Modes of Operation
Fig. 151 Electronic Control Unit Inputs and Outputs (PGM-FI Engine):
Note: The Electronic Control Unit (ECU) mounted in these vehicles is designed to analyze the input signals, make decisions, and give the output signals to operate the engine efficiently, lower emissions, and still have good performance. The basic functions are:
INJECTOR TIMING CONTROL
The ECU contains memories for the basic discharge durations at various engine speeds and manifold pressures. The basic discharge duration, after being read out of memory, is further modified by signals sent from various sensors to obtain the final discharge duration.
ELECTRONIC IDLE CONTROL
When the engine is cold, the A/C compressor is on, the transmission is in gear (A/T only), the alternator is charging, the ECU controls current to the Electronic Air Control Valve (EACV) to maintain correct idle speed.
OTHER CONTROL FUNCTIONS
1. Starting Control
When the engine is started, the ECU provides a rich air-fuel mixture.
2. Fuel Pump Control
a. When the ignition switch is initially turned on, the ECU supplies ground to the main relay which supplies current to the fuel pump for 2 seconds to pressurize the fuel system.
b. When the engine is running the ECU supplies ground to the main relay which supplies current to the fuel pump.
c. When the engine is not running and the ignition is on the ECU cuts ground to the main relay which cuts the current to the fuel pump.
3. Fuel Cut-Off Control
a. During deceleration with the throttle valve closed, current to the injectors is cut-off at speeds over 1,200 rpm, to improve fuel economy.
b. Fuel cut-off action also takes place when the engine speed exceeds 6,600 rpm regardless of the position of the throttle valve, to protect the engine from over running.
4. Ignition Control Solenoid Valve (ICSV)
The ICSV is operated by the ECU which receives signals from the engine speed, coolant temperature, and manifold vacuum to provide maximum performance and fuel economy, while protecting the engine by preventing detonation from occuring.
5. EGR Control Solenoid Valve (EGR CSV)
When the EGR is required for the control of oxides of nitrogen (NOx) emissions, the ECU supplies ground to the EGR CSV which supplies regulated vacuum to the EGR valve.
6. Purge Cut-Off Control Solenoid Valve (PCCSV)
When the engine coolant temperature is above the set temperature of the TW sensor, the purge cut-off solenoid valve is activated by the control unit receiving signals from each sensor.
ECU BACK-UP FUNCTIONS
1. Fail-Safe Function
When an abnormality occurs in the signal from a sensor, the ECU ignores that signal and assumes a pre-programmed value that allows the engine to continue tto run.
2. Back-Up Function
When an abnormality occurs in the ECU itself, injectors are controlled by a back-up circuit independent of the system in order to permit minimal driving.
3. Self-Diagnosis Function (Check Engine Warning Light and LED Indicator)
When an abnormality occurs in a signal from a sensor, the ECU lights the Check Engine warning light, stores the failure code in eraseable memory and indicates the code with a LED on the ECU anytime the ignition is on. When the ignition is initially turned on, the ECU supplies ground for the Check Engine warning light for about 2 seconds.
ON-BOARD DIAGNOSTICS
The On-Board diagnostic system is a built in function of the electronic control unit. The ECU constantly monitors all input and output functions and compares the various readings against a programmed set of standard values. When the reading from a component is significantly out of the "normal" range, the ECU will illuminate the "CHECK ENGINE" light in the instrument cluster to notify the driver that a problem exists.
When the CHECK ENGINE warning light has been turned on, turn the ignition on, move the front driver's seat to the front position and observe the LED on the rear of the ECU. The ECU indicates a system failure code by its blinking frequency.