AutoFocus: Ford multiplexing challenges and a Crown Vic quick fix

Sept. 7, 2010

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. It not only increases the requirement of diagnostic equipment, since many of these problems can only be diagnosed with factory scanners, but also 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.

Recently, I took a call on a 2007 Ford Crown Victoria. The complaint was that the 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.

1. 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.

2. Further tracing of the circuit found a broken wire in the harness. 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.

(For information on Identifix, visit