I'll apologize right now, this is likely to be long....

So not following the KISS principle bit me this week. For the past few weeks my 2018 Charger Daytona 392 would randomly start up and inform me that the lane keep assist was off, the backup camera wouldn't work, the HVAC blower motor was dead, no park assist, no blindspot monitoring, and the SRS would shutoff. There may be others I'm forgetting. This didn't happen all the time, and sometimes it would all come back on within a city block or two of driving, sometimes it might take several miles of driving. But once things turned on, they never had an issue for the remainder of that drive. Lately it seemed to turn into a situation where it fails more often than not which made the troubleshooting easier. When I'd scan for diagnostic codes there would be anywhere from 12 to 23 codes across several modules. Clearing the codes didn't seem to have much impact on how soon the issue would return. Over time though I started to notice that a vast majority of the codes were related to communication issues, and several modules were reporting "lost communication to the Forward Facing Camera Module" or FFCM. So I started to seek a Youtube education on the CAN bus system.

Typical CAN topology and failures when in a bad state

Per the service manual, the FFCM CAN connection was at the front STAR connector mounted ahead of the glove box. I pulled that down and inspected it. Everything looked fine. I hooked my oscilloscope across the CAN-H and CAN-L signals and used the math function to add the differential signals together, leaving me with what should be a flat line. Randomly I noticed some wiggles on the math trace so I thought for sure that would be the FFCM communication issue. I confidently disconnected the FFCM from the Star connector, but the noise was still present. I then proceeded to disconnect each module from the STAR connector and nothing caused the noise to disappear. So I progressed to the rear Star connector and tried the same thing. Nothing caused the noise to go away. But this method did allow me to map the connections to their respective modules. This is when I realized that the network topology map was showing that I did not have communication to the FFCM, the left and right blind spot sensors (LBSS and RBSS), the Park Tronic System (PTS), or the Heated Seat Module (HSM).

Noise on CAN bus

Back to service manual to look at the wiring diagrams for these devices. Relatively quickly I noticed that the FFCM received power from fuse F45, the LBSS, RBSS, and PTS were all powered by F44, and the HSM received fused ignition from F43. These 3 fuses are all right in a row in the rear power distribution panel. I checked for voltage to and through these three fuses. All three fuses were good, but the power to and through each was hovering around 2.8 volts DC. That couldn't be right, so I confirmed there was 12VDC to the distribution block, there was, and measured a few more fuses. All fuses in a particular section of the panel were 2.8V, but the remainder were showing battery voltage. So I knew my meter and meter ground were good. The image below shows the section of fuses that were all reading low voltage. This is where the service manual created a bit of extra work for me. Per the wiring diagrams, fuses F44 and F45 were powered by the Run Relay, but F44 is direct to battery. Since all three of these fuses were measuring low voltage, I figured I needed to get to the back side of the distribution block to find a couple of bad solder joints. Once I finally had the block apart, it was pretty obvious the diagram was wrong and that whole section of fuses were on one bus bar to the Run Relay. I sure wish I had thought to just check for continuity from the relay terminal to all the fuses before going to the work of getting the distribution block out. In the image below, I had taken out the Run Relay to inspect the contacts.


Location of the removed relay and the fuses it feeds


Back side of the distribution block and affected circuits


I bench tested the relay and it tested fine, so then I checked the connections at the distribution block, assuming a bad contact somewhere. I had good voltage to the relay control and ground, as well as good battery voltage to the feed side of the switched relay contact. Everything looked clean and it appeared everything was making good connection to the relay terminals. So next I opened up the relay (cover just snaps on) and while everything looks good inside, I noticed there's a resistor across the relay coil. I was expecting to see a diode here, but as I did more research for a replacement relay, it appears resistors are not uncommon for this 70A relay. I installed the relay into the distribution block with the cover off. With the start button in the Run position, I had battery voltage to the supply side of the relay, but still was measuring low voltage on the fuses. However, if I pushed slightly on the end of the relay paddle that gets sucked in by the magnetic field, modules dinged and I had full battery voltage to the fuses. I tried cleaning the relay contacts and flowed solder onto the coil connections and resistor connections (which originally were spot welded). This got the relay to where it worked better. I was now getting 10.3 volts to the fuses, which happens to be enough for them to turn on. I have a new relay on order (the engineer in me isn't willing to pay $30 for a generic relay from the parts store or $40 from the dealer, when I can get a Bosch online (hopefully not counterfeit) for $15) but even with the "cleaned" relay, I now have a clean bill of heath from the CAN topology!

I didn't find much info online/on Youtube pertaining to voltage causing these CAN bus communication faults, or for the Run Relay failing in this way, so I thought I'd make this long to hopefully get some sympathy, err, possibly save others from disassembling as much as I did only to find it was a relatively simple issue.


CAN topology after fixing the issue


Bad Run Relay part number

-Rod