Network communication diagnostics

Sept. 1, 2017
Regarding operation and diagnostics, today, if you press a button to pop a SUV’s hatch, your switch is likely sending a LIN bus message to the BCM to broadcast another message on CAN to a lift gate module so it can activate an output driver circuit that provides the voltage to the hatch release solenoid.

You’re probably an old movie buff if you remember Paul Newman’s 1967 movie Cool Hand Luke. There’s a famous line in that classic prison film where the captain of the prison guard (Strother Martin) beats the rebellious inmate Luke (played by Paul Newman) and says, “What we’re dealing with here is a failure to communicate!” If you’ve ever felt beaten, figuratively speaking, by a vehicle experiencing a “failure to communicate” this article is for you!

Brief history of automotive multiplexing

Serial data bus communications, or multiplexing as it’s sometimes referred to, has been on vehicles dating back earlier than the 1980s. The entire point of multiplexing (or MUX for short) is to eliminate the need for a wire for every component’s purpose on the vehicle. An instrument cluster, for example, could have dozens of circuits connected to it to provide information such as VSS, RPMs, fuel level and engine temperature. To lighten the wiring harnesses throughout the vehicle, the instrument cluster, in this example, could simply connect to a serial bus transmitting the needed sensor values with one or two data bus circuits. Besides weight reduction, more complex system interactions can happen as in the case of stability controls. When the ABS module senses wheel slip on acceleration it can notify the PCM of the lack of traction. The PCM then reduces the electronic throttle body’s throttle blade angle to lessen torque applied to the wheels until the wheel slip condition has ceased.

Regarding operation and diagnostics, today, if you press a button to pop a SUV’s hatch your switch is likely sending a LIN bus message to the BCM to broadcast another message on CAN to a lift gate module so it can activate an output driver circuit that provides the voltage to the hatch release solenoid. If you long for the ‘good ole days’ when you could just trace the wires from the switch to the lift gate release solenoid, you’ve not been taking advantage of technology. You may be able to use your scan tool’s ability to monitor the lift gate switch’s status, BCM lift gate activation request input, and lift gate module output status. Furthermore, you may also be able to use your scan tool’s bi-directional capability to send a request to the lift gate module to power the lift gate release solenoid. A few button presses and you’ve pinpointed the most likely problem for the lift gate not working. If you like fast diagnostics, data buses make these days the “good old days!”

Bus architecture and commonly used terms

  1. Nodes

Modules that are connected to a serial bus are sometimes referred to as “Nodes.” Remember when a scan tool is connected to the DLC, it becomes a “node on the bus,” too. That also applies to those aftermarket telematics dongles that insurance companies provide to customers in order to track mileage/times on the road to verify vehicle use behaviors that afford better rates. The problem is these “nodes” can sometimes corrupt the bus messages causing a very wide variety of possible symptoms.

  1. Protocols & Gateway Modules

The protocol of the bus pertains to the structuring of bits of data. You might say protocols are like languages. Many written languages use the A through Z alphabet. Data buses simply use 0’s and 1’s of voltage shifts or light (for fiber optic buses) states. In language, the same alphabet can be arranged in many different manners to present words and phrases in English, French or German while the same binary states (1’s and 0’s) in a data bus can be used for UART, CAN, Flexray, MOST, etc. Gateway modules (Figures 1, 2, 3) are comparable to language translators; they can communicate on more than one protocol/language.

Figure 1 Figure 2
Figure 3
  1. U-Codes

If P-Codes are Powertrain, B-Codes are Body, C-Codes are Chassis, what are U-Codes? U-Codes are communications DTCs that are set in a module when that module determines that another module is not communicating on the bus. It’s important to remember in diagnostics that the least likely module to suspect as the cause of a U-Code is the module that set the U-Code.

