Relative compression via voltage
Posted to Technical Theory Forum on 2/19/2014
20 Replies
In the tool equipment forum there was some discussion about
BOB uses and it was mentioned about using it for relative
compression testing.
http://members.iatn.net/forums/read/msg.aspx?f=forum8&m=152854&fv=0&ar=0
It is possible to do a relative compression test via just
battery voltage. Pico diagnostics has had this setup for a
while. The earlier version could also display what was
connected on channel B. So, I decided to see how much of a
variance there was between the battery voltage and BOB
voltage.
First up is my connections:
2000 Dodge Dakota, BATT/Charging/Starting Photo
2000 Dodge Dakota, ECM/Inputs/Outputs Photo
My first test was using the Pico diagnostic relative
compression test. I use this a fair amount as a go, no go on
mechanical, combined with listening to the cranking cadence.
2000 Dodge Dakota, BATT/Charging/Starting Photo
In this case PD display the second channel as AC coupled and
the peaks are relatively even.
OK, next up is connect all four channels two DC coupled, two
AC coupled, the test points are the battery and the BOB pin
16. I did a vertical zoom on the DC coupled, and filtered
all four channels. In this case looks like the results are
very similar.
2000 Dodge Dakota, BATT/Charging/Starting Waveform
psd:
2000 Dodge Dakota, BATT/Charging/Starting Waveform
And just in case anyone is thinking I am a Pico snob, here
are some Red tool (Modis) captures. As far as I know there
is not a vertical zoom so I just did an AC couple and again
the connection is the battery on one channel and BOB pin 16.
2000 Dodge Dakota, BATT/Charging/Starting Photo
2000 Dodge Dakota, BATT/Charging/Starting Photo
2000 Dodge Dakota, BATT/Charging/Starting Photo
The results again seem very close. I can not say it will be
the same on every vehicle, but it seems like something to
play with. Also worth noting is there is a relationship
between the battery voltage and the current. A bad cylinder
will be opposite of what a current probe would.
Anyone else try this out yet? if so, feel free to add your
captures.
Sam
Samuel from Colorado
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