Functional Operation

FUEL CONTROL (Gas)
The PCM controls the air/fuel ratio of the engine by varying fuel injector on time. Mass air flow is calculated by the speed density method using engine speed and manifold absolute pressure (IAT is a modifier in Speed Density).

Different fuel calculation strategies are used depending 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 determined 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 any circuit or system that has an effect on vehicle emissions, or is used by the PCM to determine the proper functionality of these systems. 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 detected 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 preprogrammed 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 OBDII. These OBDII Diagnostics control the functions necessary to meet the requirements of California OBDII 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 PCM.




The table summarizes the various OBDII monitors operation.

TRANSMISSION CONTROL
The PCM also controls the 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 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 converter clutch solenoid, transmission temperature sensor, 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 third. However, default mode will not allow 4th gear or any EMCC operation.

Once the DRBIII(R) is in the RE transmission portion of the diagnostic program, it constantly monitors the PCM, updating the screens 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 set the maximum governor pressure. Current is regulated by modulation of the pulse width of a 512 Hz driver frequency. The transmission control relay supplies electrical power to the solenoid valve. Operating voltage is 12 volts (DC) and is provided through the relay's fused B+ contact. The solenoid is polarity sensitive. The PCM energizes the solenoid by grounding it through the power ground terminal on the PCM.

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 PCM. This feedback is needed to accurately control pressure. The unit is an absolute pressure device and the output is calibrated to be 0.35 to 0.65 volts at 14.7 psi (normal barometric pressure). Since this is an absolute pressure device, 0 psi calibration 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 10W temperature governor pressure is needed.

- Transfer Case Low-Range Operation - On four-wheel drive vehicles operating in low range, the engine can accelerate to its peak more rapidly than in Normal range, resulting in delayed shifts and undesirable engine "flare." The low range governor pressure curve is also higher than normal to initiate upshifts sooner. The PCM compares the electronic vehicle speed signal to the transmission output shaft speed signal to determine when the transfer case is in low range.

- 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. 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 speed and throttle position is used to determine target pressure. Actual governor pressure is read from the sensor and the difference between the target pressure and actual pressure is 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. Output shaft speed, throttle position, controller calculations, and shift lever position, all determine different governor pressure curves. Governor pressures can be different at the same output shaft speed. The desired governor pressure is determined by many things; including the acceleration of the vehicle. There is no need for concern if the same output shaft speed has 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 Temperature Sensor
Transmission fluid temperature readings are supplied to the PCM by the trans temp sensor. The temp sensor is located 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 PCM 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 Output Shaft Speed Sensor
The output shaft 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 PCM for processing.

Torque Converter Electronics
The torque 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 in some cases, through the PCM and supplied to one of the generator field terminals (Generator Source +). All others, the Generator field is connected directly to the ASD output voltage. The amount of current produced by the generator is controlled by the Electronic Voltage Regulator (EVR) circuitry, in the PCM. Battery temperature is determined either by IAT, Ambient or Battery temperature sensor. This temperature along with sensed line voltage is used by the PCM to vary battery charging. This is accomplished by cycling the path to ground to the other generator field terminal (Generator field driver).

Speed Control
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 turn OFF, shifting into neutral, excessive RPM (tires spinning) or ignition key OFF.

Fuel Vapor Recovery System (Duty Cycle Purge Control) - Gas Engine
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 changes in flow due to varying engine vacuum.

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 or Cam Sensor.(*)

- 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 a clogged fuel injector, a sticking pintle, or that an incorrect injector is 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 in Section 6.0 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.

SKIS OVERVIEW
The Sentry Key Immobilizer System (SKIS) is an immobilizer system designed to prevent unauthorized vehicle operation. The system consists of Sentry Key Immobilizer Module (SKIM) sends a PCI Bus message to the engine controller indicating ignition key status. Upon receiving this message the PCM will terminate engine operation or allow the engine to continue to operate.

SKIM ON-BOARD DIAGNOSTICS
The SKIM has been programmed to transmit and monitor many different coded messages as well as PCI Bus messages. This monitoring is called "On- Board Diagnosis".

Certain criteria must be met for a diagnostic trouble code to be entered into the SKIM memory. The criteria may be a range of Input voltage, PCI Bus message, or coded messages to the SKIM. If all of the criteria for monitoring a circuit or function are met and a fault is sensed, a diagnostic trouble code will be stored in the SKIM memory.

SKIS OPERATION
When ignition power is supplied to the SKIM, the SKIM performs an internal self-test. After the self- test is completed, the SKIM energizes the antenna (this activates the transponder chip) and sends a challenge to the transponder chip. The transponder chip responds to the challenge by generating an encrypted response message using the following:

- Secret Key - This is an electronically stored value (identification number) that is unique to each SKIS. The secret key is stored in the SKIM, PCM, and all ignition key transponders.

- Challenge - This is a random number that is generated by the SKIM at each ignition key cycle. The secret key and challenge are the two variables used in the algorithm that produces the crypto algorithm to receive, decode and respond to the message sent by the SKIM. After responding to the coded message, the transponder sends a transponder ID message to the SKIM. The SKIM compares the transponder ID to the available valid ignition key codes in the SKIM memory (8 key maximum). After validating the key, the SKIM sends a PCI Bus message called a "Seed Request" to the engine controller then waits for a PCM response. If the PCM does not respond, the SKIM will send the seed request again. After three failed attempts, the SKIM will stop sending the seed request and store a trouble code. If the PCM sends a seed response, the SKIM sends a valid/invalid key message to the PCM. This is an encrypted message that is generated using the following:

- VIN - Vehicle Identification Number
- Seed - This is a random number that is generated by the PCM at each ignition key cycle.

The VIN and seed are the two variables used in the rolling code algorithm that encrypts the "valid/ invalid key" message. The PCM uses the rolling code algorithm to receive, decode and respond to the valid/invalid key message sent by the SKIM. After sending the valid/invalid key message the SKIM waits 3.5 seconds for a PCM status message from the PCM. If the PCM does not respond with a valid key message to the SKIM, a fault is detected and a trouble code is stored. The SKIS incorporates a warning lamp located in the instrument cluster. The lamp receives power and ground from the instrument cluster. The lamp is actuated when the SKIM sends a PCI Bus message to the instrument cluster requesting the lamp on. The SKIM will request warning lamp illumination for:
- Bulb checks at ignition on
- To alert the vehicle operator to a SKIS malfunction
- Customer key programming mode

For all faults except transponder faults and VIN mismatch, the lamp remains on steady. In the event of a transponder fault the light flashes at a rate of 1 Hz (once per second). If a fault is present the lamp will remain on or flashing for the complete ignition cycle. If a fault is stored in SKIM memory which prevents the system from operating properly, the PCM will allow the engine to start and run (for two seconds) up to six times. After the sixth attempt the PCM will not allow engine to start.