Information Bus: Description and Operation

CCD BUS (+/-) CIRCUITS - PCM OUTPUTS
The Powertrain Control Module (PCM) sends certain output signals through the CCD bus circuits. These signals are used to control certain instrument panel located items and to determine certain identification numbers.
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 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." This network consists of the Mechanical Instrument Cluster (MIC) that supplies both bias and termination and the PCM that provides a second termination. Bias is the part of the voltage divider network that places both bus+ and bus- at 2.5 volts. Termination in the circuitry is required to complete the voltage divider network and it also provides some electromagnetic protection for the bus.

NOTE: COMMUNICATION OVER THE BUS IS ESSENTIAL TO THE PROPER OPERATION OF SOME VEHICLE SYSTEMS, ON BOARD DIAGNOSTIC (OBD) SYSTEMS AND COMMUNICATION WITH THE DRB. PROBLEMS WITH THE OPERATION OF THE BUS OR DRB MUST BE CORRECTED BEFORE PROCEEDING WITH.

CCD 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 to ground or a specific ground may be open.

Bus (+) or Bus (-) Shorted Together - The two bus wires are shorted together.

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 Termination - The bus system has lost connection with all of its terminators. The Instrument Cluster (MIC) and Power Train Controller (PCM) supplies termination for the 1997 Jeep Cherokee.

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 1997 Jeep Cherokee.

Not Receiving Bus Messages Correctly - The DRB cannot communicate over the bus and does not know why.