No Crank CAN System Troubleshooting Part 1 and 2(Ford Escape)

Just watched this series with interest. I have in my shop at this moment a vehicle that has a very similar issue. The vehicle I had was a 2009 Chevy Silverado 2500HD with a 6.6L Duramax that came from another shop as a no communication with ECM.

Story was that the other shop had put a new OEM Remanufactured ECM because the old ECM had been programmed for a DPF delete and they were attempting to put the vehicle back to stock to sell. I was told that when they installed the new ECM the truck would not communicate. After they loaded up the parts cannon and replaced the ABS, Trailer Brake and Glow plug modules, they gave up and towed the truck to my shop for me to look at.

Now that the back story is told, I wanted to share some of my observations while I was diagnosing this truck.

In the video's Paul says that he couldn't prove that the PCM on the Escape was bad and that he didn't know if there was a single module that was responsible for the CAN signals.

When I started my testing I found a good 5V supply, 60 ohms on the CAN lines and a good waveform on the BUS but still no communication from the ECM. In my testing I found a way to tell if a Module is talking on the BUS. All you have to do is isolate the module in question from the rest of the BUS and scope the CAN lines from that module. Do this by either disconnected other modules from the BUS or at a splice. All the Modules will produce the same waveform, from what I can tell there is no single module responsible for making the signals. It's kind of like all the modules are "plugged" into the CAN circuit just like you connect a Scanner.

When I isolated the ECM from the BUS I could see no waveform on the CAN lines. But when I connected it back to the CAN lines I had a good signal going to the ECM.

Yes although two modules on the can bus have built in 120 ohm terminating resistors the can bus isn't controlled be any particular module acting as master. Each module on the bus already has its' own unique identifier so dosen't have to be allocated an address by any master module acting as a router, each module on the bus listens for a free space in the bus traffic and simply broadcasts its' message packet.
The two 120 ohm terminating resistors are only there to damp signal echoes on the bus and any stray interference. 1980s' style office computer 10b2 networks that used BNC connectors also needed two terminating resistors but these were separate 50 ohm resistors simply fitted on at ends of LAN cables usually with a Tee fitting.
What is important is identifying which modules have a terminating resistor (I would guess the PCM and ABS because these will be present on every vehicle) before doing a module disconnect test.
It looks like 120 ohm resistors have now become become a must have in the tool box.

Andy.MacFadyen wrote: Yes although two modules on the can bus have built in 120 ohm terminating resistors the can bus isn't controlled be any particular module acting as master. Each module on the bus already has its' own unique identifier so dosen't have to be allocated an address by any master module acting as a router, each module on the bus listens for a free space in the bus traffic and simply broadcasts its' message packet.
The two 120 ohm terminating resistors are only there to damp signal echoes on the bus and any stray interference. 1980s' style office computer 10b2 networks that used BNC connectors also needed two terminating resistors but these were separate 50 ohm resistors simply fitted on at ends of LAN cables usually with a Tee fitting.
What is important is identifying which modules have a terminating resistor (I would guess the PCM and ABS because these will be present on every vehicle) before doing a module disconnect test.
It looks like 120 ohm resistors have now become become a must have in the tool box.

Good info. Ill have to watch that video again to refresh... Don't quite recall how and where to install and test with those resistors. Have similar problems with a vehicle with no CAN com that needs diagnosis! I Also need to order some online seeing as our local Radio shacks have closed shop.

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I like your thinking here. Kinda like Class 2 networks, using SP205 to isolate a suspect module. So, just a junk reman ECM, then?

Tyler wrote: So, just a junk reman ECM, then?

I don't know if I would call it a junk ECM out of the box. When I got the truck it had a new O.E. Reman ECM and the original ECM that had the same problem. So I would have to guess that the other shop somehow "bricked" 2 ECMs. Not sure what took them out but the truck has a third ECM (OE) and the truck runs fine.

The hardest part about isolating the suspect module from the network is in how the network is wired up. In the vehicle I had each module except for the ECM had 4 CAN wires (2 connections)to each module and a separate 120 ohm terminating resistor in the harness. I looked at the diagram for the Ford Escape and that system had 2 wires (1 connection) going to each module. So trying to isolate the PCM would be a lot harder to isolate.

In fact I had a Dodge Journey in two days ago that had a no comm to the cluster, I was having a difficult time finding the locations of the connectors to isolate the cluster. I guess stuff like that will come with more experience as I see more of these faults.

CAN bus problems are going to become more common and many shops will just throw random parts at them. Everybody here is trying to stay ahead of the learning curve but a lot the shops out there have no chance it took them 10 years plus to learn how to test basic 3 wire alternators.