Functional Operation
ON-BOARD DIAGNOSTICSThe PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "on-board diagnosis".
Certain criteria must be met for a trouble code to be entered into the PCM memory. The criteria may be a range of: engine rpm, engine temperature, and/or input voltage to the PCM. If all of the criteria for monitoring a system or circuit are met and a problem is sensed, then a trouble code will be stored in the PCM.
It is possible that a trouble code for a monitored circuit may not be entered into the PCM memory even though a malfunction has occurred. This may happen when the monitoring criteria have not been met.
The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria are met, a trouble code will be stored in the PCM memory.
PCM OPERATING MODES
As input signals to the powertrain control module (PCM) change, the PCM adjusts its response to output devices. For example, the PCM must calculate a different injector pulse width and ignition timing for idle than it does for wide open throttle. There are several different modes of operation that determine how the PCM responds to the various input signals.
There are two types of engine control operation: open loop and closed loop.
In open loop operation, the PCM receives input signals and responds according to preset programming. Inputs from the heated oxygen sensors are not monitored.
In closed loop operation, the PCM monitors the inputs from the heated oxygen sensors. This input indicates to the PCM whether or not the calculated injector pulse width results in the ideal air-fuel ratio of 14.7 parts air to 1 part fuel. By monitoring the exhaust oxygen content through the oxygen sensor, the PCM can fine tune injector pulse width. Fine tuning injector pulse width allows the PCM to achieve the lowest emission levels while maintaining optimum fuel economy.
The engine start-up (crank), engine warm-up, and wide open throttle modes are open loop modes. Under most operating conditions, closed loop modes occur with the engine at operating temperature.
Ignition Switch ON (Engine OFF) Mode
When the ignition switch activates the fuel injection system, the following actions occur:
- The PCM monitors the engine coolant temperature sensor input. The PCM modifies fuel strategy based on this input.
When the key is in the "ON" position and the engine is not running (0 rpm), the MFI relay (ASD relay) and fuel pump relay are not energized. Therefore, voltage is not supplied to the fuel pump, IAC motor and fuel injectors.
Engine Start-Up Mode
This is an open loop mode. The following actions occur when the starter motor is engaged:
1. The MFI (ASD) and fuel pump relays are energized. If the PCM does not receive the crankshaft signals within approximately one second, these relays are de-energized.
2. The PCM energizes all fuel injectors until it determines crankshaft position from the camshaft and crankshaft signals. The PCM determines crankshaft position within one engine revolution. After the crankshaft position has been determined, the PCM energizes the fuel injectors in sequence. The PCM adjusts the injector pulse width and synchronizes the fuel injectors by controlling the fuel injectors' ground paths.
3. Once the engine idles within 64 r/min of its target engine speed, the PCM compares the current MAP sensor value with the value received during the ignition switch on (zero rpm) mode. A diagnostic trouble code is written to PCM memory if a minimum difference between the two values is not found.
Once the MFI (ASD) and fuel pump relays have been energized, the PCM determines the fuel injector pulse width based on the following:
- engine coolant temperature
- manifold absolute pressure
- intake air temperature
- engine revolutions
- throttle position
The PCM determines the spark advance based on the following:
- engine coolant temperature
- crankshaft position
- camshaft position
- intake air temperature
- manifold absolute pressure
- throttle position
Engine Warm-Up Mode
This is an open loop mode. The PCM adjusts injector pulse width and controls injector synchronization by controlling the fuel injectors' ground paths. The PCM adjusts ignition timing and engine idle speed. The PCM adjusts the idle speed by controlling the idle air control motor.
Cruise Or Idle Mode
When the engine is at normal operating temperature, this is a closed loop mode.
Acceleration Mode
This is a closed loop mode. The PCM recognizes an increase in throttle position and a decrease in manifold vacuum as engine load increases. In response, the PCM increases the injector pulse width to meet the increased load.
Deceleration Mode
This is a closed loop mode. The PCM recognizes a decrease in throttle position and an increase in manifold vacuum as engine load decreases. In response, the PCM decreases the injector pulse width to meet the decreased load.
Wide Open Throttle Mode
This is an open loop mode. The throttle position sensor notifies the PCM of a wide open throttle condition. Once a wide open throttle is sensed, the PCM de-energized the A/C compressor clutch relay for various time periods.
MONITORED CIRCUITS
The PCM is able to monitor and identify most driveability related trouble conditions. Some circuits are directly monitored through PCM feedback circuitry. In addition, the PCM monitors the voltage state of some circuits and compares those states with expected values. Other systems are monitored indirectly when the PCM conducts some type of a rationality test to identify problems.
Although most subsystems of the powertrain control module are either directly or indirectly monitored, there may be occasions when diagnostic trouble codes are not immediately identified.
For a trouble code to set, specific conditions must be met and unless these conditions are encountered, a code will not set.
When a trouble code identifying a problem in an emission related system is set, the Service Engine Soon/Malfunction Indicator lamp in the dash will turn on. Conversely, any other trouble code will set without illuminating the Service Engine Soon/Malfunction Indicator lamp.
ELECTRONIC PINION FACTOR
The vehicle speed sensor supplies distance pulses to the powertrain control module (PCM), which are used to calculated speed and mileage. A pinion factor is stored in the transmission control module (TCM) in order to provide the appropriate distance pulses for the vehicle. The pinion factor is programmed into the TCM at the assembly plant.
DIAGNOSTIC TROUBLE CODES
Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of trouble codes as well as no trouble code problems. It is not necessary to perform all of the tests in this book to diagnose an individual code.
Always begin by reading the diagnostic trouble codes using the scan tool. Refer to Computers and Control Systems / Testing and Inspection / Procedures / Trouble Code Descriptions. This will direct you to the specific test(s) that must be performed.