Relative Compression Testing - Automotive Education

In this video, we talk about Relative Compression Testing. This can be a very useful diagnostic step which can save you a lot of time and money when working on a vehicle. We outline the tools needed and the process followed to perform this test, as well as talk about what information you can get from the scope capture afterwards.
Thank you for watching, I hope you enjoy the video. Please 'like" and Subscribe for more content.

[@DIYAutoHomeschool]

Thank You for watching the video, feel free to leave a comment with any thoughts or questions.

[@eyesalooking]

The clearest explanation I have seen on relative compression using an amp clamp and scope. Thank you. You explained it in enough detail that I am adding this to my scope knowledge base.

[@sheerwillsurvival2064]

Excellent diagnosis.timing chain I was thinking head gasket at first but then realized they were all about the same value. Genius 👊🏻👍🏻

[@SchrodingersBox]

Really good video and channel.

[@DIYAutoHomeschool]

Schrodingers Box Thank you sir, I've been a fan of your channel for a while now.

[@EshbanAutoGarage]

Very informative. I'm a mechanic in Nairobi Kenya and I'm thankful for this information.

[@Graham_Wideman]

I greatly appreciate KZbinrs such as you who put in so much effort. I would like to contribute a revision to your explanation of the initial current spike. That initial peak is not attributable to counter EMF. Its due to the starter motor being initially stationary ("stalled"), which makes it act like almost a short circuit across the battery (well, very small resistance and inductance of the windings). It jumps up to the amount of current that would flow if you just applied the battery to the armature winding, sitting alone and stationary outside the starter motor. But as the motor starts to turn, _then_ it produces reverse EMF (because it's simultanously working as a generator) , partially opposing (essentially subtracted from) the battery voltage, and reducing the current. As the motor speeds up, so the "generator effect" increases in opposition to the battery voltage, so the current decreases to the ongoing plateau level you have sketched.

[@DIYAutoHomeschool]

Thank you for your input. I would have to agree that your explanation is a better one for the initial current spike. I appreciate the info and the help. And thank you for your comment.

[@REPOOCNITSUJ]

Brilliant video!! You have a new subscriber. Please, please, please post more videos like this. I just bought my first scope and I'm fumbling around trying to learn how to use it. This video was very clear and instructive to a new scope user like me. Thanks a million!

[@66crush9]

Seeing how this is hooked up and several clear examples that resulted from this test makes this the most informative video I have seen on this topic...thank you for posting this...

[@TheDerekeder]

Wow! Excellent video - thank you so much!

[@pctuterman1]

Relative compression test explained the way it should be explained. Thanks for such a great video. Now I know for sure why I subscribed to your channel.

[@nickayivor8432]

What do you call INTELLECTUAL AND PRODIGY AND PROFICIENT AND SUBSTANTIAL ADD ONE MORE KNOWLEDGEABLE THE ANSWER IS DIY AUTO HOMESCHOOL My teacher Thanks for sharing it, start to finish loved it . take care and have a great Evening with all your family around you ACCOMPLISHED DIY AUTO HOMESCHOOL From Nick Ayivor from London England UK 🇬🇧

[@kenricrose]

Great video! Was looking for some automotive diagnostics I could do with my new scope and this video was a big help. It makes perfect sense but I had never thought of doing this.

[@DIYAutoHomeschool]

Thanks for the comment. I'm glad you found the information useful. In time, we are going to have an entire series of videos on different scope tests with both voltage waveforms, as well as current waveforms. I'd suggest trying it out on some known good vehicles just to get a feel for how to set it up. Thanks again for the input, hope to see you in the future.

[@jerradguillory7909]

👏 subscribed! Alot of mechanic vids i watch they only just walkthrough what they do without explaining what they doing they going through their diagnostic motion with camera on

[@bobkins270]

New subscriber. I did enjoy the video. And look forward to the rest. Brad

[@whitetiger8652]

I like your explanation. Thanks!

[@hustlerforgita7538]

You answered some of my question I had with this test thank you very simple explanation thumbs up

[@rubenvor]

Outstanding and simple explanation! Good job. Thanks a lot/

[@philh9238]

nice work, like the vids. hope to see more from you!

[@NewLevelAuto]

phil holbrook agreed

[@ThunderbirdRocket]

Excellent 🔥👍🏼 ! Thanks 🙏🏼

[@hammerhead6537]

Excellent video tutorial.

[@kevinedward1234]

Great video!!!!

[@rrmech11]

Hey buddy. Excellent explanation of a relative compression test.

[@samadam4722]

I love you draw pic on board explains it is best . I watched many videos they good too but they don't explain it on board hope they learn from you . We watch because we want to learn and skip tech school.you best and wish you best I never commented on KZbin but I liked pic on board and made to find away to comment..

[@Tek_Nician]

Thanks for the video. It was very informative

[@RobHaag71]

I first thought head gasket on the Cadillac, but I'm sure I wound perform a leak down test, but then I might be thinking valves after that. That one could get you into trouble if you didn't know about the timing chain, or be puzzling if the valves seat for you during a leak down. It's nice to have the web to turn to these days for sure. Thanks for the video!

