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Relative compression via voltage
Posted to 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

Files Referenced:

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car Vehicle Data

2000 Dodge Dakota 4.7L

Engine4.7 L
Trans4-speed Automatic (Electronic)