Transmission Control Systems: Description and Operation
TRANSMISSION CONTROLThe 45RFE/545RFE electronic transmission has a fully adaptive control system. The system performs its functions based on continuous real-time sensor feedback information. The control system automatically adapts to changes in engine performance and friction element variations to provide consistent shift quality. The control system ensures that clutch operation during upshifting and down- shifting is more responsive without increased harshness.
The Powertrain Control Module (PCM) continuously checks for electrical problems, mechanical problems, and some hydraulic problems. When a problem is sensed, the PCM stores a diagnostic trouble code (DTC). Some of these codes cause the transmission to go into "limp-in" or "default" mode. The 45RFE/545RFE has three default modes:
(I) Immediate shutdown - The PCM deenergizes the transmission control relay. This causes the transmission system to immediately default to third gear if shift lever is in the "D" position, or 2nd gear if it is in the "2" or positions. Park, Neutral, and Reverse arc still available.
(0) Orderly Shutdown - If the PCM recognizes a problem that does not require an immediate shutdown, the transmission will maintain the current gear and the transmission control relay will remain energized until de-energizing it will not overspeed the engine. When the vehicle speed reaches d reasonable level the PCM de-energizes the transmission control relay. This causes the transmission system to immediately default to third gear if shift lever is in the "D" position, or 2nd gear if it is in the "2" or "U" positions. Park, Neutral, and Reverse are still available.
(L) Logical Shutdown with Recovery - The PCM does not de-energize the Transmission Control Relay. Instead, the transmission will utilize 1st and 3rd gears while in "D", and will use 2nd while in "2" or "U'. All transmission operation in this mode will be at a preset line pressure (open loop). The transmission will resume normal operation (recover) if the detected problem goes away. Three recoveries are permitted in a given key after the fourth occurrence the operation described above will be maintained.
Once the DRBIII is in the "EATX" portion of the diagnostic program, it constantly monitors the PCM to see if the system is in limp-in mode. If the transmission is in limp-in mode, the DRBIII will flash the red LED.
TRANSMISSION OPERATION AND SHIFT SCHEDULING AT VARIOUS OIL TEMPERATURES
The transmission covered in this manual has unique shift schedules depending on the temperature of the transmission oil. The shift schedule is modified to extend the life of the transmission while operating under extreme conditions.
The oil temperature is measured with a Temperature Sensor on the 45RFE/545RFE transmission. The Temperature Sensor is an integral component of the Transmission Range Sensor (TRS). If the Temperature Sensor is faulty, the transmission will default to a "calculated" oil temperature. Oil temperature will then be calculated using engine coolant temperature, battery/ambient temperature, and engine off time from the Body Control Module (BCM). These inputs are received from the communication bus periodically and are used to initialize the oil temperature at start up. Once the engine is started, the PCM updates the transmission oil temperature based on torque converter slip speed, vehicle speed, gear, and engine coolant temperature to determine an estimated oil temperature during vehicle operation. Vehicles using "calculated oil temperature" track oil temperature reasonably accurately during normal operation. However, if a transmission is overfilled, a transmission oil cooler becomes restricted, or if a customer drives aggressively in low gear, the calculated oil temperature will be inaccurate. Consequently the shift schedule selected may be inappropriate for the current conditions.
LINE PRESSURE CONTROL
Proper control of the transmission line pressure is essential for proper operation. The 45RFE/545RFE normally uses closed loop line pressure control, where actual line pressure (reported by the line pressure sensor) is continuously monitored. The PCM determines the desired (target) line pressure which is required, and adjusts the Pressure Control Solenoid (PCS) until the actual line pressure matches the desired line pressure value. In the event of a line pressure sensor failure DTC P0932(CA), the PCM changes to an open loop control at an essentially constant line pressure.
Proper diagnosis of line pressure systems is facilitated by the use of a special tool (T-fitting - Miller # 8259) which allows the use of a mechanical pressure gauge to compare the line pressure sensor reading on the DRBIII to the gauge pressure.
Technicians should compare the mechanical gauge reading with the "actual" and "desired" line pressure reading on the DRBIII. All three readings should closely match in pressure. Because the mechanical and actual line pressure may not match the desired at low engine speeds (due to low pump output RPM), line pressure should always be checked at 1500 - 2000 RPM.