  1. Bus Architecture – Loops and Stars

Understanding how modules are wired is helpful in the diagnosis of a problem. Buses that connect all the modules to a single point/shorting bar is referred to as a Star Configuration. Buses that connect all of the modules in a chain is referred to as a Loop Configuration. A combination of both is often used. Regardless all modules are connected in a parallel fashion, electrically speaking. However, when trying to isolate a bus circuit with a suspected short within a module or section of wiring, the star configuration provides a faster method. With star configurations, you simply go to the one or two shorting bars and remove branches/modules from the bus one at a time to see if the condition is corrected. In loop configurations, you must locate individual modules and disconnect them at their connectors to isolate them from the bus.

Data bus ABCs

CAN is CAN right? Not exactly. Data buses are divided into three speeds starting from the slowest to the fastest.  Meaningless tech trivia? Not really. Chrysler and Mercedes are two examples of OEMs that use bus speed classifications in their service information to differentiate high speed from medium speed CAN bus circuits (Figure 6).

Note: Bus speeds can be expressed in baud (i.e. 8192 baud) or 8.192 kbps (thousand bits per second).

Class A —  Mostly older vehicles, 160 up to 10 kbps. Older GM UART (160 and 8.192 kbps) and Chrysler CCD (Chrysler Collision Detection) at 7.8125 kbps are examples.

Class B — Current vehicles. Communicates module to module from over 10k baud of up to 125 kbps. GM, Ford and Chrysler J1850 (GM Class 2, Chrysler PCI and Ford SCP), GM LS (Low Speed) LAN (CAN), Ford MS (Medium Speed) CAN and Chrysler IHS (Interior High Speed) all work within this speed classification.

Class C — Current vehicles. Communicates module to module from 125 kbps up to 1 Mbps (1 million) for powertrain and chassis modules.

Figure 4

Diagnostic Tech Tip: If you’re not sure which bus is which on a vehicle (schematics aren’t always 100 percent accurate) you can use a DSO (Digital Storage Oscilloscope) to clock out the bus’s speed  (Figure 4).

Arbitration – Data bus hierarchy

Each module on a data bus is referred to as a node and has a unique identifier. All modules on the bus are electrically wired in parallel and see the same messages sent but only respond when a message is pertinent to them. To prevent modules from attempting to communicate simultaneously, nodes and select types of messages are assigned priority levels. For safety (stability controls) powertrain and chassis modules are given highest priority (1); audio and navigation devices are often medium (2) priority; and simple activation of lighting may be lowest priority (3). A process known as arbitration is used to give the module with the longest dominant status (pulling the bus low) the priority to speak first. Because the speed of CAN is so high, everything seems instant.

Understanding and diagnosing CAN bus errors

A major player on any vehicle’s CAN bus is the CAN controller chip in each module on the CAN bus. Each module (or node) on the network can communicate with other modules on the vehicle with messages that are relative to it. The signal is differential: each of the CAN lines is referenced to the other line, not to vehicle ground. This has significantly better noise rejection when used in electrically noisy environments like vehicles.  However, in order for a CAN bus chip inside an electronic module to work flawlessly at transmitting and receiving messages a few things must be right.

  1. Good Power and Ground Feeds — Power and ground circuits to modules on the bus must be capable of sourcing sufficient current for the module to work correctly. Ensure this by checking for excessive voltage drops via a substitute load in place of a module that’s being checked for communications problems.
  1. Proper EMI (Electro Magnetic Interference) Housekeeping  — Use a scope to test the power/ground circuits connected to module suspected of causing bus problems to check for transient voltage spikes and excess AC (ripple voltage). Also inspect the wiring on HS CAN bus circuits to ensure the two bus wires are in a twisted pair arrangement to prevent data corruption. Twisted pair wiring allows dual wire CAN bus signals to not run parallel to other circuits that are in close physical proximity and might have EMI inducing potential. About one twist per inch for dual wire CAN bus circuits seems to be the average.
  1. COP (Computer Operating Pulse) Free of Resets  — Use a scope to monitor for complete data bus signal drop outs. On vehicles such as Chrysler using an ASD (Auto Shut Down) relay ensure that relay is not turning off and on rapidly as COP resets occur. Bus signal drops out that appear as if the ignition was switched off are also indicators of COP resets.
Figure 5
Figure 6
  1. Proper Terminating Resistors on High Speed Buses — High speed buses such as 2-wire Class C CAN use a terminating resistor at each end of the bus (Figures 5, 6) in order to reduce reflected power. If the resistance is not correct, CAN signal errors become more likely. With the ignition off and the bus asleep, CAN buses should have approx. 60 ohms measured between pins 6 & 14.     
  1. Corrupted Software — Sometimes a communications bus experiences intermittent issues due to software problems. Searching for software updates via TSBs applies to most vehicles.  Frequently on GM models and occasionally on some other OEMs and, there are software updates for modules that are not documented in normal TSBs. This is where some reprogramming prep knowledge comes in for knowing the various OEM websites that give you the ability to look up the most current software p/n’s for any module in question.

