Functional Operation

FUEL CONTROL
The PCM controls the air/fuel ratio of the engine by varying fuel injector on time. Mass air flow is calculated using the speed density method using engine speed, manifold absolute pressure, and air temperature change.

Different fuel calculation strategies are used dependent on the operational state of the engine. During crank mode, a prime shot fuel pulse is delivered followed by fuel pulses determined by a crank time strategy. Cold engine operation is deter mined via an open loop strategy until the O2 sensors have reached operating temperature. At this point, the strategy enters a closed loop mode where fuel requirements are based upon the state of the O2 sensors, engine speed, MAP, throttle position, air temperature, battery voltage, and coolant temperature.

ON BOARD DIAGNOSTICS
The PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "on-board diagnosis "

Certain criteria, or "arming conditions," 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 a problem is sensed with a monitored circuit, and all of the criteria or arming conditions are met, 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 because one of the trouble code criteria (arming conditions) has 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 (arming conditions) are met, a trouble code will be stored in the PCM memory.

The On Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBD II. These OBD II Diagnostics control to the functions necessary to meet the requirements of California OBD II and Federal OBD regulations These requirements specify the inclusion of a Malfunction Indicator Light (MIL) located on the instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of the MIL is to inform the vehicle operator in the event of the malfunction of any emission systems and components which can affect emissions and which provide input to, or receive output from, the Powertrain Control Module.

OBD II MONITOR INFORMATION:

The table summarizes the various OBD II monitors operation.

TRANSMISSION CONTROL
The PCM operation includes control of 4 speed automatic transmissions utilizing electronic governor pressure control, eliminating the need for a separate transmission controller. Transmission control is achieved through regulation of governor pressure using a Governor Pressure Solenoid valve. Valve position is controlled by pulse width modulation. Torque converter clutch and overdrive solenoids are also controlled by the PCM, as are the transmission protection relay and dashboard overdrive OFF lamp. PCM inputs affecting transmission operation include the throttle position sensor, output shaft speed sensor, vehicle speed, engine speed sensor (CKP), brake switch, ignition, overdrive ON/OFF switch, torque convertor clutch solenoid, sump fluid temperature thermistor, and governor pressure sensor.

The PCM continuously checks for internal transmission problems, electrical problems, and some hydraulic problems. When a problem is sensed, the PCM stores a diagnostic trouble code. Any of these codes cause the transmission to go into "default" mode. When the PCM detects a problem, the transmission will default to third gear. When this happens, the only transmission functions are:
- PARK and NEUTRAL
- REVERSE
- THIRD GEAR
- MANUAL SHIFTING of FIRST, SECOND and THIRD GEAR

No upshifts or downshifts are allowed. The position of the manual valve alone allows the ranges that are available. Although engine performance is seriously degraded while in this mode, it allows the owner to drive the vehicle in for service. The transmission can be shifted manually by quickly down-shifting into 1st to achieve 1st gear, then shifting to 2nd, then to 3rd. However, default mode will not allow 4th gear or any electronically controlled converter clutch operation.

Once the DRB III is in the RE transmission portion of the diagnostic program, it constantly monitors the Powertrain Control Module, updating the DRB III screen with switch, sensor, and input/ output states, as well as displaying diagnostic trouble codes and default status.

Transmission Identification
The transmission part/identification numbers and codes are stamped on the left side of the case just above the oil pan gasket surface. The first letter/number group is the assembly part number. The next number group is the transmission serial number. Refer to this information when ordering replacement parts.

Governor Pressure Solenoid Valve
The solenoid valve generates the governor pressure needed for upshifts and downshifts. It is an electro-hydraulic device and is located in the governor body on the valve body transfer plate.

The inlet side of the solenoid valve is exposed to normal transmission line pressure while in forward gears. The outlet side of the valve leads to the valve body governor circuit.

The solenoid valve regulates line pressure to produce governor pressure. The average current supplied to the solenoid valve controls governor pressure. One amp current produces zero psi governor pressure. Zero amps sets the maximum governor pressure. Current is regulated by modulation of the pulse width of a 512 Hz driver frequency (512 cycles per second) .

The Powertrain Control Module (PCM) supplies electrical power to the solenoid valve. Operating voltage is 12 volts (DC) and is provided through the battery terminal on the module.

The solenoid is polarity sensitive. The PCM energizes the solenoid by grounding it through the power ground terminal on the Powertrain Control Module.

Governor Pressure Sensor
The governor pressure sensor measures output pressure of the governor pressure solenoid valve.

The sensor output signal provides the necessary feedback to the Powertrain Control Module. This feedback is needed to accurately control pressure. The unit is an absolute pressure device and the output is calibrated to be .35 to .65 volts at 14.7 psi (normal barometric pressure). Since this is an absolute pressure device, 0 psicalibration is required often to compensate for changing atmospheric pressure or altitude. This voltage measured at 0 psi is referred to as zero pressure offset.

Governor Shift Schedules
The electronic governor has several governor curves possible as opposed to a conventional governor which has a single governor curve with two stages. These transmissions are mechanically and hydraulically the same as the ones they replace.

As with all-hydraulic transmissions, the vehicle shift speeds are determined by balancing a hydraulic pressure signal proportional to transmission output speed (called governor pressure) against a pressure signal determined by throttle position (called throttle pressure). The four curves are used during the following operating conditions.

