Part 2 of 2
Vehicle Speed ControlThe PCM controls vehicle speed by operation of the speed control servo vacuum and vent solenoids. Energizing the vacuum solenoid applies vacuum to the servo to increase throttle position. Operation of the vent solenoid slowly releases the vacuum allowing throttle position to decrease. A special dump solenoid allows immediate release of throttle position caused by braking, cruise control turn OFF, shifting into neutral, excessive RPM (tires spinning) or ignition key OFF.
Fuel Vapor Recovery System (Duty Cycle Purge Control)
Duty Cycle Purge is a system that feeds fuel gases from the purge canister and gasoline tank into the throttle body for mixing with incoming air. Metering of the gases is performed by duty cycling the purge solenoid by the PCM.
The system is disabled during wide open throttle conditions and while the engine is below a specified coolant temperature. When engine temperature becomes greater than a calibrated parameter, duty cycle purge is delayed for a calibrated time. Once purge delay is over, purge will be ramped in to soften the effect of dumping additional fuel into the engine.
The PCM provides a modulated 5 Hz signal (at closed throttle) or 10 Hz signal (at open throttle) to control this system. Modulation of the signal is based upon a calculated air flow (based upon known fuel flow through the injector at a given pulse width and RPM) and is adjusted to compensate for charges in flow due to varying engine vacuum.
Leak Detection Pump System
The leak detection pump is a device that pressurizes the evaporative system to determine if there are any leaks. When certain conditions are met, the PCM will activate the pump and start counting pump strokes. If the pump stops within a calibrated number of strokes, the system is determined to be normal. If the pump does not stop or stops too soon, a DTC will be set.
PCM OPERATING MODES
As input signals to the 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 optimum fuel economy combined with low emissions.
The engine start-up (crank), engine warm-up, and wide open throttle modes are open loop modes. Under most operating conditions, the acceleration, deceleration, and cruise modes, with the engine at operating temperature, are closed loop modes.
Ignition Switch ON (Engine OFF) Mode
When the ignition switch activates the fuel injection system, the following actions occur:
1. The PCM determines atmospheric air pressure from the MAP sensor input to determine basic fuel strategy.
2. The PCM monitors the engine coolant temperature sensor and throttle position sensor input. The PCM modifies fuel strategy based on this input.
When the ignition key is in the "ON" position and the engine is not running (zero rpm), the auto shutdown relay and fuel pump relay are not energized. Therefore, voltage is not supplied to the fuel pump, ignition coil, 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 auto shutdown and fuel pump relays are energized. If the PCM does not receive the camshaft and crankshaft signal 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.
Once the auto shutdown 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. During certain idle conditions, the PCM may enter into a variable idle speed strategy. At this time, the PCM adjusts engine speed based on the following inputs:
- throttle position
- battery voltage
- engine coolant temperature
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. The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.
NON-MONITORED CIRCUITS
The PCM does not monitor the following circuits, systems, and conditions even though they could have malfunctions that result in driveability problems. A diagnostic code may not be displayed for the following conditions. However, problems with these systems may cause a diagnostic code to be displayed for other systems. For example, a fuel pressure problem will not register a diagnostic code directly, but could cause a rich or lean condition. This could cause an oxygen sensor, fuel system, or misfire monitor trouble code to be stored in the PCM.
Engine Timing - The PCM cannot detect an incorrectly indexed timing chain, cam shaft sprocket, or crankshaft sprocket. The PCM also cannot detect an incorrectly indexed distributor.(*)
Fuel Pressure - Fuel pressure is controlled by the fuel pressure regulator. The PCM cannot detect a clogged fuel pump inlet filter, clogged in-line filter, or a pinched fuel supply.(*)
Fuel Injectors - The PCM cannot detect if a fuel injector is clogged, the pintle is sticking, or the wrong injectors are installed.(*)
Fuel Requirements - Poor quality gasoline can cause problems such as hard starting, stalling, and stumble. Use of methanol-gasoline blends may result in starting and driveability problems. (See individual symptoms and their definitions (Glossary of Terms).
PCM Grounds - The PCM cannot detect a poor system ground. However, a diagnostic trouble code may be stored in the PCM as a result of this condition.
Throttle Body Air Flow - The PCM cannot detect a clogged or restricted air cleaner inlet or filter element.(*)
Exhaust System - The PCM cannot detect a plugged, restricted, or leaking exhaust system.(*)
Cylinder Compression - The PCM cannot detect uneven, low, or high engine cylinder compression.(*)
Excessive Oil Consumption - Although the PCM monitors the exhaust oxygen content through the oxygen sensor when the system is in a closed loop, it cannot determine excessive oil consumption.
(*)NOTE: Any of these conditions could result in a rich or lean condition causing an oxygen sensor trouble code to be stored in the PCM, or the vehicle may exhibit one or more of the driveability symptoms listed in the Table of Contents.
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 DRB III.
