Recently a customer called to tell us that her 2006 Mazda Tribute was acting strange. The check-engine light, ABS light, traction-control light, cruise-control light, battery light and brake lights were on or flashing. In addition, the speedometer needle was jumping up and down, and the odometer was flashing.
When the car arrived at the shop, I went to confirm the complaint. The speedometer head looked like a Christmas tree, so we knew we were going to have to pull error codes by hooking the car up to our computer. When we tried to pull codes from the data-link connector (DLC), there was no communication with any module.
The DLC – usually located under or near the steering wheel – is how we access the car’s onboard diagnostic II system. The DLC is a multi-pin connector port that enables a scanner to see live and stored data. This is done through a control area network (CAN bus), which is designed to allow many small computers or devices to communicate with each other without a master computer.
These CAN systems use a message-based protocol. This is done through multiplexing, a method in which multiple digital or analog signals may be shared into one signal over a shared medium. The primary reason for this was to save weight. The best day-to-day example of using a multiplex wire is the ethernet cable that connects your computer to a modem. Without such a cable, the wire harness would be much bulkier and heavier.
Back to our Mazda. When we can’t access the car’s network through the DLC, it’s because something on the CAN bus has failed or shorted. But first, we checked all the basics. We made sure the battery was correctly installed and properly charged. We made sure the alternator was operating correctly and checked the main fuse box. To make sure we didn’t have a problem with our scanner, we performed a protocol check on it and then connected it to a different car; it worked fine.
Realizing a module or component is pulling the system down is not the problem – it’s finding the culprit.
Figuring out a diagnostic strategy is key. We knew the instrument cluster was having a majority of the problems, but that was to be expected. The instrument cluster is like a switch or a gateway. Many systems have to report back to the instrument cluster to provide the driver with information. It’s used as a gateway to communicate with most of the CAN bus.
The next step is to disconnect each circuit or module, depending on their accessibility. If a module is difficult to disconnect, we can pull its fuse and hope it will become disabled. After we disconnect each model or circuit, we try to communicate with the DLC.
After disconnecting the ABS module, we were able to communicate with the car. We pulled the following codes: general module (GEM) U2033 vehicle communication network fault; ABS U1900 ABS CAN communication bus fault; powertrain control module (PCM) fault; P0462 fuel level sensor low input; U0121 lost communication with ABS module; and U0155 lost communication with instrument cluster. We then inspected the wire harness and connector to the ABS module. The lock on the connector had some damage, but the wiring was OK. Something had shorted inside the ABS model and was corrupting the CAN communication on the whole system.
We tried to order a new ABS module, only to find out Mazda discontinued them. So we ordered a used one and installed it. We got a code for mismatched vehicle identification number (VIN), but since we have the Mazda factory scanner we rewrote the old VIN number to our customer’s car. We then test-drove the car and all was OK.