Labscope Relative Compression Testing

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[mobilemechman]

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!!

[TheWet]

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.

[videosenutube]

Best one yet on this subject.thanks Mr wayne.

[jeffreywilsonrubikscubeonb]

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

[davelowe]

@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.

[DiyMech]

@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.

[DiyMech]

@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,

[DiyMech]

@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.

[DiyMech]

@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.

[sureshbabu-flkx]

Thank you for very help full video and information

[DiyMech]

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.

[DiyMech]

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.

[DiyMech]

@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.

[fredshead]

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

[DiyMech]

@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!)

[meiermotorsports]

@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

[Hashim-iobv]

Thank you for your helpful information

[DiyMech]

@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.

[jackfoster]

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

[ksauto]

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!!!