System Description
DIAGNOSIS SYSTEMWhen troubleshooting On-Board Diagnostic (OBD II) vehicles, the vehicle must be connected to the hand-held tester (complying with SAE J1987). Various data output from the vehicle's ECM can then be read.
OBD II regulations require that the vehicle's on-board computer illuminates the Malfunction Indicator Lamp (MIL) on the instrument panel when the computer detects a malfunction in:
1. The emission control system/components
2. The powertrain control components (which affect vehicle emissions)
3. The computers
In addition, the applicable Diagnostic Trouble Codes (DTCs) prescribed by SAE J2012 are recorded in the ECM memory. If the malfunction does not reoccur in 3 consecutive trips, the MIL turns off automatically but the DTCs remain recorded in the ECM memory.
To check DTCs, connect the hand-held tester to the Data Link Connector 3 (DLC3) of the vehicle. The hand-held tester displays DTCs, the freeze frame data and a variety of the engine data.
The DTCs and freeze frame data can be erased with the handheld tester.
In order to enhance OBD function on vehicles and develop the Off-Board diagnosis system, CAN communication has been introduced in this system (CAN: Controller Area Network). It minimizes the gap between technician skills and vehicle technology. CAN is a network, which uses a pair of data transmission lines, spanning multiple computers and sensors. It allows high speed communication between the systems and simplification of the wire harness connection.
Since this system is equipped with CAN communication, CAN VIM (VIM: Vehicle Interface Module) connecting with handheld tester is necessary to display any information from the ECM on the tester. (Also communication between the handheld tester and the ECM uses a CAN communication signal.) When confirming DTCs and any data of the ECM, connect the CAN VIM between the DLC3 and the hand-held tester.
NORMAL MODE AND CHECK MODE
The diagnosis system operates in "normal mode" during normal vehicle use. In normal mode, "2-trip detection logic" is used to ensure accurate detection of malfunctions. "Check mode" is also available to technicians as an option. In check mode, "1-trip detection logic" is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent malfunctions.
2-TRIP DETECTION LOGIC
When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the ignition switch is turned OFF and then ON again, and the same malfunction is detected again, the MIL will illuminate.
FREEZE FRAME DATA
Freeze frame data records the engine conditions (fuel system, calculated load, engine coolant temperature, fuel trim, engine speed, vehicle speed, etc.) when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.
The ECM records engine conditions in the form of freeze frame data every 0.5 seconds. Using the hand-held tester, five separate sets of freeze frame data, including the data values at the time when the DTC was set, can be checked.
- 3 data sets before the DTC was set
- 1 data set when the DTC was set
- 1 data set after the DTC was set
These data sets can be used to simulate the condition of the vehicle around the time the occurrence of the malfunction. The data may assist in identifying of the cause of the malfunction, and in judging whether it was temporary or not.