Hard Code
A diagnostic trouble code that comes back within one cycle of the ignition key is a "hard" code. This means that the defect is there every time the powertrain control module checks that circuit or function. Procedures in this verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an "intermittent" test must be performed.
Codes that are for OBDII monitors will not set with just the ignition key on. Comparing these to non-emission codes, they will seem like an intermittent. These codes require a set of parameters to be performed (The DRB III pre-test screens will help with this for MONITOR codes), this is called a "TRIP". All OBDII DTC's will be set after one or in some cases two trip failures, and the MIL will be turned ON. These codes require three successful, no failures, TRIPS to extinguish the MIL, followed by 40 warm-up cycles to erase the code. For further explanation of TRIPS, Pre-test screens, Warm-up cycles, and the use of the DRB III, refer to the On Board Diagnostic training booklet # 81-699-97094.
Intermittent Code
A diagnostic trouble code that is not there every time the PCM checks the circuit is an "intermittent" code. Most intermittent codes are caused by wiring or connector problems. Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them. The following checks may assist you in identifying a possible intermittent problem:
- Visually inspect related wire harness connectors. Look for broken, bent, pushed out, or corroded terminals.
- Visually inspect the related harnesses. Look for chafed, pierced, or partially broken wire.
- Refer to any Hotline Newsletters or technical service bulletins that may apply.
- Use the DRB III data recorder or co-pilot.
Starts Since Set Counter
This reset counter counts the number of times the vehicle has been started since codes were last set or erased. This counter will count up to 255 start counts.
The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on the DRB III as STARTS since set.
When there are no trouble codes stored in memory the DRB III will display "NO TROUBLE CODES FOUND" and the reset counter will show "STARTS since set XXX."
HANDLING NO TROUBLE CODE PROBLEMS
Symptom checks cannot be used properly unless the driveability problem characteristic actually happens while the vehicle is being tested.
Select the symptom that most accurately describes the vehicle's driveability problem and then perform the test routine that pertains to this symptom. Perform each routine test in sequence until the problem is found. For definitions, see Glossary of Terms.
SYMPTOM DIAGNOSTIC TEST
Hard Start Checking The Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Checking IAT Sensor
Engine Stall In Gear Check TCC Operation
Start And Stall Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Hesitation/Sag/Stumble Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Surge Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Lack of Power/Sluggish Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Spark Knock/Detonation Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Cuts Out/Misses Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Backfire/Popback Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Runs Rough,
Unstable/Erratic Idle Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Checking IAT Sensor
Poor Fuel Economy Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking IAC Motor Operation
Checking EVAP Emission System
Checking IAT Sensor
NO START INFORMATION
Important Note:
If the Powertrain Control Module has been programmed, a DTC Will be set in the ABS and Air Bag modules. In addition, if the vehicle is equipped with a Sentry Key Immobilizer Module (SKIM), Secret Key data must be updated to enable starting.
For ABS And Air Bag Systems:
1. Enter correct VIN and Mileage in PCM.
2. Erase codes in ABS and Air Bag modules.
For SKIM Theft Alarm:
1. Connect the DRB III to the data link connector.
2. Go to Theft Alarm, SKIM, Misc. and place the SKIM in secured access mode, by using the appropriate PIN code for this vehicle.
3. Select Update the Secret Key data, data will be transferred from the SKIM to the PCM (This is required to allow the vehicle to start with the new PCM).
4. If three attempts are made to enter secured access mode using the incorrect PIN, secured access mode will be locked out for one hour. To exit this lock out mode, leave the ignition key in the Run/Start position for one hour. Ensure all accessories are turned off. Also monitor the battery state and connect a battery charger if necessary.
After reading (System Description and Functional Operation), you should have a better understanding of the theory and operation of the on-board diagnostics, and how this relates to the diagnosis of a vehicle that may have a driveability related symptom or complaint.
WARNINGS AND CAUTIONS
Road Test Warnings
Some complaints will require a test drive as part of the repair verification procedure. The purpose of the test drive is to try to duplicate the diagnostic code or symptom condition.
CAUTION: BEFORE ROAD TESTING A VEHICLE, BE SURE THAT ALL COMPONENTS ARE REASSEMBLED. DURING THE TEST DRIVE, DO NOT TRY TO READ THE DRB III SCREEN WHILE IN MOTION. DO NOT HANG THE DRB III FROM THE REAR VIEW MIRROR OR OPERATE IT YOURSELF. HAVE AN ASSISTANT AVAILABLE TO OPERATE THE DRB III.
Vehicle Damage Cautions
Before disconnecting any control module, make sure the ignition is "OFF". Failure to do so could damage the module.
When testing voltage or continuity at any control module, use the terminal side (not the wire end) of the connector. Do not probe a wire through the insulation; this will damage it and eventually cause it to fail because of corrosion.
Be careful when performing electrical tests so as to prevent accidental shorting of terminals. Such mistakes can damage fuses or components. Also, a second code could be set, making diagnosis of the original problem more difficult.