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Home >> Isuzu Truck >> 1999 >> Hombre XS Space Cab 2WD V6-4.3L >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Testing and Inspection >> Initial Inspection and Diagnostic Overview >> Description of On-Board Diagnostics >> The Diagnostic Executive

The Diagnostic Executive

The Diagnostic Executive is a unique segment of the software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:
- Monitoring the Diagnostic Test Enabling Conditions
- Requesting the Malfunction Indicator Light (MIL)
- Illuminating the MIL.
- Recording Pending, Current, and History DTCs
- Storing and Erasing Freeze Frame Data
- Monitoring and Recording Test Status information

Trip
The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as a key ON and key OFF cycle in which all the enabling criteria for a given diagnostics as been met allowing the diagnostic to run vehicle operation, followed by an engine OFF period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

Warm-Up Cycle
A Warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen greater than 40 °F (4 °C) from the start-up temperature and reached a minimum engine coolant temperature of 160 °F (71 °C). If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information
The diagnostic Tables and the functional checks are designed in order to locate a poor circuit or a malfunctioning component through a process of logical decisions. The Tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures. The language of communicating the source of the Malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the Malfunction Indicator Lamp (MIL) will illuminate on some applications.

Malfunction Indicator Lamp (MIL)
The MIL (Malfunction Indicator Lamp) is on the instrument panel. The MIL has the following functions:
- The MIL informs the driver that a fault that affects the emission levels of the vehicle has occurred. The owner should take the vehicle in for service as soon as possible.
- As a bulb and system check, the MIL (Malfunction Indicator Lamp) comes ON with the key ON and the engine not running. When the engine is started, the MIL turns OFF if no DTCs are set.

When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.


MIL Requests and History Codes
The Diagnostic Executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:
- Type A - Is emissions related, and turns ON the MIL the first time the diagnostic executive reports a fault.
- Type B - Is emissions related, and turns ON the MIL if the fault is active for 2 consecutive driving cycles.
- Type Cycles if - Is non-emissions related, and does not turn ON the MIL.

When a type A diagnostic test reports a failure, the Diagnostic Executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the Diagnostic Executive turns on the MIL for that diagnostic test. The Diagnostic Executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:
- The load conditions must be within 10% of the vehicle load present when the diagnostic executive reported the failure.
- The engine speed conditions must be within 375 RPM of the engine speed present when the diagnostic executive reported the failure.
- The engine coolant temperature must have been in the same range present when the diagnostic executive reported the failure.

When the Diagnostic Executive requests the service light to be turned ON or a type Cycles if diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.
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