RAM 1500 Truck 2WD V6-3.9L VIN X (1999)
Information Bus: Description and Operation
Standard Cab
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 measurable 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 1999 Ram
Truck, the instrument cluster supplies bus bias and termination and the PCM supplies an additional point of termination.
Bus Failure Messages
Short to Battery - Either or both of the bus wires are shorted to the battery potential.
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 Powertrain Controller (PCM)
supply termination for the 1999 Ram Trucks.
Bus Bias Level Too Low - Either of both of the bus wire potentials are significantly below their normal 2.5 volts.
Bus Bias Level Too High - Either of 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 1999 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 cannot communicate over the bus and does not know why.
