I really look forward to these types of videos from you guys. These videos help people like me who are trying to learn the ins and outs of scope function. It really helps to be shown how to perform the test and to see the results with live examples. Keep up the good work!!

Love this test… it has proved itself in the field for me many times and saved me a ton of time. The only thing I would have done different would have been to set another channel on your scope to trigger off of something (coil/injector etc) on an accessible cylinder that you know the number of. That way, when you compare the trigger cylinder to your dead cylinder in the firing order, you will know exactly what cylinder has the problem. In this video, you revealed a compression issue for sure, but not which cylinder had the issue…setting up another channel to monitor a known cylinder # will pinpoint the cylinder number in one test. Just a thought

That is true if there is very low compression in a cylinder like he had here with a plug removed. However, if you have a smaller loss of compression causing a drivability problem you can not hear that. This test is meant to tell you if there is a compression issue without pulling plugs and installing a gauge. If this test shows low compression then you can do a normal compression test, but that can take a very long time on some cars. So this test saves you from doing a compression test in vein.

@Jallandhara Each cylinder compresses in turn. As the starter motor requires more amperage to cause the compression, its current rises (the peaks in the graph). Since a bad cylinder that will not hold compression, it requires a lower current to 'compress', the amperage is lower - as reflected in the periodic absence of a peak on the graph. The test only tells you that one (in this example) cylinder is bad. It doesn't reveal which one is at fault.

Good video!

@romeoadryan It's the 80 Amp AC range that is used as the current alternates as can be seen from the waveform. Now the waveform you linked to does spike up to 420 amps on first crank, but as i stated the verus AC amp range on this video can only do +/-20 amps so you would never see the 420 amp spike, but it won't damage the clamp probe. the waveform in your link settles between 120 and 180 amps eg +/-30 amp draw. Hope this clarifies.

Best one yet on this subject.thanks Mr wayne.

@romeoadryan To further clarify, the clamp meter detects current via inductance (e.g. through a clamp/coil), this then relates to a millivolt per amp ratio of the measured current, so current in it's own right cannot blow the clamp (at least not at automotive current levels, even at 500 or 600 amps) as current is converted to a milli-voltage level.

Yes, but, the whole idea of "relative" compression testing is the nature of the test itself. We conduct a "relative" test as it's easy and quick (If you have the right test equipment) and at this stage don't wish to remove anything, it points us down a particular route in order to exclude or not a compression related fault. e.g from this test it may point us to clearly to cylinder 3 (out of 8 cylinders say) and from there we can focus on that area, e.g. confirm with a manual compression test.

@romeoadryan Further, he could only set his scope to a 40amp range, e.g to +20 /-20 amps, when the car fired you can see the trace disappear off the trace, so all you know is it was greater than 20 amp initial draw, could well have been 80 or 100 amps, but after the spike the waveforrm settles to a +12/ -8 amp trace, with plug removed +12/-12 (on failed cylinder). So the scope range limit of 40 amps was adequate to cover for this test. Again hope this helps,

@romeoadryan I had a look at the pico auto site and there is a diesel cranking amp waveform that shows amp swing from -325 to -200 e.g 125 amp swing, so I don't think our 80 amp clamp probes will cope, really need a 400 amp clamp probe as a minimum to see complete waveform, exempt the start spike.

Thats what he is doing, the "snap-on verus" is a scope with the clamp on one channel, it's a petrol car so the low amp current probe does 10mv per Amp, but you will need a high current probe to see the full waveform of a diesel.

@cpfreakfree1 i would say it just steers you in the right direction as you said. but as he pulled one plug and knowing the firing order you could identify each peak as its cylinder in the firing order. but you would want to know all cylinder pressures wet and dry anyways if you found a problem

very educative. Thank you regards

Thank you for your helpful information

@romeoadryan The reference to a low amp current probe means that the probe (not the circuit) can measure low amperage values e.g. from 0 to 20 amps at say 100mv per amp, in this case his "low amp" probe can also measure an 80 amp range at 10mv per amp. These ranges are really "low amp" ranges as many clamp meters can measure 200, 400 and 600 amps. All relative really ! (sorry had to get relative in somewhere). The 80 amp range is more than adequate for this test. Hope this helps.

Ford foot to floor flood-mode would open throttle fully and keep from starting by shutting off injectors.

That's a really good way of doing that.

Thorough and fast.

The Short Scraped Knuckle "Load Test" of pulling a SP and checking the engine Responce is Dirty and somewhat quick as well. This shows the associated Cyls that can be culprits

@romeoadryan My guess is possibly yes, I haven't tried it as I have the same probe, but what I think may happen is that the trace if measured on a scope may reach say 100 amps (e.g. 1v as my guess is 1v may be it's max swing, there again it maybe 0.8v (80amps)) and flatline till it comes back down below a 100 or 80 amps. The only way to do this is obtain a standard oscilloscope and give it a try.

just a couple of things I would like to mention, with a lot of non starters I had it was down to excessive fuel in the bores, due to excesvive cranking etc with low battery, that done 2 things, it soaked the plug in fuel so it wouldn't fire, and the Petrol/gas got into the bores and washed all the oil off the cylinders and lost the compression, I cleaned the plugs and put some oil down the bores which gave it a bit of compression and it started......Fred

@writerofwriters69 Amp clamp probe (or similar) can be bought in the uk for £40, I bought same one. Low end 2 channel digital oscilloscope can be bought for £250 and upwards. "new ark USA" sells one for $369 (or go second hand). So although the equipment used in the video is expensive (it's paid for itself as far as the business is concerned) the test methodology can be easily replicated for little cost e.g. circa $400 rather than a $6000 verus. (relatively speaking, sorry!)