Diagnostic help

  1. Using a DMM (Digital Multi Meter) at the DLC can be useful in detecting a bus that is:
  1. Experiencing a continuous short to ground
  2. Experiencing a continuous short to power
  3. Totally inactive
  4. Not connected to correct value termination resistors (high speed CAN applications)
  1. DSO use is preferred for conditions where the bus is intermittent:
    1. Intermittent loss of signal
    2. Ground Offsets (intermittent ECU ground issues)
    3. Intermittent single or double opens on the bus
    4. Intermittent bus shorts to power or ground
    5. Error Messages.  Sometimes bus errors don’t set DTCs but might be identified with a bus decoding device such as the one built into Pico Scope software (Figure 9).
Figure 7 Figure 8
Figure 9

With both scopes and meters, a BOB (Break Out Box) for the DLC makes good sense to prevent DLC terminal damage while testing (Figures 7, 8).

  1. Using a factory scan tool

A factory OEM dealer scan tool is preferred in cases where your aftermarket “OE level” scan tool will not communicate with more than one module. Building your vehicle as a year older or newer might shed light on the issue but sometimes you just have to have the OEM dealer scanner to scan some vehicle /some buses. While a few aftermarket OE Level scan tools can shed light on what modules are communicating and what ones are not (Figure 10) many dealer tools do a decent job with bus diagnostics. Many OEM tools provide;

Figure 10 Figure 11
  1. Higher degrees of vehicle bus communications reliability
  2. Utility features that perform data bus diagnostics (Figures 11, 12, 13, 14)
  3. Bus activation/pinging
Figure 12 Figure 13
Figure 14

Advanced Diagnostic Tip: This tip for those who are fortunate enough to own a micro amp clamp (the $700 kind – NOT the less expensive milliamp clamp). A module that is intermittently corrupting the bus can sometimes be identified by the fact that it will draw more current (in micro amps) on its branch of the data bus circuit than other modules operating properly draw on their branches of the data bus. 

Figure 15

Future bus challenges

Going forward you’re going to see more wired (GM) and fiber optic (European) MOST buses (Figure 15), along with buses such as Flex Ray giving manufacturers a stop gap towards simplified Ethernet networks (Figure 16) to further reduce wiring weight and complexity in the vehicle. Rest assured that Motor Age will be keeping you updated as technology changes so you can deal with that Cool Hand Luke “failure to communicate” without feeling like you’ve taken a beating!

Figure 16
About the Author

Dave Hobbs

Dave Hobbs is a senior technical trainer and curriculum developer for Delphi Technologies Aftermarket at BorgWarner Inc. He's Master ASE-certified with L1 (advanced engine performance) & L3 (hybrid) specialist certifications.

He has extensive OEM service and field engineering expertise, with more than 30 years of experience in troubleshooting vehicle systems electronics, with 15 of those years in the independent aftermarket repair business.  He has 20 years of experience in training engineers (worldwide) and service technicians in both the OEM and aftermarket arenas, as well as experience in working with postsecondary vocational / community college students as an adjunct instructor.

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