Having a hard time diagnosing a no communication or vehicle network issue? In this Diagnostic Quick Tip, National Field Trainer Jason Gabrenas shows you how to locate the source of the problem, using a lab scope to check the CAN network system.
Diagnosing communication problems on a vehicle network can be a little bit of a challenge if you're not sure how to hook up or what you're really looking at.
The only way you can test the CAN network is by using a lab scope. So I'm going to go over here and show you how we can test it and how the pattern is supposed to look.
As you can see here we have both our channels hooked up to a DLC breakout box. This has got a short cord that plugs in to the DLC and it breaks out to a box that has 16 individual banana jacks and you can plug in to channel six and 14 if it's CAN network vehicle, because all CAN network vehicles, that's a standard they have to terminate on pins six and 14. And then pins four and five are also ground so you can see we have the ground plugged into pin four on this vehicle.
Looking back to the lab scope , how it's set up, you can see both channels are set to 10 volt scale. So that'll give us a nice, good size window.
And then we have to set our time base down to 100 microseconds. That's 100 millionths of a second wide. That's pretty short amount of time. We're dealing with some really fast stuff here.
So let me pause it and then I will make some measurements and we can see what we're talking about here when we're transmitting data.
All of these little squares here, these are what they would call data packets. This is the data going back and forth between computers.
Now it really doesn't matter what's inside those data packets, it's just a matter of a few different voltages that we want to see.
So we see here on cursor number one, that's when it's not transmitting. We want to see somewhere around about two and a half volts right there. So we see both of them are at about 2.43, 2.42.
Then on cursor two is where it's actually transmitting data and it should go up by about a volt and down by about a volt on both lines.
So we're seeing one and a half volts on the yellow line and 3.45 volts on the green line. We're looking at about a volt in either direction. You want to make sure you don't see any weird noise, any weird fuzziness going on in the line that can indicate a bad module. Also see where zero is way down here.
Well, if you're down to zero volts, that's going to be a bad ground or ground issue in one of your modules probably or in the line. And we could also be shorted to power.
Five volts is well outside of the pattern, 12 volts is right off the screen. So if we were shorted to power, that would be another thing that we would need to look at.
So as you can see, this is a pretty easy way. Pretty standard hook-ups. Just hook right up and that'll help you diagnose any communication issues on these CAN vehicles.