Overview of ATC System Diagnostics With DRB

AUTOMATIC TEMPERATURE CONTROL DIAGNOSTICS
The ATC controller is designed with on-board diagnostics which are capable of troubleshooting each input and output circuit of the controller. When a fault is detected and in memory, an "Er" is momentarily displayed, but only once during an ignition cycle. There are three different groups of testing features that this system is capable of:

1. Fault Codes
2. Input Circuit Testing
3. Output Circuit Testing/Actuator Tests


The ATC system is fully addressable with DRB III and it still retains the on-board diagnostics from the past years. The ATC system will report 21 Diagnostic Trouble Codes. All of the inputs to the ATC head can be read with DRB and the outputs can be actuated. Diagnosis of the ATC system should begin with reading active codes on the DRB. If no active codes are displayed, read the stored codes next.


If any of the actuators are replaced or the A/C, heater housing is replaced, the system should be recalibrated. This happens automatically at battery disconnect or it can be accomplished using the DRB III. Without recalibration, the ATC head will not be sure of the actuators positions and may result in poor air control.

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 mulitplex 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, most 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 and also provides some electromagnetic protection for the bus. In this vehicle, the bias is supplied by the Body Computer and door modules, termination is provided by the PCM and BCM. Without termination, voltage on one bus wire goes to approximately 5 volts and 0 volts on the other wire.


NOTE: Communication over the bus is essential to the proper operation of the vehicle's on board diagnostic systems and the DRB. Problems with the operation of the bus or DRB must be corrected before proceeding with diagnostic testing.