I am working on a 2016 vw jetta. This is a question about the network on this car in particular.
Is it a valid test to cut the two can wires going to a module, and expect to see communication on the module
side of the cut wires? Lets say the transmission module for example….
I have normal can signals on those wires going to the trans controller.
I cut the wires and have no CAN comms on the module side, just around 2.5 volts on both hi and lo can.
The reason for this test is because the trans controller is a no comm with many modules stating that as well.
I am wondering if this is a valid test? If so I can continue my testing with powers and grounds etc.
You can always try your experiment on a known working module.
Your question doesn’t make sense to me… cut both comm wires and expect to see communication? Do you mean waveform activity on a scope or going direct to module outside of vehicle so to speak
If the system only has two can wires going to the module and you have on the trans controller side with the wires cut a bias voltage then the module Is not sending out any communication messages. Powers and grounds would be ok if there is a bias voltage wake up signal might be in question if you have com going to the module with scope connected just unplug the connector with the can wires and…
Sorry, but yes you are correct…..waveform activity is what I meant to say.
No it is not a valid test. The module has to be connected to the network in order to have communication. Only specific modules initiate communication. A valid test with a module that has no communication would be to disconnect the module and check that it is receiving communication to it from the rest of the network at the connector. If it is then you check you powers, grounds and pin fit at…
Usually a module will broadcast a “ping” to the network, kind of ‘ “CAN you hear me now?” (Punny,, no?). But I can’t say always. Some modules are awoke by a separate wire and some are awoke by a CAN signal. You can connect a scan tool (6-14) to the module can wires and have the scan tool ping it. That is if the normal configuration does not have a gateway module. ‘Least, that’s my experience.
Yes this a valid test. Done it many times. If the module has a terminating resistor the waveform will be similar to normal CAN. If it does not the waveform will look something like a 0-5 sine wave. I'm not sure if I have an example, my laptop is at the shop.
Interesting timing - I was just in a 2014 Jetta no-start/no-comms 2 days ago. These vehicles have 3 HS CAN busses, interconnected by a CAN gateway to the DLC. On the car in question, it would crank and die (immo), so we knew the individual CAN busses were talking, but we were wondering why the TCM was the only module showing up on a scan (ODiS, Autel, etc). So we opted to separate the 3 CAN…
So no scantool communication possible without gateway. However a scope test on those individual busses may have yielded some resemblance to network activity as Caleb suggests?
No - just voltages, unless you hook them to a scan tool and interrogate them, at which time you would see the tool's out-going messages as a wave form. This is only useful to show that the buss lines can support comms. The modules wont respond until they see the gateway's unique identifiers.
Right. Ok, do you know would they be high? 5 volt or low, .5v or not specifically set?
Both lines sat at 3.5v on a DC VOM. Putting a terminator across them had no effect.
Correct Jeff, this is because the gateway processes the data and spits it out to the DLC and the other networks. That's one type of gateway. The other type is comm passes thru a gateway. It's simply a conduit. For example on a lot of Fords with gateways the HS1 and HS2 CAN are hardwired thru the gateway directly to the DLC. However the HS3 and MS CAN come into the gateway and that's were they…
However, the opposite direction (ie - out to the modules) is important in this particular application because the Gateway acts not only as a fancy busbar, but as a scheduler and translator. The individual modules in Volkswagen do not speak proper OBD CAN on their own.
Alternative: Verify whether or not the module contains a terminating resistor, if it does you will need a standalone resistor. Unplug module, load test power and ground. Verify can bus signal with lab scope at the plug with the module disconnected, this will verify can bus wire integrity. If these steps test positive you only have one thing left. Let me know how this procedure works for you.
Hi Darren. No matter what, do yourself a favour and NEVER cut any wires, especially communications, unless performing a wiring repair and then repair, preferrably using prescribed methods, to limit the number of repairs in any single conductor. If a repair is performed on a conductor, voltage drop test the repair to ensure that no undesired resistance is introduced. If you really want to open…
I suggest just measure the resistance of the wires while the Ignition is ON, and engine not running. You should see 60 ohms anywhere on the CAN bus you look if the CAN wiring is intact. If it's 120 ohms, there is either a break in the CAN wiring somewhere, or in an ECU. If its some other value, then you have to trace from module to module and see where the problem is. If it is 60 ohms, then the…
You cannot actually measure proper CAN impedance/termination resistance while the buss is hot (ie - powered). Practically you can often get away with it, but for the most part that isn't telling you anything useful since if the buss is talking the termination value isn't useful and if it isn't talking there is no real reason to measure it powered.
Might not work like that. Now this diagram is taken from an older VW Ssp. I haven’t personally checked on a ‘16. I don’t think they use the conventional method of two 120 ohm resistors volkspage.net/technik/ssp/ss….pdf
The document Justin posted clearly states the resistors can not be accurately measured with IGN off at least on the model/modules that document applies too.
Myles, where are you sourcing your information regarding testing the bus with the ignition on?
I don't know what he's doing but if it works like he says, I want to buy the DVOM he's using!
Exactly Eric! Methinks he needs to re-read what he wrote.
From experience in working on CAN bus issues. Not all ECU's may active or communicating with key OFF. With key ON and engine OFF, there will be active CAN bus traffic, but less than with engine running, Some vehicles will have CAN traffic with key off as well. I often check it OFF, Key ON, and Running. The purpose of the impedance test is to look for an obvious CAN related wiring problem (wires…
Sorry Myles, but checking the resistance of a powered circuit is just a bad idea. It has the potential to damage test tools and may or may not give accurate results based on the ohmmeter's measurement methodology. Cheers, Bob
Thanks for your explanation Myles. I've been working primarily on GM CAN networks, from inception through current VIP CAN FD diagnostics. I have never needed to, or considered using the method that you describe. Yes, I do understand impedance and how to use a wide array of tools, including those above in your reply. Quite honestly, it doesn't make any sense to me to test a powered circuit…
On a VW or Audi with no comm I always like going into gateway module, Measuring value block 125 and up will be network info. Modules on the bus have a 1, modules not on bus have 0 it breaks it up between power train, covenience, and infotainment. Also those blocks, i know if using ross tech VCDS can give clues to whether bus is in single wire operation.