And if it's an all in one unit, were the ECM, BCM, TCM & ABS is integrated as one as I seen being used lately, the whole unit hast to be replaced. Now back to the test light. I am going to give you an example how can you smoke an ECM, BCM, ABS or a TCM with a test light. New Tech, fresh out of tech school, or an inexperienced Tech. is using a test light, test light bulb, blows out and the Tech needs the test light now, so tech substitutes a dome light bulb. since it similar in size and it's a good fit in test light. The dome light build draws a little over an amp of current [ see the danger] and then uses it to check if the ECM is grounding an actuator. This is how an ECM, BCM TCM or ABS unitl is smoked... And on Chryslers ABS wheel speed circuit, ,don't know is its true on units designed By Fiat/Chrysler, but on units prior. Try using a test light, one that its bulb hasn't been replaced w/a dome lamp bulb. A you will smoke the ABS wheel speed circuit, Also, if use for more than a few minutes on a Chrysler EGR control side circuit, which is the postive side, You will smoke the EGR valve driver... I know I got carried away with this explanation, but I felt is important in understanding how electricity behaves...

HEY... What's up? I am missing your vids. I enjoy your vids - viewing from Trinidad and Tobago

My favorite youtuber. Great video!

Hey! Love you channel! Very cool information.

Hello From Panama I like so much your videos Your concepts are right. I suggest always use a voltmeter when checking sensor or stuff relared to PCM. In analog voltmeters you can see in the left side of scale a value such ohmsX volts. It usually is 20komhs per volts. For sensor troublehooting 5v bias your analog voltimetr will present 100,000 ohms to circuit in electronics it is call impedance. That value is ok for the test and should not affect the circuit operation. IF you want more impedance you must use a MOSFET voltmeter o simply a digital voltimeter because they use mosfet coupling and shows more than 1Megohm impedance. The lamp test is always ok to test electromechanical devices such relays contacts or to isolate bad conecctions in wiring.

Bill Nye The Car Guy? Very informative vid. See you next year!

I finally subscribed and as an old electrician voltage is like the pressure in a water pipe current is like the volume (size) of the pipe. Voltage does not change speed. I like your stuff. Mike

Interesting video. Thanks for sharing!

Wow! Great explanation. I watched it multiple times and will refer to it in the future.

Thank you for another technically informed video keep up the good work... please.

Thank you, thank you, that was a great explanation! Thank you

I used a power probe and injected 12 v on a 5v ECT sponsor connector like a dummy and had to get a new module… well a used one actually and had it cloned with the old one .. 🤦🏻‍♂️ lesson learned lol never inject voltage into just any wire especially if you don’t know what your doing..

Ohhh so is that what electrical engineers spend their time doing? Another great video, thanks!

I love you channel:)...wondering if you could make some videos similar to what CarWizard is doing? Go through each car manufacturer and say which cars and trucks you would buy and which ones to stay away from? I greatly value your opinion. Cheers!

great video and great infomation thank you.

Good video. Happy Motoring and Happy New Year.

Excellent video, thanks for sharing your knowledge, 👍👌😎😎

It does depend on how big your test light is. Go for one with a small bulb. All the pcm drivers that could be damaged will require more than about 500 mili amps before they may fail so a 200 milli amp test light is is fine.

The big danger with a test light like that is that you can provide voltage to a coil somewhere on a solenoid and "jump start" your engine or some other system that you have your arm sticking into or are laying under. Be cautious what you test with the lights, it is really just a wire and you are basically connecting things to the battery to see if the other side of the device goes to ground or power. Better to use a meter with one of the additional probes that will show you what is going on if you know what you are seeing on the meter. Also, never push the pointy end through the insulation. You will wish you didn't in a few months when that wire starts to corrode and the resistance goes up and you keep changing "bad" sensors or whatever and it just never acts right again.

Despite the high resistance of intact skin, I still wouldn't rely on it to safely limit current flow if I had a pacemaker or similar device implanted. There are still old unipolar systems out there that could be more sensitive than one might think.

Great video!

What has happened to the channel? Is he still making videos?

Great video man thank you

No new videos in 6 months! 🥺 I miss your videos!

Good video. Thanks...👍

Voltage is pressure, current is flowrate

Omg I love your sense of humor

I have been an electrician for over 40 years and I am telling you that some of your info is spot on but on some other stuff you could be steering folks wrong on some of the stuff that you are saying. For one thing when using your test light in series with your DMM you say there is no current flowing. WRONG. Just because the light does not light up does NOT mean there is no current flowing. True that the light bulb will limit the current but there is enough current flowing and voltage to damage some electronic components.Just saying, buyer beware.