Low Transmission Fluid Temperature - When the transmission fluid is cold at or below 30 °F the conventional governor can delay shifts, resulting in higher than normal shift speeds and harsh shifts. The electronically controlled low temperature governor pressure curve is higher than normal to make the transmission shift at normal speeds and sooner. The PCM uses a temperature sensor in the transmission oil sump to determine when low temperature governor pressure is needed.

Wide-Open Throttle Operation - In wide-open throttle (WOT) mode, adaptive memory in the PCM assures that up-shifts occur at the preprogrammed optimum speed. WOT operation is determined from the throttle position sensor, which is also a part of the emission control system. The initial setting for the WOT upshift is below the optimum engine speed. As WOT shifts are repeated, the PCM learns the time required to complete the shifts by comparing the engine speed when the shifts occur to the optimum speed. After each shift, the PCM adjusts the shift point until the optimum speed is reached. The PCM also considers vehicle loading, grade and engine performance changes due to high altitude in determining when to make WOT shifts. It does this by measuring vehicle and engine acceleration and then factoring in the shift time.

Normal Operation - Normal operation is refined through the increased computing power of the PCM and through access to data on engine operating conditions provided by the PCM that were not available with the previous stand-alone electronic module. This facilitated the development of a load adaptive shift strategy - the ability to alter the shift schedule in response to vehicle load conditions. One manifestation of this capability is grade "hunting" prevention - the ability of the transmission logic to delay an upshift on a grade if the engine does not have sufficient power to maintain speed in the higher gear. The 3-2 downshift and the potential for hunting between gears occurs with a heavily loaded vehicle or on steep grades. When hunting occurs, it is very objectionable because shifts are frequent and accompanied by large changes in noise and acceleration.

Governor Operation
The electronic governor control system replaces the old centrifugal governor pressure control and is located on the valve body. The control system uses a governor pressure solenoid that can vary pressure, a pressure sensor, and the output shaft speed sensor.

The electronic governor control system regulates pressure to control shifts in the first three gears. Output shaft and throttle position are used to determine target pressure. Actual governor pressure is read from the sensor and the difference between the target pressure and actual pressure are used to determine duty cycle correction.

The duty cycle is the amount of time the governor pressure solenoid needs to be OFF to meet the target pressure.

Speed of the output shaft, throttle position, controller calculations, and shift lever position determine different governor pressure curves.

Governor pressures can be different at the same output shaft speed. The desired pressure is determined by many things including the acceleration of the vehicle. There is no need for concern if at the same output shaft speed there are different requested pressures. There is a need for concern if the target pressure and actual pressure are not within three PSI for five seconds or more. If this occurs the control system could result in erratic shifting.

The only time the governor control system stays at zero is when the gear selector is in park, neutral, reverse or drive with the vehicle at a stop. When the transmission is in park, neutral, or reverse no line pressure is supplied to the governor pressure solenoid making governor pressure zero.

Transmission Fluid Temperature Thermistor
Transmission fluid temperature readings are supplied to the Powertrain Control Module by the thermistor. The thermistor location is in the governor pressure sensor connector. The temperature readings are used to control engagement of the overdrive clutch, the converter clutch, and governor pressure. Normal resistance value for the thermistor at room temperature is approximately 1000 ohms.

The Powertrain Control Module (PCM) prevents engagement of the converter clutch and overdrive clutch, when fluid temperature is below approximately 30 °F.

If fluid temperature exceeds 260 °F, the Powertrain Control Module will cause a 4-3 downshift and engage the converter clutch. Engagement is according to the third gear converter clutch engagement schedule.

The overdrive OFF lamp in the instrument panel, also illuminates when the shift back to third occurs. The transmission will not allow fourth gear operation until fluid temperature decreases to approximately 230 °F.

Transmission Speed Sensor
The speed sensor is located in the overdrive housing. The sensor is positioned over the park gear and monitors transmission output shaft rotating speed.

Speed sensor signals are triggered by the park gear lugs as they rotate past the sensor pickup face. One revolution of the output shaft produces 23 pulses. Input signals from the sensor are sent to the Transmission Control Module for processing.

Torque Converter Electronics
The converter contains a converter clutch mechanism. The converter clutch is an electronically controlled mechanism. It is engaged in fourth gear, and in third gear only when the overdrive control switch is in the OFF position, and also, in third gear over temp mode.

The torque converter is not a serviceable component. It should be replaced as an assembly when:diagnosis indicates a malfunction has occurred, or when a major malfunction allows debris to enter the converter.

OTHER CONTROLS
Charging System
The charging system is turned ON when the engine is started and ASD relay energized. When the ASD relay is ON, ASD output voltage is supplied to the ASD sense circuit at the PCM. This voltage is connected through the PCM and supplied to one of the generator field terminals (Gen Source +). The amount of current produced by the generator is controlled by the Electronic Voltage Regulator (EVR) circuitry, in the PCM. A battery temperature sensor, located in the battery tray, is used to sense battery temperature. This temperature along with sensed line voltage, is used by the PCM to vary the battery charging rate. This is done by cycling the ground path to the other generator field terminal (Gen field driver).

Speed Control System
The 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 switch turned 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 EVAP 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 fuel flow (based upon known fuel fuel flow through the injector at a given pulse width and RPM) and is adjusted to compensate for changes in fuel 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 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, camshaft 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.

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 stream 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.