how to read data on wire

I have an o-scope and need to know how to check to see if a wire (any wire) in a vehicle has data on it. Can someone help? I am new to this so any help would be greatly appreciated. Thanks!

What are you trying to find? If looking for digital data, just probe the wires, if there's data there, you'll see it instantly. For that matter, you'll see analog data (power, speaker leads, etc.) as well...

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I am looking for 12v constants and 12v switched wires in multiple vehicles. I am using a portable Fluke 190 O-scope/meter, so it has BNC fittings and meter fittings. I believe I need to use the probe with the BNC connector. I also believe I need to use the ground clamp on the BNC probe to see the data. Correct? How would I know if there is data? I believe it will be a square wave. Is it also true that data could happen once in a 24hr period. I guess I would have to leave the scope connected for 24hrs on record? I am connecting GPS and tracking systems so I need wires that don't have communication on them. Thanks!

12V constant, and 12V switched are just that... there WON'T be any data on them. They are 12V DC wires, and that's ALL the "data" they carry. The wires you are desiring to probe can be probed with any VOM, a 'scope is overkill... WAY overkill.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

haemphyst wrote:

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12V constant, and 12V switched are just that... there WON'T be any data on them. They are 12V DC wires, and that's ALL the "data" they carry. The wires you are desiring to probe can be probed with any VOM, a 'scope is overkill... WAY overkill.

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I know all this. I probe alot of wires and I need to know when a wire is caring data or not. I don't want to tap into a data wire. If I just use a VOM, then I won't know if there is data. right?

If you are only lookging for 12vdc wires you can probe with a meter.  If there is data on the wire, it will NOT read 12vdc.  Data wires will usually read under 3vdc with a normal meter.

A 'scope isn't exactly the easiest piece of equipment to use.  You need to make sure a lot of things are set correctly to measure what you are trying to measure, starting with time period and voltage level.  There will probably be channel selections and trigger modes that if not set right won't allow you to use the 'scope.  Your best bet would be to read the book front to back and understand it all.  I've had my 'scope for 3+ years, and honestly, I don't know how to use all the features on it.

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Kevin Pierson

KPierson wrote:

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If you are only lookging for 12vdc wires you can probe with a meter.  If there is data on the wire, it will NOT read 12vdc.  Data wires will usually read under 3vdc with a normal meter.

A 'scope isn't exactly the easiest piece of equipment to use.  You need to make sure a lot of things are set correctly to measure what you are trying to measure, starting with time period and voltage level.  There will probably be channel selections and trigger modes that if not set right won't allow you to use the 'scope.  Your best bet would be to read the book front to back and understand it all.  I've had my 'scope for 3+ years, and honestly, I don't know how to use all the features on it.

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So, all the data wires have a voltage of 3volts or less?

It would actually depend on the signal, but I would say that would be a good generalization.

Most data signals are 0-5vdc signals, however you will occasionally come across a 12vdc 'data' signal, like a fuel injector signal (technically is a data wire) or even possibly a crank angle signal). 

I would say, more importantly, just don't tap in to any wire that doesn't read a constant 12vdc when you probe it.  Also, use a good wiring reference for the wires you need to tap.  You should never go in to a car without and idea of the color and location of the wire you intend to tap in to.  Once you have that information then you verify it with your DMM.

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Kevin Pierson

KPierson wrote:

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It would actually depend on the signal, but I would say that would be a good generalization.

Most data signals are 0-5vdc signals, however you will occasionally come across a 12vdc 'data' signal, like a fuel injector signal (technically is a data wire) or even possibly a crank angle signal). 

I would say, more importantly, just don't tap in to any wire that doesn't read a constant 12vdc when you probe it.  Also, use a good wiring reference for the wires you need to tap.  You should never go in to a car without and idea of the color and location of the wire you intend to tap in to.  Once you have that information then you verify it with your DMM.

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It's not that easy. I work on brand new vehicles that aren't even on the showroom floor yet. There are no wire diagrams either. I have to go in there blind and find constant and switched. I can't use wires on the driver side so the steering column is out of the question. I can't just hope that a 12v wire has no data I have to be sure.

1: Why is it so important that there be no data? I can't see how this would affect a GPS power supply requirement.

2: If you're THAT worried about it, build yourself a low-pass filter (LC) for the tapped leg, and shunt the "data" off the 12V.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

trobins5 wrote:

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It's not that easy. I work on brand new vehicles that aren't even on the showroom floor yet. There are no wire diagrams either. I have to go in there blind and find constant and switched. I can't use wires on the driver side so the steering column is out of the question. I can't just hope that a 12v wire has no data I have to be sure.

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That would definately complicate things. 

You would have to make sure you don't tap the - side of solenoids or relays, as they will read 12vdc when they are waiting for a (-).  So, basically, you would need to tap a wire, and then drive the car and test every single function of the car before making sure you have the right wire.  You 'may' be able to just look for thicker wires, as thicker wires are generally feed wires, but even that isn't fool proof.

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Kevin Pierson