[h=3]CAN System Faults[/h]CAN-compliant vehicles are just as vulnerable to electronic faults as older vehicles. Though CAN systems use fewer wires and fewer connectors to save weight and cost, they also use more modules and more complicated modules. Communication problems can occur if module connectors become corroded or loose, if wires become grounded, shorted or break, or system voltage is below specifications. Some modules may even forget their settings or locations if the battery is disconnected or goes dead.
On some Chrysler minivans, for example, the automatic climate control system will quit working if battery power is lost. This happens because the electric stepper motors that control the position of the blend doors forget their locations. The system has to be put into a "relearn" mode to re-establish all the motor locations and settings.
Various kinds of problems can occur on other CAN-equipped vehicles when the battery is disconnected or goes dead. The modules in the CAN system require a certain amount of voltage for their Keep Alive Memory settings. If this is lost, the module will forget these settings and may not function properly until it has time to relearn the lost data. In some cases, this requires a special relearn procedure using a scan tool because the module can't do the relearn by itself. And on some vehicles, the module may go to sleep and not wake up until it is pinged by a scan tool or the main gateway module (usually the body control module). Relearning procedures typically require a factory scan tool or a professional level aftermarket scan tool.
One of the features of CAN and other network systems is that modules can send and receive "ok" signals to let the main control module know if they are working or not. In theory, this makes diagnostics easier. On the other hand, it also means that one misbehaving module may generate enough noise to disrupt the entire network causing a complete shutdown of the vehicle!
When a serial bus communication problem occurs, it will usually set a "U" diagnostic trouble code (DTC) and turn on the Malfunction Indicator Lamp (MIL). Depending on the fault, the vehicle may or may not start, or it may only operate in a "limp-in" mode with limited capabilities. Loss of communication between the engine controller and transmission controller (code U1026 on a GM, for example) may put the transmission into a limp-in mode where it will only operate in one or two gears.
Loss of communication codes may indicate a wiring problem on the bus, or a fault with a module. Isolating the fault may require unplugging modules one at a time until the fault is found. Just remember that all modules on a bus network need three things to function properly: power, ground and a serial data connection.
When diagnosing bus or module communication problems, you usually start by checking for voltage at the module, then the ground connection, and finally the data line. If all three are good but the module isn't working, the module needs to be replaced.
On GM applications, a code U100 or U1255 means a general loss of communication on the data bus. With a Tech 2 scan tool, you can go to Diagnostic Circuit Check, then Message Monitor to see a list of active modules and compare it to the list of modules that are supposed to be on when the key is on.
To minimize the parasitic current drain on the battery when the vehicle is off, a "sleep" signal is sent to the modules on the network. Some may remain on for a short period of time after the ignition is switched off (air bag module, for example), and some may never go to sleep (anti-theft module and keyless entry receiver, for example) but most are put into a sleep mode to save battery power. If the sleep signal is never sent, or a module fails to recognize the sleep signal, it may remain active and pull power from the battery. Depending on the current draw, this may run down the battery if the vehicle sits for a period of time.
Looks expensive Jay.
