The first network protocol is Chrysler's Chrysler Collision Detection (CCD) Multiplex bus. The CCD bus was first installed in 1988 and was phased out slowly beginning in 1998.
The CCD bus consists of "CCD chips," which are inside the various control modules on the bus. The bus is a two-wire, twisted pair parallel configuration with all modules joined to the bus in a parallel circuit. The two parallel bus wires are called CCD Positive (+) and CCD Negative (-) and they are tied together with either one or two 120-ohm resistors that reside separately inside one or two modules on the bus. These resistors serve as termination points. If there are two termination resistors, they are wired in parallel to each other, giving the CCD bus about 60 ohms total resistance.
The CCD bus is also supplied a 5V bias voltage by one of the modules on the bus. This bias voltage is applied to both CCD bus wires as a series circuit. There is a 5V supply applied through a 13K-ohm resistor within the CCD chip, dropping the 5V to 2.51V on the CCD- bus wire circuit. Voltage from CCD- flows through a 120-ohm termination resistor to the CCD+ bus wire circuit. The CCD+ circuit is then grounded through another 13K-ohm resistor within a CCD chip. The voltage drop through the termination resistor creates 2.51V on CCD- and 2.49V CCD+.
During data transfer, the voltages on each of the individual CCD bus wires will vary from their idle 2.5V, with the CCD- being pulled "low" and the CCD+ being pulled "high." The normal idle voltage that resides on CCD+ and CCD– are important specs from a diagnostics point of view. If the voltages are too far off, say above 2.8V or below 1.8V, then the communication on the bus stops completely or becomes invalid.
The typical way to diagnose CCD bus faults is to check the voltages on the CCD+ and CCD- wires individually during communication. You can use a two-channel graphing meter or scope to do this, using pins 3 and 6 at the OBDI DLC connector, or pins 3 and 11 if the vehicle has an OBDII DLC connector, and measuring each separately to ground. Then, if the bus voltages are incorrect, begin unplugging various modules one at a time until the bus voltages come back to normal range restoring normal communication.
If the bus voltage never comes back to normal range, the bus wires themselves may be shorted, open or subjected to outside electrical interference. The tricky part of this technique is that service information doesn't always tell you which module(s) are the bias voltage supply modules and which are the termination resistance supply modules. Obviously, the bus needs at least one bias module and one termination module in order to operate. If you unplug the circuit's only bias or only termination module during your diagnostics, then the entire bus goes down. The DRBIII scan tool functions help in this situation.
The data network diagnostics functions within the DRBIII are under the "System Monitor" menu selection. For CCD Bus, there are two subsections that enable CCD bus diagnostic functions. The screen from the first function is listed as "CCD Bus Test" shown in Figure 1.THE OUTCOMES OF THIS AUTOMATED DRBIII BUS TEST ARE:
1. Bus shorted to battery
2. Bus shorted to 5 volts
3. Bus shorted to ground
4. Bus (+) shorted to Bus (-)
5. Bus (+) and Bus (-) open
6. Bus (+) open
7. Bus (-) open
8. No Bus bias (no voltage)
9. Bus bias level too high (above 3.5 volts)
10. Bus bias level too low (below 1.5 volts)
11. No Bus termination
12. Not receiving Bus messages correctly
The next DRBIII CCD bus diagnostic feature is shown in Figure 2. This function will read and display the voltage level found on the CCD bus at that moment. This continuous live reading can be used during a wiggle test or to read the voltage levels while unplugging various modules.Another part of this test is an actual bias voltage output, which the DRBIII can toggle on and off at your command. In the event the bias voltage is not present on the vehicle bus due to a defective biasing module, the DRBIII can take its place and power the bus right from the DLC.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.
