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
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.
- 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|
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.
- 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.