Diagnostic Communication

DIS/MoDiC to control module communication is dependent on the integrity of the Diagnostic Link (D-Bus). The diagnostic link is a wire circuit in the vehicle that connects to all control modules designed to communicate with the DIS tester. The connection point for the DIS with the diagnostic link in the vehicle is at the 20 pin diagnostic connector in the engine compartment.

The diagnostic link has evolved through the model years as follows:

RXD/TXD: The wires in the 20 pin connector are identified as RXD (pin 15) and TXD (pin 20). This configuration is used with all M1.x and M3.x systems.

- RXD is a one way communication link from the DIS/MoDiC to the control module which initiates the start of communication.

- TXD is a two way communication link where the diagnostic data is transferred back and forth between the DIS/MoDiC and the control module.

TXD II: With the introduction of OBD lithe diagnostic link changed to meet the needs of the OBD II Data Link Connector (DLC). The DLC is located in the passenger compartment and provides a connection for an aftermarket scantool for powertrain systems only. Scantool communication with these control modules is only possible when the 20 pin diagnostic connector cap is installed. A circuit bridge in the cap completes the DLC circuit.

The original RXD/TXD link is still in the vehicle for non powertrain systems. But, (control module dependent) RXD may not be used at all since later control modules communicate with the D2 protocol (different language) which only requires TXD (single wire communication).

TXD II (pin 17) was added to the 20 pin diagnostic connector to provide a separate connection to Drivetrain control modules (DME, AGS, EML) for OBD II compliance. The separate connection is for security reasons. The DIS/MoDiC share the same TXD II wire though the scantool and DIS/MoDiC communicate with different "languages".

- Scantool "speaks" ISO 9141 protocol.
- DIS/MoDiC "speak" D2 protocol.

If problems are encountered trying to establish communication, consider the following:
- Battery voltage too low (connect a battery charger prior to communication).
- Power and Ground circuits for the control module possibly open.
- Diagnostic link circuits possibly open or shorted.