Description of On-Board Diagnostics
Overview
The AJ6 4.0 Litre Engine Management System (EMS) is an integrated engine management system controlled through a digital Electronic Control Module (ECM) containing a microprocessor. The system maintains optimum performance over the engine operating range by precisely controlling fuel injection, ignition timing and emission control functions. In addition, the ECM provides various interface outputs and incorporates an on-board diagnostic function.
A summary of EMS function and control includes:
- Fuel delivery (fuel pump)
- Evaporative emission control (canister purge)
- Fuel injection
- Cold start
- Warm-up
- Exhaust emission
- Oxygen sensor feedback
- Overrun fuel cut-off
- RPM limitation (engine overspeed)
- Wide-open-throttle fuel cut-off
- Ignition timing
- Idle speed control
- Air injection
- Exhaust gas recirculation
- Adaptive idle fuel metering (1993 MY ON)
- On-Board Diagnostics (OBD)
- Limp home" capability
- Vehicle systems interfaces Serial communication (ISO)
Engine Control Module (ECM)
The AJ6 4.OL EMS is microprocessor based using the Lucas 15 CU ECM as the heart of the system. The 15 CU ECM microprocessor runs at 2.0 MHz. The ECM uses discrete components plus analog-to-digital circuits to interface between the microprocessor and the input sensors and output devices. Software, programmed into an EPROM, is divided into "control code" and "data" (engine calibration). Control code is common to all engine specifications; data is written for specific market specifications such as US EPA emission regulations.
The ECM contains two double sided printed circuit boards. One is a low power board and the other is a high power board. The yellow and blue 25-way connectors are therefore referred to as the low and high power connectors respectively. Most of the input signals from engine mounted sensors, and interfaces with other systems are located on the low power (Yellow) connector. The high power connector (Blue), mainly serves outputs such as fuel injector drive and relay activation. Inputs and outputs are included with the wiring diagrams.
The ECM receives sensor inputs and feedbacks, which are used to determine the optimum strategy for the prevailing conditions. The ECM's strategy has 256 memory locations containing injector pulse durations and ignition timing angles for 16 different engine loads (engine load sites) and 16 different engine speeds (engine speed sites). Canister purge, idle speed, air injection and exhaust gas recirculation are controlled by the ECM from stored strategies.
On-Board Diagnostics (OBD)
On-Board Diagnostic Facility (OBD I)
The ECM includes a fault diagnosis facility that continuously monitors the operation of the engine management sensors and components. If a fault is detected, the OBD system will activate the Malfunction Indicator Lamp (MIL) (CHECK ENGINE) warning in the instrument pack and flag a diagnostic trouble code (DTC) in the ECM memory. The ECM can be interrogated through the instrument pack LCD display (XJ6 Sedan Range) or center console LCD display (XJS Range). To display the code, switch the ignition OFF; wait 5 seconds, then switch the ignition ON (do not crank the engine). On Sedan Range vehicles, press the VCM button. On XJS Range vehicles, a code is automatically displayed 5 seconds after the ignition is switched ON. The CHECK ENGINE warning will display and the DTC, will appear 5 seconds later. If two or more DTCs are flagged in memory, only the highest priority code will be displayed. If the vehicle battery is disconnected, all stored codes will be cleared from the ECM's memory. DTCs can also be accessed via serial communication.
Limp Home Default
In order to allow vehicle operation it a malfunction occurs, "limp home" default values are incorporated as an ECM facility. If a sensor fault is detected by the OBD system, the ECM will substitute a nominal value for the missing input(s) as follows:
Missing Sensor Input Default value
Engine coolant temperature 78° F (25° C)
Intake air temperature 86° F (30° C)
Mass air flow Throttle position (voltage) Vs. pulse duration matrix; ignition timing = 8° BTDC @ idle, 20° BTDC above idle
Throttle position 1.5 volts
EGR temperature Switches off EGR
Oxygen sensor voltage Closed loop operation canceled (feedback = 2.5 volts)
Idle trim adjustment (1990-1992 MY only) 2.5 volts
Vehicle Systems Interfaces
The engine management system interfaces with the transmission control module, the instrument pack and the air conditioning compressor clutch circuit to provide operational control, sensor input and data input.
Transmission Control Module Interface
Engine torque reduction
The ECM receives two ignition timing retard inputs from the 1CM: transmission shift "up/down" signal (IGNITION SELECT) (pulsed signal), and transmission "shift in progress signal (IGNITION RETARD) (pulsed signal).
Engine speed and load
The ECM outputs engine load and speed signals (PULSED SIGNALS) to the 1CM. The 1CM uses the engine load and speed data to determine shift patterns and torque converter clutch apply and release.
Engine idle speed
The transmission decoder module (XJ6), rotary switch (XJS) outputs gear position signals to the ECM. When the gear selector is in R, D, 2 or 3, the signal is 5 volts; when the gear selector is in P or N, the signal is ground or 0 volts. The ECM uses the gear position inputs to control idle speed and for DTC 69.
Instrument Pack Interface
Vehicle road speed
The instrument pack outputs a road speed signal (pulsed signal) to the ECM. The ECM uses the signal to determine idle speed control functions.
Low fuel level (1992 MY ON [ECM part no. DBC 9622 ON])
The instrument pack outputs a fuel level voltage signal to the ECM. When the voltage drops below 5.7 volts (2.5 gallons [11 litres] fuel remaining), fuel metering diagnostics (OBDI are canceled. Cancelling the fuel metering diagnostics prevents the ECM from flagging DTCs caused by the vehicle running out of fuel.
Engine speed
Engine speed is output to the instrument pack for operation of the tachometer. Engine speed is sensed from the ignition coil primary circuit (1990 - 92 MY Sedan and XJS). The ECM outputs an engine speed signal (pulsed signal) to the instrument pack (1993 MY ON Sedan).
Fuel metering pulse
The ECM outputs a fuel metering pulse to the instrument pack for use by the trip computer (average fuel consumption, instantaneous fuel consumption).
Fuel fail warning
If the OBD system detects a fault, the ECM outputs a fuel fail warning signal (ground) to the instrument pack.
Air Conditioning Compressor Clutch Interface
When the air conditioning compressor clutch is operating, a parallel circuit applies battery voltage to the ECM. The ECM uses this signal to determine the need for idle speed control compensation.
Serial Communication
Serial communication between the engine management system and JDS or PDU is available. Serial communication is used for engine setup, accessing stored DTCs, fault diagnosis and erasing DTCs.