Typical Line Pressure problems include:
- Mechanical and "actual" readings both less than desired
- If the mechanical and "actual" readings do not increase significantly as engine speed is raised above 2000 RPM, the pressure control solenoid is usually at fault. The pressure control solenoid is usually accompanied by DTCs P0867(C8) and P0868(C9). The PCS is located in the Transmission Solenoid/TRS assembly.
- If the mechanical and "actual" readings vary with engine speed (above 2000 RPM), the fault is often a sticking main regulator valve. This valve is located in the transmission pump assembly.
- "Actual" reading on the DRBIII differs from the Mechanical Pressure reading (higher or lower) by more than 69kPa (10 psi). This is sometimes accompanied by a DTC P0869(CB). The fault is usually in the Line Pressure Sensor or the Line Pressure Sensor Wiring.
- All three readings match, but the "actual" reading exhibits momentary intermittent pressure increases to 1724 kPa (250 psi). The line Pressure Sensor is usually the problem. This will cause erratic shift quality (particularly a harsh 3-1 coast down shift), repair by replacing the Line Pressure Sensor.
DRIVE LEARN PROCEDURE
Procedure To Learn A Smooth 1st Neutral To Drive Shift
Perform this procedure only if the complaint is for a delayed or harsh shift the first time the transmission is put into gear after the vehicle is allowed to set with the engine not running for at least 10 minutes. Use the following steps to have the PCM learn the 1st N-D UD CVI.
NOTE: The transmission oil temperature must be between 80 - 110 °F (27 - 43 °C).
1. Start the engine only when the engine and ignition have been off for at least ten 10) minutes.
2. With the vehicle at a stop and the service brake applied, record the 1st N-D UD CVI while performing a Neutral to Drive shift. The 1st N-DUD CVI account for air entrapment in the UD clutch that may occur after the engine has been off for a period of time.
3. Repeat steps 1 and 2 until the recorded 1st N-D UD CVI value stabilizes.
NOTE: It is important that this procedure be performed when the transmission temperature is between 80 - 110 °F (27 - 43 °C). If this procedure takes too long to complete fully for the allowed transmission oil temperature, the vehicle may be returned to the customer with an explanation that the shift will improve daily during normal vehicle usage. The PCM also learns at higher oil temperatures, but these values (line pressure correction values) are not available for viewing on the DRBIII.
Procedure To Learn A Smooth Neutral To Drive Garage Shift:
Perform this procedure if the complaint is for a delayed or harsh shift when the transmission is put into gear after the vehicle has had its first shift. Use the following steps to have the PCM learn the NORM N-D UD CVI.
NOTE: The transmission oil temperature must be between 80 - 110 °F (27 - 43 °C) to learn the UD CVI. Additional learning occurs at temperatures as low as 0 °F and as high as 200 °F. This procedure may be performed at any temperature that experiences poor shift quality. Although the UD CVI may not change, shift quality should improve.
1. Start the vehicle engine and shift to drive.
2. Move the vehicle forward to a speed of at least 16 km/h (10 MPH) and come to a stop. This ensures no air is present in the UD hydraulic circuit.
3. Perform repeated N-D shifts at a stop while pausing in Neutral for at least 2-3 seconds and monitor NORM N-D UD CVI volume until the value stabilizes. The value will change during the N-D shift. This is normal since the UD value is different for the N-D shift then the normal value shown which is used for 4-3 coastdown and kick downs. Perform repeated shifts in this temperature range until the NORM N-D UD CVI. value stabilizes and the N-D shifts become smooth.
4. This procedure may be performed at any temperature that experiences poor N-D shift quality. Although the UD CVI may not change, shift quality should improve.
Procedure To Learn The 1st 2-3 Shift Meter A Restart Or Shift To Reverse:
Use the following steps to have the PCM learn the 1st 2-3 shift OD CVI.
NOTE: The transmission oil temperature must be above 80 °F (27 °C).
1. With the vehicle engine running, select reverse gear for over 2 seconds.
2. Shift the transmission to Drive and accelerate the vehicle from a stop at a steady 15 degree throttle opening and perform a 2-3 shift while noting the OD CVI. During the shift, a different value may appear on the screen, which is the 1st 2-3 OD CVI.