You have a PURE Funny Heart sir. your'e not joking, but still, its funny. Thanks a lot for all your video. I believe this is a great source to learn. May the God bless you.

Wow my pal very clear. Better than this explanation nowhere else. Happy new year. Best regards.

I love your videos man😂

Maybe I missed it in your overview but I didn't hear you mention the multimeter itself. Most decent ones are 10 Meg, low end ones maybe 2, analog not much better than the test light. The body thing is easy to forget and has options as two fingers on the same hand, finger to let's say elbow, or across as you demonstrated. Good video.

You’re the man in Sun Valley 😎

Man you are a riot.great job sir.

Pretty complex but interesting!

Thoroughly enjoyed the video and what's the brand name of your test light?

Thanks for your channel 👍 🙏 😀

I've heard that static electricity can blow your ecm. It was always good practice to disconnect the negative. I know that systems have gotten more sophisticated, but which one's and when. I think it's better not to take the chance. Just like giving someone a jump. There's a lot of controversy about the weakening of your electronics that can show up months later. Just my 2 cents

but maybe need be careful though. Suppose you branch the test light to the 5V output that goes to eg. map-sensor (other branch of the test lead to ground) . You will draw 250mA extra to that output. Maybe too high for that particular output (cannot be sure unless you have the specs of that ecu, in most cases you don't have it). When you connect a V-meter to that 5V output it will do no harm because the V-meter has a high resistance input and will not draw any extra current. So I would never connect a test light to a voltage output from a ecu.

Excellent video, but I’ve always understood that the convention of current flow in a car as going from the positive to the negative side of the battery, not the opposite as you stated at the beginning. That is why we should always disconnect the negative side of the battery first, so as to eliminate ground to all circuits. Please correct me if I am mistaken. TIA and, again, great video (long time subscriber here).

8:18 "Oh my god I'm dead" Now I am dead from laughing 😂

Now I know thanks for the video 🤠👍

Can you please do a top 5 Most Reliable Vans for big families.?

I thought I had damaged the PCM, but it turned out that I had forgotten to plug the idle air control valve back in on my 305 Tbi! Thanks!

Now put out a video with a scope current clamp around the .200 Amp test lite, and watch it current rush 2.0 + amps, which burns out the .200- .400-.500 mA drivers. So yes a test lite can easily fry a PCM driver!!!!!!!!!!! ( relay, etc )

Hey pal, great video as usual. I have a question for you please; Some months back I had a P0171 and P0174 CEL code on my 08 lexus IS350 to indicate that my car was running lean on bank 2. Further down the line I changed both upstream 02 sensors and the codes went away. However I noticed the SFT and LFT on bank 1 are 2.3% and 10.1% respectively whereas that of bank 2 are 2.3% and 14.8%. I've checked under the hood for hissing noises or leaks but nothing. The car runs great however the mpg (14/17mpg) isn't very good. Please what does this indicate?

You are one of my favorite people

Considering you can get a multimeter for about 10 bucks, it is a far more powerful troubleshooting tool than a test light and is totally worth it to both own and learn to use.

A test light can surely overload a low power circuit but I would much rather see a tech with one on those in his hand that a dang power probe. All those should be thrown in the garbage. I work a techline and you wouldn't believe how many modules I hear (actual beeping) get fried by 12V being pushed in a signal or control circuit by a trigger happy tech with a power probe.

YOU ARE A PROFESSOR.

Gynaecologist😀😀

Ok excellent demo

You are very informative. Thank you. However...there are other electrical forces current human understanding is not expressing.

@10:43 subtle but elegant right hand movement😉

Good video.

ecu signal wires (AVI) are just like you said " a multimeter " , But there's a lot of other wire's in engine bay that are DPO DPI SPI system's and there very easy to blow up . Be careful

What about the LED test light what's the difference worse than regular test light thank you again great video

Spoiler alert, YES, you can instantly fry your pcm with a test light. Always use a DIGITAL volt meter. An analog meter can fry a pcm the same as a test light.

For a lot of circuits can be burned with 300ma current drow. I did fix few of them. Specially for camshaft/crankshaft circuits. Try to burn a Led with 300ma 12v, you will probably burn it.

