AUTOINC. ONLINE - Data Line Communications Made Simple

Data Line Communications Made Simple

Posted 7/1/2008
By Kevin Caple

On today’s cars it’s not uncommon to find multiple computers and modules that need to communicate with each other. Several manufacturers are now using modules at each door to control the power windows, door locks and mirrors. Using a module to control these functions can save miles of wire as well as potential connection problems.

On a conventional power window system, a master switch at the driver’s door and individual switches at the other doors were connected together by several wires. These wires carried a fair amount of current that increased the chance of electro magnetic interference (EMI) with other electronics.

"On today's cars it's not uncommon to find multiple computers and modules that need to communicate with each other...The solid-state circuitry of these modules is proving to be quite reliable, and engineers have incorporated some helpful diagnostic aids in the modules."

Now, with the module system, there is a power, a ground and a data wire. When activating the “down” button for the rear window at the master switch, a message is sent from the driver’s door module to the rear door module via the data line (a low voltage, low current signal). So the rear door module now controls the window motor. The data line is typically a 0 to 5 volt or a 0 to 7 volt square wave pattern. Note: Even a lab scope cannot decipher the information the square wave message contains; a scan tool with the proper software is needed to read the data.

The solid-state circuitry of these modules is proving to be quite reliable, and engineers have incorporated some helpful diagnostic aids in the modules. If a module fails to communicate with other modules, the modules that are still working will set a code that is typically a “U” series code followed by a number that will help identify which module is not communicating. For example, in the door module, if the right rear door module has failed, the other modules on the data line will set codes indicating that they cannot communicate with the right rear door module. The diagnostics are easy – verify power and ground to the module and that the data line is not open. If it is OK, then the fault is a bad module.

Did you notice how we were able to diagnose this problem from the driver’s seat with a scan tool and a definition of codes? The last step involved meters and actual disassembly but was very precise, compared with testing the old system by pulling switches and testing several wires.

Now for the downside: If the anti-lock module fails on a vehicle with a data line (I think of it as a telephone line between modules) that connects the door modules, body computer, powertrain computer, instrument cluster, anti-lock brakes, air bag module and radio, then the other modules will set codes saying they cannot communicate with the anti-lock brakes.

But, what if the module fails in a way that grounds the data line? Now none of the modules can communicate and the scan tool becomes useless! You will need to resort to disconnecting one module at a time until communication is restored with the rest of the modules, and then replace the module that was inhibiting communications. Still not too difficult, right?

In the worst case, the data line has a problem. You disconnect all of the modules one at a time and still there is no data. There may be more than one bad module (which is not uncommon if something spiked the system such as a lightning strike) or if the data line itself is shorted to power or ground. Disconnect all modules on the data line and verify there is no voltage or continuity to ground. Be sure the ignition switch is on when testing, or a short to power may go undetected.

Some models offer a splice pack, a connecter that all the data lines come to, that makes it quick and easy to disconnect modules and isolate wiring problems. Models without this feature are much more difficult to diagnose as each module will need to be accessed and disconnected. You may need to check any sources available to you to see if your model has a history of problems with any particular module. This can save you time by allowing you to start at the module that consistently shows problems on similar models. For example, if a late model GM N body car gets water underneath the carpeting, it could ruin the airbag module and connector.

Overall, the data line system with individual modules is quite reliable. Knowing how to diagnose a data problem will help make the repairs fast and accurate.

Kevin Caple is an Identifix GM specialist. He is a certified GM driveability, diagnostics and emissions technician with ASE master and L1 certification.

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