Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Background Info
- working on a 2012 Porsche Cayenne
- customer changed his battery (under the driver's seat... not sure what the engineers were thinking there....)
- vehicle came in with various lights illuminated in the instrument cluster
- after some diagnostics, determined that I am dealing with a shorted CAN bus
What I am not sure of
- assuming there is no "splice pack" or I can't find the "splice pack", do I have to disconnect the wiring harness(es) from each module one at a time? Or, can I cheat and just pull the fuse that powers the appropriate module one at a time and save time that way?
As always, thoughts appreciated. Thanks in advance.
Pulling the fuse is not a surefire way of eliminating a module only disconnecting it is reliable , with that said you could try removing the fuse on some harder to reach modules just know if the network stays down its not proof that particular module is not the problem. have you looked at a diagram yet to see what is on the can bus and if it has splice packs or what modules are on the network?
To answer your question in a meaningful manner you would have to clarify some details, among them what kind of CAN bus are you refering to and what type of short do you experience: is it a short to plus, a short ground, a short between can high and can low if applicable....
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
- customer changed his battery (under the driver's seat... not sure what the engineers were thinking there....)
- vehicle came in with various lights illuminated in the instrument cluster
- after some diagnostics, determined that I am dealing with a shorted CAN bus
Am I understanding this correctly, the battery was changed and now the vehicle has a shorted canbus? No prior problems or codes.
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
Just wanted to update you guys on this post / question......
Here is what I thought was a shorted CAN bus. I attached my oscilloscope channels 1 and 2 to CAN high (i.e. #6 of DLC which was my Channel 1) and CAN low (i.e. #14 of the DLC which was my Channel 2). The wave form that I got for Channel 1 was a steady 2.45V as well as a steady 2.45V for Channel 2. So, based on the steady 2.5V waveform for Channels 1 and 2, I just assumed I was working with a shorted bus. After doing some research on the CAN topology of the 2012 Cayenne (i.e. I couldn't find any bus combs to work from), I decided the diagnostic approach I was going to use was "let's play unplugging the modules game!" The main problems with the 2012 Cayenne I was working on was (1) the ride height was not working and (2) the exterior lights did not illuminate. Unplugging a module in the RR related to the suspension got the vehicle to the proper ride height and the exterior lights worked once I unplugged the front BCM. However, after getting the suspension and the exterior lights working the oscilloscope still showed a shorted bus waveform at the DLC (i.e. 2.5V on both channels 1 and 2). I did some more module unplugging thinking that the job wasn't done until I had a proper CAN bus signal (i.e. 3.5V high and 1.5V low). Still nothing. Then I had a brainwave - the shop next door had a 2017 Porsche Macan as well as a 2016 Porsche Cayenne in for service. I scoped the CAN bus at the DLC (i.e. #6 and #14 of DLC) for both of these vehicle and had the same wave form (i.e. 2.5V for CAN high and CAN low). In other words, I had a 2012 Cayenne, 2016 Cayenne, and a 2017 Porsche Macan all giving me the same CAN bus waveform on pins #6 and #14 - 2.5V for both channels. When going to the wiring schematics for all 3 vehicles, you see that #6 and #14 of the DLC goes to the gateway module. So, how the heck do you get a CAN bus signal for these vehicles - go right to the module in question?
Again, thoughts appreciated and thanks in advance.
Yes you will have to measure directly on a module. The gateway is were all the can busses come together. The gateway converts messages from different speed busses so communication between different speed can busses can take place. Pin 6 and 14 from the diagnostic socket are only used by a scantool.
Use a wiring diagram to identfy can wires on the easiest reachable module and take a measurement.
Emiel - thanks for taking a moment to respond. Respectively, you personally have troubleshooted CAN bus systems on Porsche products and can confirm that for the Porsche brand one needs to go to individual modules to get CAN bus signals (please note - this is not meant to be a confrontational question - simply take this question at face value - thanks)? I don't pretend to be a Bus communication expert. However, the vehicles that I have worked with in the past have had the CAN bus waveform present at the DLC / DLC breakout box. For example, a 2006 Saab 9-3 2.0T uses the CIM (pls see attached pic) as a gateway module to bus info between the "powertrain bus" as well as the "Instrument bus". On the 2006 Saab, you can obtain the CAN bus waveform at pins 6 and 14 of the DLC - not so on Porsche products.... OR... am I missing something??
How a diagnostic CAN works isn't something porsche specific. The only thing you'll see on it is diagnostic traffic. If there ain't a scan tool connected and actively communicating it'll flatline. To see traffic on another vehicle network you have to connect to that network.
For example, a 2006 Saab 9-3 2.0T uses the CIM (pls see attached pic) as a gateway module to bus info between the "powertrain bus" as well as the "Instrument bus". On the 2006 Saab, you can obtain the CAN bus waveform at pins 6 and 14 of the DLC - not so on Porsche products.... OR... am I missing something??
That CIM isn't drawn as a gateway. The "Instrument bus" is passing thru' it. Then passes thru' 736, hits a splice then goes to two other modules. To measure the "Instrument bus" you can 'scope anywhere on that network. DLC is the easiest for that particular network but at a module, or H102-1, would also work. Can even poke holes in the wires if you want.
382/671 is drawn as the gateway. "Instrument bus" and "powertrain bus" are connected by electronics in that module. Measurement of the "powertrain bus" can only be done on the wiring to the left side of that 382/671 module.
Matt, I think the initial pic I posted (i.e. CAN bus wiring from DLC) was a little bit miss leading. Have a look at the next 3 pics - I think I made a mistake calling the CIM a gateway module. I now believe the ICM is the gateway module. Would you agree?
Matt, I think the initial pic I posted (i.e. CAN bus wiring from DLC) was a little bit miss leading. Have a look at the next 3 pics - I think I made a mistake calling the CIM a gateway module. I now believe the ICM is the gateway module. Would you agree?
I sure got misled!! After squinting at the first pic even harder it's just the powertrain network and the 382/671 module is drawn as pass thru'.
Reading the description on the last image you posted it appears the vehicle has three networks. HS-CAN P-Bus and a single wire I-Bus, which are both connected to the DLC, and a fiber optic O-Bus. The I-Bus might be GM SW-CAN??
Sounds like the ICM is one of two gateways on the vehicle. It handles P-Bus <> O-Bus and I-Bus <> O-Bus traffic. It also allows scan tools to communicate with the O-Bus via one of the wired networks.
The other gateway is the CIM which only carries P-Bus <> I-Bus traffic.
Topic Author
