On Board Diagnostics Overview
Diagnostic Connections
The diagnostic connector in the bottom edge of the driver's side fascia provides the interface between the vehicle and the Jaguar Portable Diagnostic Unit (PDU) or a universal scan tool.
Communication Networks:
A scan tool communicates only with the control modules connected to the ISO 9141/2 network, for the retrieval of any DTC they contain. The PDU communicates with the control modules connected to the CAN, the SCP or the ISO 9141/2 network (only the CAN network is used to communicate with the ECM), for:
- the retrieval of DTC
- fault diagnosis
- control module programming
- the enabling/disabling of system feature options.
Diagnostics
The ECM performs self test routines and monitors engine functions, inputs and outputs to ensure correct operation of the engine and the engine management system. Hard wired inputs and outputs are monitored for short and open circuits, and sensor inputs are also monitored for range.
Additional checks are run on the more critical sensor inputs to ensure their validity. Some of the more critical inputs have substitute or default values which the ECM adopts if the input is diagnosed as faulty.
Any faults detected are logged in the ECM memory as DTC. The ECM also outputs engine malfunction messages on the CAN and adopts a default mode of operation. Most default modes retain some degree of engine operation (limp home).
The ECM also stores OBD II related DTC detected by other control modules on the CAN. Non-OBD II related DTC are retained in the memory of the control module that detects the fault.
To prevent false DTC being logged, the monitoring of some inputs and engine functions is inhibited while the vehicle is above a given altitude:
- At altitudes of 2438m (8000ft) and above,the ECM inhibits diagnostics on: EGR valve and EGR flow; EVAP valve and EVAP purge flow; idle speed control; misfire detection; catalytic converter efficiency.
- At altitudes of 2652m (8700ft) and above, the ECM also inhibits diagnostics on: HO2S; MAFS; 02S.
ADDITIONAL INPUT CHECKS
Accelerator Pedal Position
Input
Accelerator pedal position (2 input signals) and mechanical guard position
Additional Strategy
Accelerator pedal position input 1 compared to accelerator pedal position input 2. Both accelerator pedal position inputs compared to mechanical guard position input.
Action, Substitute or Default Value
If one input fails, the input of the other two is used. If two inputs fail, ECM adopts fixed throttle mode.
Throttle Position
Input
Throttle position (2 input signals)
Additional Strategy
Throttle position input 1 compared to throttle position input 2.
Input 1 compared to target value (target value depends on operating mode, eg. normal running, cruise control, traction active).
Action, Substitute or Default Value
If either input fails, ECM adopts mechanical guard mode of operation.
Engine Coolant Temperature
Input
Engine coolant temperature
Additional Strategy
After engine start, checks that expected temperature increase occurs, then monitors for excessive temperature decrease.
Action, Substitute or Default Value
If fault detected, uses substitute value derived from transmission oil temperature.
Mass Air Flow
Input
Mass air flow
Additional Strategy
Compares input with predicted air flow (derived from a map of throttle angle against engine speed).
Action, Substitute or Default Value
If fault detected, uses throttle angle as load measurement.
Intake Air Temperature
Input
Intake air temperature
Additional Strategy
Monitors for too fast change of input and for fixed input.
Action, Substitute or Default Value
If fault detected, uses a fixed intake air temperature of 50°C ( 122°F).
DEFAULT MODES
Engine Shutdown Mode
Effect: Activates fuel cut-off to stop engine
General Warning Lamp: Red
Check Engine Lamp (MIL): ON
Message: ENGINE FAULT
Fixed Throttle Mode
Effect: Fixed throttle valve angle of approximately 2.5° set, producing maximum engine speed (unloaded) of approximately 1200 RPM.
General Warning Lamp: Red
Check Engine Lamp (MIL): ON
Message: ENGINE FAULT
Mechanical Guard Mode
Effect: Engine speed increases as throttle valve goes to mechanical guard position; idle speed increases to approximately 1400 RPM; full throttle available
General Warning Lamp: Red
Check Engine Lamp (MIL): ON
Message: ENGINE FAULT
Note:
In mechanical guard mode, fuel intervention smooths the transition from normal to default mode, to prevent sudden acceleration of the vehicle.
Also fuel intervention limits idle speed. Without fuel intervention idle speed would be approximately 2000 RPM and cause excessive shock loads on the transmission when shifting out of Park or Neutral. When engine load increases, idle speed fuel intervention is progressively withdrawn.
Engine Speed Limiting Mode
Effect: Engine speed limited to 3000 RPM
General Warning Lamp: Amber
Check Engine Lamp (MIL): ON
Message: ENGINE FAULT
Redundancy Mode
Effect: Substitute or default value adopted; no noticeable difference in performance
General Warning Lamp: Amber
Check Engine Lamp (MIL): OFF
Message: ENGINE FAULT
Exhaust Emission Mode
Effect: Substitute or default value adopted; potential difference in performance
General Warning Lamp: None
Check Engine Lamp (MIL): ON
Message: None