The On-Board Diagnostic (OBD) System
Design and function
The on-board diagnostic (OBD) system
The fuel and ignition Systems are equipped with an OBD system with three diagnostic test modes (DTMs) to facilitate fault tracing.
The DLC is located in the engine compartment by the left hand suspension tower. Output 2 is used for the fuel system and output 6 for the ignition system.
Faults are registered In DTM 1. If these faults are emission related a MIL is lit on the combination instrument. A maximum of three faults can be stored at one time (the fuel system OBD system for California models can store up to 17 DTCs).
Sensor signals are tested in DTM 2. DTM 2 can be activated manually by pressing the diagnostic button twice. The control module responds with a code if it receives a signal when the sensor is activated.
The signals to certain control components are tested in DTM 3. DTM 3 can be activated manually by pressing the diagnostic button three times. The control module then activates the control components one by one. Whether the component is operating is ascertained by listening or feeling it.
The fuel system
The control module continually checks the following modes when the car is being driven:
- The control module's own, internal functions
- HO2S and fuel trim
- ECT sensor
- MAF sensor
- Battery voltage
- TP switch
- Engine speed signal from the DI system control module
- Speedometer signal
- Knock enrichment signal from the DI system (does not apply to B 230 F, B 204 FT/GT)
- IAC valve
Checking For More Codes:
A fault in any of these functions is presented as a DTC in DTM 1.
Entering Control Function #2:
DTM 2 tests the signals from the following sensors:
^ TP switch in full load position OK 3-3-3
^ TP switch in idle position OK 3-3-2
^ Engine speed signal from DI OK 3-3-1
^ A/C control on/off OK 1-1-4
^ A/C compressor start OK 1-3-4
^ Engine speed compensation for automatic
transmission OK 1-2-4
Entering Control Function #3:
DTM 3 tests the signals to the following control components:
^ FC half speed
^ FC full speed
^ Injectors
^ IAC valve
^ The carbon filter solenoid valve
^ The cold start valve
The distributor Ignition (DI) system
The control module continually checks the following functions when the car is being driven:
- The control module's own, internal functions
- KS signal
- Fuel system load signal
- RPM sensor signal
- ECT sensor signal
- Signal from the EGR controller
- Signal from the EGR temperature sensor
Checking For More Codes:
A fault in any of these functions is presented as a DTC in DTM 1.
Entering Control Function #2:
DTM 2 tests the signals from the following sensors:
^ RPM sensor signal OK 1-4-1
^ TP switch in idle position OK 3-3-4
Entering Control Function #3:
DTM 3 tests signals to the EGR controller.