Relative compression strange issue

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jmoore2@smccme.edu
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Relative compression strange issue

Post by jmoore2@smccme.edu » Thu Jan 28, 2016 9:02 pm

I am checking to see if anyone can explain what I am seeing here.
relative comp file 2 bad.jpg
I am doing this as a demonstration for a class I was teaching and saw this.... Doing relative compression with a pico amp clamp (2000amp model with 200amp selected) I took a recording with all plugs in on a good engine (ignition disabled and fuel disabled). I then took a recording of the same test with #1 spark plug removed. Something isn't right... I am not sure if its a setting issue. Strange that with one plug out I see a rise in amperage and 3 humps.. File 2 pic. File 1 pic is showing a normal set of amperage humps.
relative comp file 1.jpg
Doing this same test with our Verus and I got exactly what I expected 3 humps all the same and #1 cylinder flat lined on amp draw.

Thanks for the help.

Alan
Pico Staff Member
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Joined: Thu May 25, 2006 8:02 am

Re: Relative compression strange issue

Post by Alan » Thu Jan 28, 2016 9:51 pm

Hi,

These look normal to me - perhaps one thing is that with the 2nd waveform the current clamp is the other way around so the current is negative.

The first waveform shows how less current is required to compress the cylinder where the plug is removed. The other, once the engine is rotating shows the same current peaks for each cylinder.

For both the waveforms I suggest you enable filtering (from the channel drop down icon) to clean them up a bit. Alternatively use the compression test in PicoDiagnostics which gives results as a bar graph.

jmoore2@smccme.edu
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Posts: 2
Joined: Fri Jan 22, 2016 10:13 am

Re: Relative compression strange issue

Post by jmoore2@smccme.edu » Tue Feb 02, 2016 12:33 am

You are right the student had move the amp clamp around on that second capture. What was strange was the large hump which has higher current. I was not sure why this was happening.

Thanks for your response.
Joe

Alan
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Pico Staff Member
Posts: 255
Joined: Thu May 25, 2006 8:02 am

Re: Relative compression strange issue

Post by Alan » Tue Feb 02, 2016 7:39 am

jmoore2@smccme.edu wrote:You are right the student had move the amp clamp around on that second capture. What was strange was the large hump which has higher current. I was not sure why this was happening.

Thanks for your response.
Joe
If by the large hump you mean the first one on the left hand side I suspect this one was just as the engine started to rotate, it takes more current (energy) to start the engine rotating from stopped than to keep it rotating once moving. The first few revolutions of the engine will have higher current peaks until it has settled into a steady cranking speed.

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