[@istvanmeissler2238]

I watch a lot of automotive videos on KZbin and learn a lot from all of them, but this was by far one of the best. You kept it to one subject and explained it very clearly and very well. I thank you.

[@DIYAutoHomeschool]

Thank you, I appreciate your comment. I do my best to put as much information and effort into these videos as i can and I really like to hear when they help people who are trying to learn this stuff.

[@elijahfocht993]

very good informative video. thank you v much

[@tiernosev]

Good explanation, great job! I need to start using an oscilloscope but I’m not sure what scope set up to get I probably do just one or two engine diags per day

[@tony-le4nz]

Very good that was great!!!!!! Thanks a lot for this class

[@DIYAutoHomeschool]

I'm glad you like it. Thank you for watching and for your comment.

[@snaponmark]

Great video, subscribed!

[@isaacstreet1965]

Great video I am working on a 2010 Ford transit which I believe has some issues I will try this method.

[@carlos92161]

Jeremy and I appreciate it for teaching us we arey very proud to find you tell us please where we can buy that tool by the way god bless you always

[@1fnjo790]

The typical budget low amp clamps found for about $100 or less only go up to a 65a capacity and might not be able to fit around any cable fatter than a 9mm (3/8") even if in theory you might get by with a 20-1 attenuator (providing the low amp clamp can fit around the starter cable)

[@robertomijango6207]

thank for this video help thank

[@matthewshanahanautomotivet4284]

I just watched a few sections of this video, but an worried about the information. You should be using a high amp clamp, and on a Snap-on scope AC coupling on one channel, DC current on another. Snap-on lacks vertical zoom, so it is necessary for both. Starter in-rush current on an engine like that is 500+ amps, with stabilized current 100-200.

[@PhillipBailey]

Good video. Just curious about your scope settings when using the low amp clamp, are you using ac coupling? Since starter current would be well over 60 amps I use a high amp clamp with 1mv equals 1 amp.

[@DIYAutoHomeschool]

Thank you, and thanks for taking the time to watch and comment. I usually start with a 2 second sweep on the scope for relative compression tests and usually a 5 volt scale. I don't use AC coupling, the low amp probe generates a DC voltage signal and I think it maxes out at 2.5 volt output. Using a high amp probe is fine, and with a 1mv - 1amp scale, just start with a scale that will how in mv, about the current you anticipate the starter drawing. It's usually an adjustment. Run it, tweak the settings a little, do it again. You'll get the feel for what settings work best for you the more you do.

[@pseudosmith9945]

And the fix for thinking it may have low compression across the board is to do a conventional mechanical compression test on a cylinder that's very easy to get to.. then perform a relative compression test on the engine.. then you have a mechanical conventional measurement to compare the other cylinders to and you can ballpark the values..

[@DIYAutoHomeschool]

I like this approach, still gets solid numbers without the need to mechanically check every cylinder. I'll have to try this specific method out next time I'm in an applicable situation. Thanks for the comment and for checking out the video.

[@pseudosmith9945]

@@DIYAutoHomeschool and thank YOU for putting out great and informative content my friend..

[@yijianggu5203]

Great video. but the low amperage clamp only has 60 amperages. The starter draws more than 200.

[@billwilliams6338]

DIY AUTO HOMESCHOOL, // when one or more pistons are NOT giving any compression. What would be the problems on the cars engine? Do you have to replace the piston or the actual engine block is cracked inside that piston cylinder and have to replace the whole engine block?

[@JoseGonzalez-rs2fm]

Nice Video but I still got one question, How do you set up the oscilloscope for a wasted spark system?

[@MechanicTechnicianRepair]

cual. es. el. punto. de. chekiar. la. compression. con osilocopio. si. siepre. se tiene. ke. chekiar de. nuebo. con el. compresometro eso. es yo. entiedo. grasia amigo

[@JoseCruz-wn8bc]

Can I use the snap on eedm570 to do relative compression

[@fire7765]

Great video, can I use a 20 40 amp clamp?

[@DIYAutoHomeschool]

gto 55 You can probably use it and be just fine. The biggest thing you need to know is the maximum reading of your clamp. My clamp says 20 and 60 amps, but it will give voltage output up to 250 amps on the 60 amp setting. So long as 20 amps and 40 amps is not the maximum possible reading limit of your clamp you should be fine.

[@MechanicTechnicianRepair]

tengo. una. preguta. amigo

[@DIYAutoHomeschool]

¿Cuál es tu pregunta? Lo siento si las palabras están mal, he usado una herramienta de traducción. Yo no hablo español.

[@dksob81]

wasn't your ignition pickup on cylinder #2? Cylinder #1 should have been on the driverside front

[@DIYAutoHomeschool]

Yeah, looking back on it now, I realize it was. I was more focused on getting the video filmed, I basically just played lazy and didn't think about what cylinder I was on. Thanks for the info and the input.

[@dksob81]

Just making sure...lol

[@jusbenzz6324]

I thought this test could only be done with a high amp clamp

[@albertsautodiagnosticsllc2130]

No you can use a low amp clamp as well the omly draw back is the low amp clamps jaws are so small they typically will not fit around a battery cable hence why a high amp clamp is typically used. But fun fact you can also do relative compression testing using the battery posts positive and negative and AC couple it no need for a amp clamp