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When ‘F.R.E.D.’ fails to communicate

What to do when the CAN can’t.
Tuesday, April 23, 2013 - 12:31
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Some CAN-equipped Chrysler models have an IHS CAN bus (Intermediate High Speed) Type B bus in pins 3 and 11 for non-powertrain/chassis modules. We bring up letter designations because you frequently will see Chrysler use the CAN C term in its service information. It also is important to note that many Chryslers use a separate HS CAN bus (termed CAN C Diagnostic) to allow for all scan tool communications, through a single module while the rest of the modules on the vehicle communicate on either their dual wire HS CAN C bus (powertrain and chassis modules) or a single wire CAN B bus for most other modules. This strategy is not unique to Chrysler’s CAN bus vehicles.

A DLC breakout box is a must have tool for bus troubleshooting.

Looking at their history, scan tool communications via a generic OBDII tool (Global OBDII) would on past Chrysler models (pre-CAN) be handled by another bus termed Serial Communications Interface (SCI) receive and SCI transmit for both the ECM, TCM and ABS, while the PCI (J1850) bus handles module-to-module communications and enhanced scan tool communications.  Some European models (Mercedes) also take the approach of having the vehicle’s entire scan tool communications take place through a diagnostic CAN bus dedicated to a single module.

Your ‘Gateway’ to Logic
If there was ever one single point on a complex subject to pass along, it would be information about gateway modules. Both J1850 vehicles and CAN bus vehicles are known to use one or more gateway modules that each connect to multiple buses used on the vehicle. Besides J1850 and CAN, there are buses call Local Interconnect Network (LIN) that operate at a lower speed for modules not requiring super fast communications. Components such as door and seat modules, HVAC, etc. might use this less expensive bus, which then connects to a gateway module in order for other modules to use and share info with modules on the LIN bus.

These modules are called gateway modules and pull together the different networks on the vehicle. They act as a translator for total inter-vehicle communications as well as scan tool communications. This is essential knowledge for the tech. If a service manual theory of operation or vehicle network communications schematic shows a module with more than one bus protocol connected to it, consider that module a gateway. 

This means if a module you want to look at PIDS or DTCs isn’t connected to a network that is pinned out in the DLC, your scanner is going to be going through a gateway module to communicate with that particular module. In the case of some Chrysler CAN bus vehicles, the Totally Integrated Power Module (TIPM) under the hood is the exclusive communications link between the only network pinned in the DLC (Pins 6 and 14 called CAN C Diagnostic) and your scanner. 

A resistance of 120 ohms indicates an open terminating resistor or an open in the circuit leading to it.

On GM and Ford CAN bus vehicles, the DLC might contain other circuits for other networks such as a single wire CAN. Late-model GM vehicles, which fall into its Global A architecture, might add an additional dual wire CAN bus called a chassis expansion bus. This bus is for modules such as electric power steering and intelligent stability control sensors. This bus will be pinned out in the DLC in pins 12 and 13 allowing some scan tools to communicate with that bus directly. 

The modules on that bus can still communicate to the modules on the main dual wire CAN bus (pins 6 and 14) via (you guessed it) a gateway module function of the ABS module. 

Knowing which module is the gateway(s) not only helps you determine the path communications is taking when your scan tool attempts to obtain information, it is also helpful in determining the most likely cause when there are multiple bus problems.

Diagnosing F.R.E.D.s That Aren’t Communicating
Your first order of business is knowing what networks are on the vehicle, which networks are involved in modules related to your vehicle’s symptoms, which networks are pinned in the DLC and which network your scan tool is attempting to communicate with if it eventually says No Communications. Everything mentioned except for that last item is easy enough to determine by studying the vehicle communications schematics. That last item might not be hard to determine depending on the scan tool.

Typically, older platforms of tools go to the network the scanner “thinks” that vehicle’s system should have (according to how you’ve built the vehicle when setting up the tool) and sends a message out on that network essentially saying, “Hey, I’m another node (engineer talk for module) on the bus and please send me some data.” You might have no idea which network(s) the scanner is trying to address. Some of the newer scan tools will show which network the tool is attempting to communicate on. If only one module is not communicating on a bus while others are OK, check for a blown fuse first followed by a lack of good power and ground to that module’s connector. You can’t communicate when you don’t have power or ground.

Next look for an open circuit in the network between the proper DLC pin and that module or between the gateway module your scanner is attempting to communicate through and that non-communicating module. Keep in mind your scan tool (unless it is a factory scan tool) simply might not be up to the task of communicating with certain body and chassis modules. If U codes are present in other modules, follow published diagnostics. As a general rule, the modules with U codes that point to failure to communicate in general (U1000 on J1850 networks for example) are the last to suspect as being the culprit. 

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