Description of On-Board Diagnostics

System Powertrain Control Module (PCM) Location

Class 2 Serial Data Bus
OBD II technology requires a much more sophisticated PCM than does OBD I 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 PCMs 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.

TROOPER 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 Tech2, 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 communication 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 driver's 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 that 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 tails 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.

Types of Diagnostic Trouble Codes (DTCs)
The Diagnostic Executive classifies Diagnostic Trouble Codes (DTCs) into certain categories. Each type has different requirements to set the code, and the Diagnostic Executive will only illuminate the Malfunction Indicator Lamp (MIL) for emissions-related DTCs. DTCs fall into four categories: A, B, C, and D; only types A and B are emission-related The following descriptions define these categories:

TYPE A
Will store the DTC and turn on the MIL ("check Engine" lamp) on the first trip in which an emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive.

TYPE B
Will store the DTC and turn on the MIL on the second consecutive trip in which an emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. After one failure, the type B DTC is "armed," or prepared to store a history code and turn on the MIL if a second failure occurs. One passed test will disarm a type B DTC. Some special conditions apply to misfire and fuel trim DTCs. For a type B DTC to store and turn on the MIL, two ignition cycles are required.

TYPE C
Will store the DTC and turn on a "SERVICE" lamp ("Check Trans" lamp) on the first trip that a non-emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. This type of DTC will be used in future applications.

TYPE D
Will store a DTC but will not turn on the MIL on the first trip that a non-emission-related diagnostic test has run and reported a "test failed" to the Diagnostic Executive. These codes can be very helpful for vehicle service when the driver may comment about a condition, but the MIL did not turn on.