I changed my test light globe from incandescent to led to limit the current flow even more. Incandescent globes draw more current than led is my understanding

Before using you want to make sure your main ground to battery and chassis grounds to chassis are good if you had a even a slightly loose battery terminal, it won't displace current right

Wow, you have a lot of subs now.

Good video

Can you make a video explaining the pros and cons on owning a 4x4 vs 2wd? And also which 4x4 are best and which ones to avoid?

When you grab the test leads you should have... Oh my God I'm dead! XD lolol naww I'm dead

I'm new at this kind of mechanics I've alway just been a parts changer so thanks for the info and dammit wish I had your hair on top of my damn near bald head !! Those were the days ...

Excellent video thanks, so what is the Best way to test 5volts reference thanks

Amazing 🤩

Some fuses in both boxes are not getting power. Is that a battery ground ? They are mainly accessory fuses but its driving me nuts. You are the only gynocologist i know that can show me where to probe

I'm an electrical engineer and you are on target.

If you have a decent, cheaply made Chinese multimeter..... like the one this gentleman features in this video..... Please don’t leave it in direct summer sunlight (especially at 108•F day), much less in a hot car. I left mine (which was just like this one here) out in the sunlight once, after having gone to lunch. I found it warped, and the display was horizontally half lit. You’ve been warned!!!!

With the older meters we worried most about pegging the needle and wrecking the meter movement.

Excessive currents are usually the reasons why ECM will fail. If voltages are greater than the battery's 12.6 volts nominal voltage can also destroy a ECM with the exception of an alternator charging voltage that can be as high as 15 volts. Reverse voltage will definitely destroy the ECM. Unprotected loads due to short circuits can destroy a PCM. Hopefully these circuits are protected with fuses that can react quickly enough. Fuel injectors with shorted coils will definitely damage the ECM. Some ECM may have protection to detect such excessive loads before there is permanent damage. There are other reasons for ECM failures that are more environmental, such as water intrusion, oxidation or corrosion. It's amazing that a vehicle's interior high temperatures are tolerated by these ECM. I'm curious how age has an affect on the failures too.

Hey brother, fellow wrencher here. Hope you're doing alright and come back soon. Checked your socials but they're quiet

Good to know you aren't a Gynecologist. That caught me off guard. Well timed, haha!

Over load told you below in his comment that inrush current on an incandescent test lamp can exceed two amps. He is correct. Just hook up you scope for a single trigger to your test light and you''ll see the inrush current. Using a meter you only see steady state current. The only place on a pcm you should be using this type of test light is when testing powers and grounds, never on a signal input or output. Besides what does a test light tell you about those pins? Absolutely nothing! You want to test the integrity of the wires going to and from your PCM? Fine, then pull the connector from the PCM and do your testing under a real load that is presented by various bulbs used in automotive. A 2 to 3 amp bulb works well for these applications to determine if a wire can handle a load. And the point where you hook up your multi meter to your test light and tell us there is no current flow because the light isn't illuminated is not true. Of course there is current flow, but because you meter has such high impedance (resistance even on the cheap meters is 1 mega ohm) the current flow is so low it won't light the lamp. Did you forget that in the beginning you told us a meter works by measuring current and showing us voltage by internal conversion to ohm's law? So, if there wasn't any current flow you couldn't measure voltage based on your explanation. Besides, that's why you use a multi meter because the impedance of that device is so high it doesn't affect the circuit you are testing. Rethink what you presented in this video and correct it before people start blowing up their PCM.

Great videos, have 2014 Silverado. It shut off while driving. Wouldn't crank them. ECM relay clicking when turn key on and try to start. Replaced relay same thing happens. Pulled ecm relay and it cranks but won't start. Any ideas? Thanks

Sill doing videos??? have not seen any lately. All Your subscribers are hoping that you are ok and the covid19 has not slowed you down. Good Luck and hang in there.

Hanging on to the ohm meter wires as you did, shows high human body resistance to current flow when you skin is dry. However, you missed a great teaching opportunity by neglecting to lick your fingers, and then grabbing the wires a 2nd time, which would have shown much less resistance (ohms) and therefore a much higher current flow with the same voltage. This lesson would show very well why you don't stand in water (bath tub, wet basement floor, etc) when working with 110 vols, or 220 volts, etc. A wet body in those conditions can get you killed very quickly, because it conducts current much better.

i always wanted to know the different plugs on the multimeter