Diagnosing Electronic Systems Can Be ChallengingPosted 3/26/2004
By Kevin Caple
In the world of automotive electronics, everything is rapidly - and seemingly forever - changing. Power window switches that made windows go up and down via hard wires have been replaced by individual modules in each door using a power and ground wire and one data line wire to connect it to other modules. When the right rear window "down" button is pressed from the master switch, it sends a message over the data line that signals the right rear window to go down, and the right rear window module then supplies power and ground to the motor to carry out that request. These types of functions are possible with far less wires running through the car (some of which used to carry substantial amounts of current), which in turn helps reduce problems caused by electro-magnetic interference.
To aid in diagnostics of these data line circuits, engineers have created several different systems. One system uses a module that is designated as the "master." This master module sends out a state-of-health request ("Are you there and ready to work if needed?") to the other modules on that same data line. These modules then respond to the master: "Yes, I'm here and ready" or maybe, "I'm here, but I have a problem." In that case, the master module might turn on a "service vehicle" light or request that the message center display a warning message.
Here's an example of this type of system in today's car: a Grand Am. The instrument panel cluster microprocessor is the "master." The electronic brake control module, diagnostic energy reserve module, body computer and power control module (PCM) are all on the same data line. The cluster sends out a message: "Hey, brake control module, are you there?" The brake control module receives the message and responds, "Yes I'm here." The cluster says, "Hey, air bag module, are you there?" Air bag module replies, "Yes, I'm here." The cluster knows the data line is OK because it is able to communicate with the modules, and it knows the modules are OK because they responded with "Yes, I'm here." If the PCM has a problem with one of its sensors it will tell the cluster, "Yes, I'm here, but turn on the 'Service Vehicle Soon' light."
Notice how the modules speak the same language and talk at the same speed. In computer terms, this would be called protocol (language of data transmission) and baud rate (speed of transmission). UART, CAN and GMLAN are examples of data line languages. This is more engineering than we need to know, but it explains why you may not be able to use different parts from different cars to make a cool street rod with a data line operated power trunk release. If the trunk release module speaks "German" and the master module speaks "Norwegian," they won't understand each other.
On some systems you may find modules that "watch" other modules. Example: If the window modules, memory seat module and body computer are all on the same line and the left rear window module has a problem, you may get codes from all the other modules on that line saying they have lost communication with the left rear window module. In this case, using the first example ...
The driver's door module says, "Hey, left rear, are you there?" No response. The seat memory module says, "Hey, window modules, are you there?" No response from the left rear. The BCM says, "Hey, window modules, are you there?" No response from the left rear. Since the left rear can't talk for itself due to a problem, his buddies on the data line will set codes alerting the technician that there is a problem with the left rear window module not responding.
Having said all that, it sure makes it simple, doesn't it? Our hair-pulling problems start when we get shorts in the data line, to ground or to voltage, or anything that disrupts the flow of data between modules so they can't talk to each other as designed. These problems require removal of one module at a time (if the module is shorted and holding the data line grounded, then when the module is removed the others on the line would be able to communicate again) and check the data line itself for opens or shorts to ground or voltage. I think "labor intense" is the key phrase when it comes to wire problems.
Your biggest challenge diagnosing these systems will be getting good information concerning which modules - and how many - are on a specific data line. Let's hope the scan tool people are hard at work trying to keep their software up-to-date so we can communicate and interface with these systems. Here's the good news ... another year goes by without having to worry about good technicians being replaced by a machine!
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