Communication

VEHICLE COMMUNICATION
The Chrysler Collision Detection multiplex system (CCD bus) consists of a twisted pair of wires. These wires run from one module to another. They receive and deliver coded information between the modules. The information is coded to identify the message as well as the importance of the message. When there are multiple messages trying to access the CCD bus at one time, the code determines the message that has the higher priority, and is then allowed to access the bus first.

The two wires of the twisted pair that make up the CCD multiplex system are called "bus +" (bus plus) and "bus -" (bus minus) respectively. Each wire has a measureable voltage level of roughly 2.5 volts. In order to maintain the 2.5 volts on each line and provide a means of transportation for the coded messages, there is a voltage divider network located in the module that supplies bus bias or voltage. Along with the module that supplies bias, some modules provide termination. Bias is the voltage necessary to make the bus operational. At least one point of termination is necessary, some systems use more than one. Some modules are capable of biasing and terminating the bus by themselves. Termination in the circuitry (a 120 ohm resistor placed across the bus) is required to complete the voltage divider network circuit and also provide some electromagnetic protection for the bus. Without termination, voltage on the bus goes to approximately 5 volts on one wire and 0 volts on the other wire. In the 2002 Ram Truck, the instrument cluster supplies bus bias and termination and the PCM supplies and additional point of termination.

NOTE: COMMUNICATION OVER THE BUS IS ESSENTIAL TO THE PROPER OPERATION OF THE VEHICLES ON-BOARD DIAGNOSTIC SYSTEMS AND THE DRB III (R). PROBLEMS WITH THE OPERATION OF THE BUS OR THE DRB III (R) MUST BE CORRECTED BEFORE PROCEEDING WITH DIAGNOSTIC TESTING. IF THERE IS A PROBLEM, REFER TO THE VEHICLE COMMUNICATIONS.

Bus Failure Messages
Short to Battery Either or both of the bus wires are shorted to a battery potential, or a specific ground circuit may be open.
Short to 5 Volts Either or both of the bus wires are shorted to a 5-volt potential.
Short to Ground Either or both of the bus wires are shorted to ground.
Bus (+) & Bus (-) Shorted Together The two bus wires are shorted together.
No Termination The bus system has lost connection with all of its terminators. The Instrument Cluster (MIC) and the Powertrain Control Module (PCM) supply termination for the 2002 Ram Trucks.
Bus Bias Level Too Low Either or both of the bus wire potentials are significantly below their normal 2.5 volts.
Bus Bias Level Too High Either or both of the bus wire potentials are significantly above their normal 2.5 volts.
No Bus Bias The bus system has lost connection with all modules that provide bias. The Instrument Cluster (MIC) provides the CCD bus bias for the 2002 Ram Trucks.
Bus (+) Open The bus (+) wire has lost connection with termination and/or bias.
Bus (-) Open The bus (-) wire has lost connection with termination and/or bias.
Not receiving Bus Messages Correctly The DRB III (R) cannot communicate over the bus and does not know why.