Communication Breakdown

This 2008 Jeep Grand Cherokee with 68,592 miles on it has an issue. The shop owner called me saying that dash gauges, turn signals, power locks, and radio were all inoperative. Furthermore, the dash had every warning lamp illuminated. The vehicle started and ran quite well. I arrived at the shop and sure enough the complaints were verified. I have seen ignition switch issues cause this same set of circumstances before. I checked voltages at a couple of places and quickly ruled that out. It was time to dig deeper.

I hooked up my WiTech system and quickly saw that one whole communication data buss was inactive. The codes I pulled from the FCMCGW (Front Control Module Central Gateway) confirmed what I saw on the topology screen. I needed to check actual voltages on the CAN B buss.

 
Here is a little network topology layout for this Jeep. Let's review operation. We will start at the FCMCGW. This is the central hub or gateway for both the CAN-C buss and the CAN-B buss. It also translates to a Diagnostic CAN-C buss to the Data Link Connector. Some earlier diagrams will show this as CAN-D for CAN Diagnostic. CAN-C modules are items that need high speed data transmissions for operation such as PCM, TCM, ORC (Airbag Module), etc. This data buss typically is not fault tolerant. In other words if we have a short to power, a short to ground, an open on one wire the buss will be compromised and will not function properly. CAN-B buss has slower speed data transmission and such has "non-essential" modules on its wiring. These include Drivers Door Module, Sunroof, Parking Aid, etc. This buss is fault tolerant. If one wire was open or shorted the modules would continue to operate. Fault codes would be set however. Both CAN-C and CAN-B are two wire twisted lead buss systems. Both buss systems "mix" at FCMCGW. I need to check CAN-B buss voltages. Hmmm, where to check.

My favorite spot and easiest on this platform is under the passenger front seat at the Heated Seat Module. I backprobe with my scope on both CAN-B buss wires at the HSM. I have absolutely no activity. I spared you the scopeshot. I also verified proper power and ground at the HSM. This can play an important factor as well into buss communication. Now, I am left with do I have a wiring issue or a module fault. I go back to my WiTech.

I go into the Loss of Comm Test. This shows all the modules that should be there and if they are reporting. As you can see all the CAN-C modules are reporting as well as the FCMCGW. All of the CAN-B modules are not reporting and more importantly they all have the same number of no responses. I leave my scope leads in at the HSM and disconnect the connector and cycle the key. Still no activity on the buss. I always try the easiest action first. So I know that my HSM is not my issue. I have a communication breakdown. There is a reason I named this post communication breakdown. It is one of my favorite Led Zeppelin songs. When I find myself getting scattered I can always count on the music of Led Zeppelin to get my on track again. So, I go back to my truck get some Zeppelin on and look at a wiring diagram for the CAN-B buss system.

Here is one leg of CAN-B. Something is corrupting the CAN-B buss. That is a lot of modules to disconnect one at a time or wiring to check. I close my eyes and let the voice of Robert Plant carry me away to even me out. What do I always say and preach? Break it down or divide and conquer. I look at the wiring diagram looking for my best course of action. What will give me the best results.

As Jimmy Page belts out yet another one of his signature flawless riffs it comes to me. This C313 will give me possibly four modules if I disconnect it. Ok, now lets look where it is. Service information says it is located left rear of vehicle behind an interior panel. Great. One thing the liftgate is inoperative. Yes, I could pop that panel off and manually unlock it but that is more work. I crawl over the seats and was able to get to the left rear interior panel that hides C313.

I pry it back a bit to uncover C313. I disconnect it and the horn starts going off. Bingo! I jump back out. Hit the remote to shut off the blaring horn and cycle the key.

I am now able to open the liftgate. Here is that panel as viewed from the open liftgate area. My scope reading at the HSM show activity. I go back to the Loss of Comm Test.

Here it is after disconnecting C313. Now, the only non reporting modules are the Sunroof and the EOM (Overhead Console). All functions are restored save the Sunroof and EOM. I inform the shop owner that further diagnostics would require dropping the headliner to disconnect said modules and inspect wiring to the modules. I am leaning heavy to a bad Sunroof module. The shop owner then tells me that last summer this vehicle had issues with sunroof drains clogging. At this point the shop owner also advises me to leave C313 disconnected until he asks the customer if they want to go further. Apparently, the customer just called and their other vehicle broke down that morning and they are without a vehicle and need a vehicle desperately. I pull out of the shop with the sounds of Led Zeppelin pouring through my speakers. Hey Hey What Can I Do.