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, 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 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 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, 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 DRB III is in the RE transmission portion of the diagnostic program, it constantly monitors the Powertrain Control Module, 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 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 transmission control relay supplies electrical power to the solenoid valve. Operating voltage is 12 volts (DC) and is provided through the relays fused B+ contact.
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 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 low 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 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 Temperature Sensor
Transmission fluid temperature readings are supplied to the powertrain control module 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 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 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 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).
Vehicle 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.
PCM AND ECM OPERATING MODES
As input signals to the Engine Control Module (ECM) change, the ECM adjusts its response to output devices. There are several different modes of operation that determine how the ECM responds to the various input signals.
Ignition Switch ON (Engine OFF) Mode
When the ignition switch is in the ON position, the following actions occur:
1. When the PCM and ECM receive an ignition sense input, they power up. Battery voltage is supplied through the battery voltage input.
2. The wait-to start, water-in-fuel, check engine, check gauges, and transmission temperature (auto trans only) lamps are turned ON by the PCM, ECM or BCM for at least 2 seconds as a bulb test. This is done by direct wire or through the Bus to the smart cluster.
3. The ECM monitors the intake air temperature signal. If the intake air temperature is below a predetermined temperature, the ECM will start the air intake heater preheat cycle. The ECM will energize the intake heaters by grounding the air intake heater relays. The ECM will turn the wait-to-start lamp ON until the preheat cycle is over. If the ECM receives a crank signal before the preheat cycle is over, the preheat cycle will be aborted.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.
NOTE: IF THE WATER-IN-FUEL LAMP REMAINS ILLUMINATED AFTER THE 2-SECOND BULB CHECK, THE VEHICLE SHOULD NOT BE STARTED UNTIL THE FUEL FILTER HAS BEEN DRAINED AND PRIMED.
Engine Start-up Mode
When the ignition key is held in the START position, the following occurs:
1. The ECM receives the crank signal input from the crank position sensor.
2. The air intake heaters are not energized during cranking.
3. The ECM monitors engine speed and when the engine speed is above 650 rpm, the ECM determines that the engine is running.
4. If the ECM receives a crank signal before the preheat cycle is over, the heaters will be deenergized. However, the post-heat cycle will begin if the engine is in the run state or speed is above 1200 rpm.
Engine Warm-up Mode
1. The ECM may start the air intake heater post-heat cycle depending on intake air temperature. If manifold air temperature was at 66 °F or less when the ignition was turned ON, the cycle is started.
2. If the transmission temperature sensor shows it is below 59 °F, the automatic transmission will not be allowed to enter overdrive.
3. If the transmission temperature sensor shows it is below 69 °F, the transmission's torque converter clutch will not be allowed to engage.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.
Cruise or Idle Mode
During the cruising speed, the following inputs are received by the ECM:
1. The ECM monitors intake manifold air temperature through the intake air temperature sensor.
2. The intake air heater post-heat cycle will be complete, if it is not already over.
3. The vehicle speed, and throttle position sensors are used by the PCM to control the transmission overdrive and torque converter clutch operation.
4. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
5. The water-in-fuel sensor signal is used by the ECM to request the water-in-fuel lamp ON, if water is sensed in the fuel.
Acceleration Mode
When there is an abrupt demand for increased engine output and vehicle acceleration, the following occurs:
1. The vehicle speed, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. If the transmission temperature sensor shows it is below 59 °F, the torque converter clutch will not be allowed to engage.
3. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. If the speed control system Resume/Accelerate function is being used, the PCM will only allow the vehicle to accelerate at a predetermined rate. If a speed control has been set and the RESUME/ACCEL button is momentarily pushed in, the PCM will increase vehicle speed by two miles per hour.
5. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to neutral (automatic transmission)
- the speed control OFF switch is activated
Deceleration Mode
During deceleration, the following inputs are received by the PCM:
1. The vehicle speed, crank position, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. The torque converter clutch will disengage, if the brakes are applied.
3. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to Neutral (automatic transmission)
- the speed control OFF switch is activated
Ignition Switch OFF Mode
When the ignition switch is turned to the OFF position, the PCM and ECM still stores in memory trouble codes and the Throttle Position Sensor value from the previous key-ON.
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 a fuel system, or misfire monitor trouble code to be stored in the ECM.
Engine Timing - The ECM cannot detect incorrectly indexed timing gears.
Fuel Pressure - Fuel pressure is controlled by the fuel injection pump. The ECM cannot detect a clogged fuel pump inlet filter, clogged in-line filter, or a pinched fuel supply.
Fuel Injectors - The ECM cannot detect if a fuel injector is clogged, or if the pintle is sticking.
Fuel Requirements - Poor quality or air in fuel can cause problems such as hard starting, stalling, and stumble.
PCM Grounds - The PCM and ECM cannot detect a poor system ground. However, a diagnostic trouble code may be stored as a result of this condition.
Air Cleaner Air Flow - The PCM and ECM cannot detect a clogged or restricted air cleaner inlet or filter element.
Exhaust System - The PCM and ECM cannot detect a plugged, restricted, or leaking exhaust system.