Understanding CAN/OBDII - How DTCs Cause a MIL

DIAGNOSIS SYSTEM
1. DESCRIPTION
When troubleshooting OBD II (On-Board Diagnostics) vehicles, the intelligent tester (complying with SAE J1987) must be connected to the DLC3 (Data Link Connector 3) of the vehicle. Various data in the vehicle's ECM (Engine Control Module) can be then read. OBD II regulations require that the vehicle's on-board computer illuminate the MIL (Malfunction Indicator Lamp) on the instrument panel when the computer detects a malfunction in:
(a)The emission control system components
(b)The powertrain control components (which affect vehicle emissions)
(c)The computer itself
In addition, if the applicable DTCs (Diagnostic Trouble Codes) prescribed by SAE J2012 are not recorded on 3 consecutive trips, the MIL turns off automatically but the DTCs remain recorded in the ECM memory.
To check DTCs, connect the intelligent tester to the DLC3. The tester displays DTCs, freeze frame data, and a variety of the engine data. The DTCs and freeze frame data can be erased with the tester (See page ES-32).
In order to enhance OBD function on vehicles and develop the Off-Board diagnosis system, CAN (Controller Area Network) communication is introduced in this system. 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 simplifies the wire harness connection.
Since this system is equipped with the CAN communication, connecting the CAN VIM (Vehicle Interface Module) to the intelligent tester is necessary to display any information from the ECM. (Also the communication between the intelligent tester and the ECM uses CAN communication signals.) When confirming the DTCs and any data of the ECM, connect the CAN VIM between the DLC3 and the intelligent tester.

2. 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 as an option for technicians. In check mode, 1 trip detection logic is used for simulating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent problems (intelligent tester only).

3. 2 TRIP DETECTION LOGIC
When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the same malfunction is detected during the next subsequent drive cycle, the MIL is illuminated (2nd trip).

4. FREEZE FRAME DATA
The ECM records vehicle and driving condition information as freeze frame data the moment a DTC is stored. When troubleshooting, freeze frame data can be helpful in determining whether the vehicle was running or stopped, whether the engine was warmed up or not, whether the air-fuel ratio was lean or rich, as well as other data recorded at the time of a malfunction.

5. DLC3 (Data link Connector 3)
The vehicle's ECM uses ISO 15765-4 for communication protocol. The terminal arrangement of the DLC3 complies with SAE J1962 and matches the ISO 15765-4 format.

View attachment DLC3 Connector.bmp
Symbols (Terminal No.) Terminal Description Condition Result
SIL (7) - SG (5) Bus "+" line During transmission Pulse generation
CG (4) - Body ground Chassis ground Always Below 1 Ω
SG (5) - Body ground Signal ground Always Below 1 Ω
BAT (16) - Body ground Battery positive Always 11 to 14 V
CANH (6) - CANL (14) CAN bus line Ignition switch off 54 to 69 Ω
CANH (6) - CG (4) HIGH-level CAN bus line Ignition switch off 200 Ω or higher
CANL (14) - CG (4) LOW-level CAN bus line Ignition switch off 200 Ω or higher
CANH (6) - BAT (16) HIGH-level CAN bus line Ignition switch off 6 kΩ or higher
CANL (14) - BAT (16) LOW-level CAN bus line Ignition switch off 6 kΩ or higher