3. Repeat steps 1 and 2 until the 1st 2-3 upshift becomes smooth and the 1st 2-3 OD CVI stabilizes.
Procedure To Learn A Smooth 2-3 And 3-4 Upshift:
Use the following steps to have the PCM learn the OD and 4C CVI's.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. Accelerate the vehicle from a stop at a steady 15 degree throttle opening and perform multiple 1-2, 2-3, and 3-4 upshifts. The 2nd 2-3 shift following a restart or shift to reverse will be shown during the shift as a value between the 1st 2-3 OD CVI and the normal OD CVI. Updates to the normal OD CVI will occur after the 2nd shift into 3rd gear, following a restart or shift to reverse.
2. Repeat step 1 until the 2-3 and 3-4 shifts become smooth and the OD and 4C CVI become stable.
Procedure To Learn A Smooth 4-3 Coastdown And Part Throttle 4-3 Kickdown:
Use the following steps to have the PCM learn the UD shift volume.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. At a vehicle speed between 64-97 km/h (40-60 MPH), perform repeated 4-3 kickdown shifts.
2. Repeat step 1 until the UD volume becomes somewhat stable and the shift becomes smooth.
Procedure To Learn A Smooth 1-2 Upshift and 3-2 Kickdown:
Use the following steps to have the PCM learn the 2C shift volume.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. With a vehicle speed below 48 km/h (30 MPH) and the transmission in 3rd gear, perform multiple 3-2 kickdowns.
2. Repeat step 1 until the 3-2 kickdowns become smooth and the 2C CVI becomes stable.
Procedure To Learn A Smooth Manual 2-1 Pulldown Shift As Well As A Neutral To Reverse Shift:
Use the following steps to have the PCM learn the LR volume.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. With the vehicle speed around 40-48 km/h (25-30 MPH) in Manual 2nd, perform manual pulldown to Low or 1st gear at closed throttle.
2. Repeat step 1 until the LR CVI become stable and the manual 2-1 becomes smooth.
Procedure To Learn A Smooth Neutral To Reverse Shift:
Perform the following shifts.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. With the vehicle at a stop, perform Neutral to Reverse shifts until the shift is smooth. An unlearned Neutral to Reverse shift may be harsh or exhibit a double bump.
If any of the shifts are still not smooth after the clutch volume stabilizes, an internal transmission problem may be present.
Procedure To Learn A Smooth 4-5 Upshift for 545RFE:
Use the following steps to have the PCM learn the ALT 2C CVI.
NOTE: The transmission oil temperature must be above 110 °F (43 °C).
1. Accelerate the vehicle through 88 km/h (55mph) at a steady 10-15 degree throttle opening and perform multiple 4-5 upshifts.
2. Repeat step 1 until the 4-5 shift become smooth and the ALT 2C CVI become stable. Note: There is a separate ALT 2C CVI volume used and learned for 4-5 shifts. It is independent of the 2C CVI learned on 3-2 kickdowns.
TRANSMISSION CONTROL (48RE)
The Cummins Controller 848 (ECM) 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 ECM, as are the transmission relay and dashboard overdrive off lamp. ECM 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 ECM continuously checks for internal transmission problems, electrical problems, and some hydraulic problems. When a problem is sensed, the ECM stores a diagnostic trouble code. Any of these codes cause the transmission to go into "default" mode. When the ECM 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 is in the transmission portion of the diagnostic program, it constantly monitors the ECM, 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 ECM energizes the solenoid by grounding it through the power ground terminal on the ECM.
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 ECM. 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 ECM 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 ECM 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 ECM 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 ECM 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 ECM adjusts the shift point until the optimum speed is reached. The ECM 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 ECM and through access to data on engine operating conditions provided by the ECM. 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 down- shift and the potential for hunting between gears occurs with a heavily loaded vehicle or on steep grades. when hunting occurs, it is very objection- able 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 ECM 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 engine control module prevents engagement of the converter clutch and overdrive clutch, when fluid temperature is below approximately 30° F. If fluid temperature exceeds 260° F, the ECM 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 ECM 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.