Thank you for very help full video and information

@DJDevon3 that is true, this guys is very very informative and i have learned a bit even though i do stuff like this to, but your everyday joe isnt going to have this scope, but it still interesting

thanks. Great vid. can also do with vacuum test on exhaust pipe and oil tube dipstick

@romeoadryan I see where you are coming from, although I haven't tried it with extreme amperages if the 80 amp range is 10mv per amp then 80 amps would indicate 0.8v if you have a standard scope on 1v per div that is, so even if 100 amps were measured e.g. 1 Volt I don't think this would blow the probe, it would only flatline at say at 1v (100 amps) may be greater 2 or 3v even, never tried it.

God vid, but I'm not sure I agree with Duane's explanation of the DC coupling. He said the DC coupling would show only the POSITIVE humps of the ac or sine wave. Is this correct? I believe the AC coupling blocks DIRECT current, either positive or negative, and lets the alternating current thru and the DC coupling blocks the alternating current, above or below the 0 current line on the graph.

You can do this test by ear. Pull the ignition fuse or the fuel pump fuse, crank, listen for one cylinder to turn over faster than the others. If you get a nice even crank and all the pulses sound the same and keep an even rhythm, then your compression is relatively even across all cylinders. If you have a bad cylinder, you will hear the fluctuation while cranking. Listening to this guy's test, I would have said the same as him without using the instrument.

Superb sir.thanku

I wish Snap-On would let you have another channel active while doing any kind of current draw test. The reason being for this is cause if you had that, then you could use one channel to record something as #1 injector pulse or spark plug firing, use that as a trigger, then follow the firing order of the engine so that not only can you tell if you have a compression problem or not, but also be able to tell which cylinder is having the problem without having to put a compression gauge on each one.

perfect video .. thanks

Great video!! I do have a question though. You used a low amp probe and set the verus on 40amps. How many amps did the starter actually draw? I though you needed to use a high amp clamp because the stater draws approx 400 amps?? Does it hurt the verus or the amp clamp when it spikes off the chart?? Thanks for you help!!!

Another great video! Thank you.

@DiyMech I have the same type of low current probe and I was afraid because starter motor needs up to 400 amps to turn the engine and the current probe measures 80 amp max .thanks

great video

Should add channel two, trigger on that channel and connect to number one sparkplug. That will give you a sync to number 1 cylinder so you can establish which cylinder actually has the problem. Obviously you have to have a functional ignition system so disabling the fuel system would be required or on some engines you could use Clear Flood mode during cranking. I don't completely agree with your explanation of AC coupling though. Otherwise good.

Wish I could afford a Verus. Though my Fluke scope works good in finding a weak cylinder. Work is very slow now at the dealer.

@DiyMech you mean it can measure more than 80A with my probe ? I forgot to specify I work more with Disel engines .

As always nice video..very informative

If all you use is a scope probe, all you will see is voltage drop on startup e.g. if 12v goes to 10v it's OK if it goes to 6v it's gubbed (that's only voltage).

Excellent video, I am having idle and hesitation issues on a 97 corvette. I have drained the gas, checked for vacuum leaks, torqued the intake manifold, checked all plugs and wires, changed the fuel filter twice (clean), checked the o2 sensor wires, cleaned the MAF, cleaned the TB, changed the PCV, tested the fuel pressure (good). The plugs are a pain to get to. Is there and inexpensive way to do this test? What components would be needed.

Okay, so here's a question: Why does the amperage change on the same cylinder every cycle? For instance, the first cylinder to have compression after the one without the spark plug, its level of amperage isn't consistent every time the starter sends currant that way.

So it could still be a bad plug? Or electrical problem in the ignition? There may be no real problem with compression?

you dont need to on most of them any vehicle with an iac valve, air is bypassing the throttle plate

omg....I just dropped my jaw on the floor :)

That was my first though. Couldn't you use 2 channels measuring voltage, since it's an inductive probe? (Sorry if this is wrong, I don't own the Snap On equipment and thus don't know exactly how it operates.)

awsome, you are th man,

Hey D, I understand that this is a relative compression check in regards to amp's at each cylinder and that's great, but let's say we do this test and find what you found (not knowing about removing plug at 1) ...how do we determine which cylinder is the culprit based on the waveform? I'm assuming that you won't be able to, correct? I assume that this test is a quick way that would steer you toward your next move in diagnosing or at least eliminates other possibilities of the problem? Thanks

I do not understand what you use a low current probe when the circuit is over 400 amps .someone can explain me ?

muito bom

on my modus ultra my low amp clamp don't always work I think I need a hi amp clamp I was thinking if the starter was putting out more than 60amps it won't work with a low amp clamp I don't even have a selection above 60amps

@3:32 Did he clamp it on the starter wire or ALL the wires coming out of the POSITIVE post?

@DiyMech I think I found the dilemma ,I was thinking about DC current and with my probe should I use AC not as pictured in this link . "picoauto.com/waveforms/ChargingandStarting/StarterCurrent/wave52.html" thank you for this and for good videos you post

I wish I had the money to buy me a Verus

few things wrong in this video you may want to check

This video is WRONG. DC current does not go "negative". No need for AC test as this is a DC system from battery to starter, only measuring current draw. GRRR!

Beside the fact that we know u pulled #1 spark plug out, how would we know which cylinder is number one?

no offense but you still have to pull the plugs anyway now that you know it has somthing not right, which most times you can do this without doing this and by hearing it