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  • Truck Trend Shop Class: All About CAN Bus

Truck Trend Shop Class: All About CAN Bus

CAN Bus?

Alex Steele
May 22, 2019
Today's passenger vehicles can utilize about 70 onboard computers (ECUs, actuators, and so forth). So let's consider this group an extended family, and just like any family, communication means a lot. All members rely on each other, and without talking back and forth and exchanging information, a family can become dysfunctional.
The need for a robust communication network was inevitable when car and truck manufacturers began adding ECUs on a regular basis. It all started with a single Powertrain Control Module and hasn't stopped since. A primary concern in the beginning of all this technology was a reduction in wiring due to mass, cost, weight, and lack of reliability.
A basic example is a vehicle speed sensor. The powertrain control module requires that information to enhance its algorithm determining air/fuel ratio, spark timing curves, and so on. The transmission control module also needs the data to determine shift timing and force. And, yeah, the cruise control module can't maintain a pre-set speed without it.
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The use of an individual speed sensor for each module made no sense at all; an alternative was wiring the one speed sensor directly to the three ECUs. Another choice, using about the same amount of wiring, was a speed sensor circuit to the PCM (main computer), and separate speed data output circuits from the PCM to the transmission and cruise modules.
But just think about how overwhelming the number of circuits and volume of wiring would become with 70 ECUs and 100 sensors, as compared to this simplistic case in point? The wiring harness could downsize a portion of the passenger compartment, vehicle weight goes up, and the error potential due to way too many connectors, terminals, and splices could get out of hand.
Welcome the Controller Area Network (CAN bus)! Origination began in the mid- '80s by German electronics/engineering company Bosch, reached production in 1991 with Mercedes-Benz, and became part of the OBD-II standard by 1996.
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CAN bus is based on the digital signals our electronic culture was built on, used by computers, phones, and almost any device you can think of. Bits (binary digits) are values comprised by a "0" or a "1" (high or low voltage). Bytes are a set of 8 bits grouped together. This simple yet complex binary code can transmit huge amounts of data deciphered by ECUs, and ECUs can transmit commands using the same language and path.
The CAN bus uses a High and Low circuit, meaning only two wires connecting each ECU to the network. This enables ECUs to output and receive important data between each other with fewer circuits, less problems, and lower cost.
Additional networked systems include supplemental restraint, electric power steering, electric brake booster, parking brake, collision avoidance, body control (windows, door locks, and so on), HVAC, parking assist, auto start/stop, hybrid and EV systems, entertainment... You get the idea.
LIN (Local Interconnect Network)
LIN is interconnected with the CAN bus network to further reduce wiring and money spent. Not as high speed or durable as CAN bus, LIN only requires one wire between less complex components to achieve relatively simple communication. A model would be a window switch, door lock switch, and sideview mirror switch, all connected by a one-wire network to the Body Control Module (BCM). So all switches can send on/off, up/down, or left/right inputs to the BCM. From there the BCM analyzes/relays the data and sends commands through the CAN bus network.
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Communication
This part may be outside the norm, but what else is new? Hopefully you'll get a better understanding of this form of communication.
Here's a conversation between CAN bus networked ECUs, in English as opposed to bits and bytes, dealing with a system failure.
Powertrain Control Module (PCM)
I just set a Diagnostic Trouble Code (DTC) because the accelerator pedal position sensor in the electronic throttle control (ETC) system is giving an incorrect signal. This could be dangerous so I'm minimizing the allowed amount of throttle opening to avoid an accident. I also turned on the Check Engine Light to notify the driver.
ABS/Electronic Stability Control (ESC) module
Got it. Stability Control requires electronic throttle control to back off the throttle, while applying specific brake pressure to each wheel during a loss of traction. Seeing there's an ETC failure, I'll eliminate throttle control from the potential command and adjust braking accordingly. I'll also turn on the ESC warning light and set a DTC to alert the driver that the system went into a fault mode due to a relevant failure in the powertrain system.
Transmission Control Module TCM
To be safe, dealing with an ETC failure, I'll send a late-shift command and increase shift pressure to avoid any transmission damage.
The ability of software within these ECUs to analyze data, make decisions, and send appropriate commands, at super-high speeds, is imposing, and a lot of it wouldn't be possible without CAN bus communication.
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You might have noticed when a failure occurs on a lot of later-model vehicles, more than one warning indicator may light up on the instrument panel. Sometimes it looks like a Christmas tree. That's due to the effect ECUs have on each other, and manufacturers feel it's relevant enough to set a DTC and post a warning from each module involved.
Note on the conversation: auto manufacturers are using a redundant accelerator pedal position sensor, meaning two independent sensors must agree on accelerator position, or a fault will be recorded and a safe default throttle position command will be implemented. This is intended to avoid the danger of unintended acceleration. Software upgrades, referred to as "brake override", were also installed to shut down electronic throttle opening when the driver has his foot on the brake.
Diagnostics
From a technician's point of view, CAN bus is a necessity. Not only does the network enable communication between the ECUs, but also diagnostic equipment plugged into the OBD-II diagnostic link connector (DLC).
Scanners come in handheld designs, but more commonly laptop software is used at dealerships with an interface device that plugs into the DLC. It's more than just reading and erasing trouble codes. Detailed information can be obtained from all ECUs on the CAN bus network including signals from LIN linked components. Everything is bidirectional, so a large number of valuable tests can be performed, commands sent to test/activate components, calibrations, registrations, and a lot more.
A failure within the CAN bus system itself can become very involved. It all depends on where and what in the circuit failed. An open circuit near a specific ECU can be simple to find going by the "U" trouble code (communication failure) and basic circuit testing, plus no data coming from the suspect ECU. But an open circuit where the network branches off to several ECUs makes it more difficult. The worst-case scenario is a short (typically to ground) in the system. This pulls down the CAN bus circuit voltage, and nobody gets any information, including a technician just trying to view a trouble code. A failed ECU may be the cause, and can require unplugging each one in the system, one at a time, until communication comes back online. A CAN bus wire shorted to ground? Good luck with that one.
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CAN Bus Don'ts
We know a lot of folks can't resist buying aftermarket electronic toys, but shy away from anything that taps into a CAN bus High or Low circuit, please. This means of communication is voltage-sensitive and a lot of aftermarket components don't have the integrity to deal with it on a long-term basis.
Insurance companies are offering better rates if you install their DLC plug-in device which records and transmits data from your CAN bus network. There are also similar aftermarket plug-ins that allow you to view vehicle data on your phone. Both have the potential to affect CAN bus communication and cause a wide range of intermittent symptoms. It's more common than you might think. Vehicle owner's manuals specify the DLC to be used for system diagnosis, only!
When It All Goes Wrong: Worst Case
Latest news on Boeing 737 MAX aircraft indicate that an error by a single angle of attack (AOA) sensor may have played a role in the Lion Air and Ethiopian airline crashes. The sensor inputs are used by the Maneuvering Characteristics Augmentation System (MCAS) software, which may pitch the nose of an aircraft down to avoid a potential stall condition. There are two AOA sensors (left and right) at the nose of the aircraft. Their purpose is to analyze wind speed in relation to wing angle to determine a potential stall condition.
According to reports, these two AOA sensors are considered partially redundant. If one's reading significantly differs from the other, there's most likely a failure in one of the sensors. The obvious software command should be immediate notification to the pilot, put the system in a safe default mode, or disable the system entirely. Otherwise the system may accept the faulty signal as true and unnecessarily pitch the nose down to avoid a stall.

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