CAN Communications | Multiplex Systems

Controller Area Network Diagnosis

Electronics in today’s vehicles have grown to an unprecedented level of technology. From a handful of computers just a decade ago, today’s vehicles contain dozens of computers arranged in one or more main and sub-networks with interconnecting gateways. This new found complexity makes diagnosis vehicle electronics a challenge. Controller Area Network (CAN) is the new standard communication protocol between the scan tool and the vehicle that will be fully implemented by 2008. Between 2003 and 2005, this robust new system will be phased in to approximately 25% of he vehicle fleet. After 2005, the phase-in will continue at a rate of 25% per year until all new vehicles are CAN compliant by 2008. CAN has been around for many years on European vehicles and will provide many new enhancements to, your diagnostic capabilities. This course will cover the basics of CAN, how to recognize a CAN system, and will focus on changes that will affect your diagnostic strategies.

Communication Protocols

  • UAR
  • KWP 2000
  • ISO 9141
  • J-1850 VPW
  • J-1850 PWM
  • CAN

There have been many protocols used for vehicle to scan tool communications over the years. An OBD-II compliant vehicle can use any of the listed communication protocols: J1850 PWM, J1850 VPW, IS09141-2, Keyword Protocol 2000, and more recently, IS015765-4/SAE J2480 or CAN. Some OEMs still use UART as a scan tool communications protocol for non-emissions related components. U.S. car manufacturers began implementing the CAN protocol in model year 2003. ISO 9141 and J-1850 were implemented in an attempt to standardize OBD-II scan tool communications. However, each vehicle manufacturer implemented the protocol slightly differently. (ISO 9141 is a J-1850 protocol used mainly by European vehicle manufacturers.) The result was three different OBD-II network protocols that all meet the J-1850 standard but are not compatible with each other. So much for a standardized scan tool communications protocol. As of 2004 federal law mandated all vehicle manufacturers to standardize the communications.

UART | Universal Asynchronous Receive and Transmit

  • Used since the early 1980’s
  • Allows simple communication between the ECM and scan tool
  • 8.2 Kbps bandwidth
  • Allows some bi-direction controls

Multiplex System


The solution to much of this problem was in-vehicle networking, or multiplexing. Multiplexing is the method in which multiple controllers transmit and receive data on a single circuit. That circuit can be a single wire or a pair of wires. Sensors may still be hard wired to a specific controller, but that controller can share the data by broadcasting the information to the multiplex bus. Information is transmitted digitally encoded, not as an analog voltage. One of the first vehicles to use multiplexing was the 1981 Chrysler Imperial and these systems were limited to electronic instrument cluster only. This module had two micro-computers mounted on one circuit board. They communicated via a simple serial bus within the electronic cluster. Unknown to many technicians, vehicle networks have been around for quite a while. It is usually not until a communications error occurs that the technician ever realizes the vehicle uses one or more network protocols for module-to-module or module-to-scan tool communication.

Dodge Caravan CAN Wiring

The images above are of the transmission controller in a 1999 Dodge Caravan. On first glance there is no indication of a network, but on closer inspection the twisted pair wiring is visible.

Computer Network Illustration

What is a Network? A network is a collection of computers or electrical devices interconnected in such a way as to allow communication and or data sharing. The CAN network in a vehicle is considered a small area network or SAN in PC terminology. A SAN is similar to a LAN or local area network where multiple PC’s are interconnected in a single building.


This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


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