OBD II Diagnostic Management System

Powertrain Control Module (PCM) Location

Class 2 Serial Data Bus
OBD II technology requires a much more sophisticated PCM than does OBD II technology. The OBD II PCM diagnostic management system not only monitors systems and components that can impact emissions, but they also run active tests on these systems and components. The decision making functions of OBD II PCM have also greatly increased. To accommodate this expansion in diagnostic complexity, Isuzu engineers have designed the Class 2 serial data bus, which meets SAE J1850 recommended practice for serial data.





"Serial Data" refers to information which is transferred in a linear fashion - over a single line, one bit at a time. A "Data Bus" is an electronic pathway through which serial data travels.
RODEO previously used a 5 volt data bus called UART, which is an acronym for "Universal Asynchronous Receive and Transmit". When neither the vehicle's control module nor the diagnostic tool, such as a Tech 2, are "talking," the voltage level of the bus at rest is 5 volts. The two computers talk to each other at a rate of 8,192 bits per second, by toggling or switching the voltage on the data bus from 5 volts to ground.





Class 2 data, which is used on OBD II vehicles, is quite different. Data is transferred at a rate of 10.4 kilobits per second, and the voltage is toggled between zero and 7 volts.
Class 2 data is also pulse width modulated. Each bit of information can have one of two lengths: long or short. On the other hand, UART data bits come in only one length (short). The pulse width modulation of Class 2 data allows better utilization of the data line.
The message carried on Class 2 data streams are also prioritized. This means that if two devices try to communicate on the data line at the same time, only the higher priority message will continue. The device with the lower priority message must wait.

Note: The Class 2 data wire is always terminal 2 of the new 16-terminal Data Link Connector (DLC).












16 - Terminal Data Link Connector (DLC)
OBD II standardizes Data Link Connector (DLC) configurations. The DLC, formerly referred to as the ALDL, will be a 16-terminal connector found on the lower left side of the drivers side instrument panel. All manufacturers must conform to this 16-terminal standard.

Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) looks the same as the MIL you are already familiar with ("CHECK ENGINE" lamp). However, OBD II requires for it illuminate under a strict set of guidelines. Basically, the MIL is turned on when the PCM detects a DTC that will impact the vehicle's emissions.
The MIL is under the control of the Diagnostic Executive. The MIL will be turned on if a component or system which has an impact on vehicle emissions indicates a malfunction or fails to pass an emissions- related diagnostic test. It will stay on until the system or component passes the same test, for three consecutive trips, with no emissions- related faults.

DTC Types
Each DTC is directly related to a diagnostic test. The Diagnostic Management System sets DTC based on the failure of the tests during a trip or trips. Certain tests must fail two (2) consecutive trips before the DTC is set. The following are the four (4) types of DTCs and the characteristics of those codes:

TYPE A
- Emissions related
- Requests illumination of the MIL of the first trip with a fail
- Stores a History DTC on the first trip with a fail
- Stores a Freeze Frame (if empty) (DTC Information for 6VD1 engine)
- Stores a Fail Record
- Updates the Fail Record each time the diagnostic test fails

TYPE B
- Emissions related
- "Armed" after one (1) trip with a fail
- "Disarmed" after one (1) trip with a pass
- Requests illumination of the MIL on the second consecutive trip with a fail
- Stored a History DTC on the second consecutive trip with a fail (The DTC will be armed after the first fail)
- Stores a Fail Record when the first test fails (not dependent on consecutive trip fails)
- Updates the Fail Record each time the diagnostic test fails

(Some special conditions apply to misfire and fuel trip DTCs)

TYPE C (if the vehicle is so equipped)
- Non-Emissions related
- Requests illumination of the Service Lamp or the service message on the Drive Information Center (DIC) on the first trip with a fail.
- Stores a History DTC on the first trip with a fail.
- Does not store a Freeze Frame
- Stores Fail Record when test fails
- Updates the Fail Record each time the diagnostic test fails

TYPE D
- Non-Emissions related
- Not request illumination of any lamp
- Stores a History DTC on the first trip with a fail
- Does not store a Freeze Frame
- Stores Fail Record when test fails
- Updates the Fail Record each time the diagnostic test fails

IMPORTANT: Only four Fail Records can be stored. Each Fail Record is for a different DTC. It is possible that there will not be Fail Records for every DTC if multiple DTCs are set.