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  Tech Tips

Ford Multiplexing

Posted 5/17/2010
By Bob Mordorski

Fixing Multiplexed Vehicle Not Always Easy

“… module-to-module communication often makes
our job significantly more difficult.”

From an engineering standpoint, most technicians can see the logic behind module-to-module communication.

In theory, it makes sense to have a single input such as a vehicle speed sensor feed just one module, and then have that module share the information over a pair of data bus wires to other modules. Unfortunately for the technicians faced with the challenge of diagnosing and repairing these problems, module-to-module communication often makes our job significantly more difficult. Not only does it increase the requirement of diagnostic equipment, since many of these problems can only be diagnosed with factory scanners, but it requires today's technicians to be computer network diagnosticians as well.

Of course, with increased module-to-module communication comes increased unique and multiple simultaneous problems, and any one module shutting down can affect everything from interior lights malfunctioning to the vehicle not starting.

“… the best plan is to pick
one problem and concentrate
on resolving it.”

Recently, I took a call on a 2007 Ford Crown Victoria. The complaint was that the antilock braking system (ABS) light was on and the power steering was lacking assist. This vehicle model uses a variable assist power steering system that defaults to minimal assist in the event of a system failure. While trying to retrieve codes from the ABS module, we found there was no communication to the scanner. The first plan of action was to check the powers and grounds to the ABS module; the ignition power feed to the module was not showing battery voltage as it should. The fuse was checked and found to be OK. Further tracing of the circuit found a broken wire in the harness (yes, a broken wire on a vehicle just one year old!). Once the wire was repaired, communication was restored to the ABS module, the ABS light went out, and the power steering returned.

When dealing with a multiplexed vehicle with multiple problems that began at the same time, the best plan is to pick one problem and concentrate on resolving it. If too many problems are tackled at once, it's very easy to lose direction and start going in circles on a diagnosis. One useful test, if available, is to perform a "network" or automated test, where the scan tool attempts to communicate with all the modules on the vehicle's networks. If a particular module is not communicating, addressing that concern is normally a good place to start.

Bob Mordorski Bob Mordorski is an Identifix Ford team leader. He is an ASE master with L1 certification.
Experience Identifix This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.

For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time.

© 2011 IDENTIFIX. All Rights Reserved.


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