Transmission System Description and Operation
Transmission System Description and Operation (M36 and MXE)
The AF 40-6 automatic transmission (1) is an electronic 6-speed automatic transmission with lockup.
The transmission is mounted directly against the engine and has a final drive with integrated differential. The power unit is transverse mounted with all-wheel drive. Gear positions P-R-N-D are selected with the selector lever mounted in the centre console. Manual up/downshifting is possible.
Transmission functions can be divided into three parts:
- Mechanical
- Hydraulic
- Control system
Mechanical
The main mechanical components of the AF40-6 automatic transmission:
- Torque converter with lock-up clutch
- 2 planetary gear units, of which the rear is double
- 3 disc clutches
- 1 band type brake
- 1 multi-disc brake
- 1 free wheel
- Final drive with differential
Torque Converter Components
Note
The car must not be stalled (car held with brakes and full throttle in D position) for longer than 5 seconds.
The torque converter is bolted to the engine flywheel (flexplate). The converter is filled with oil via pressure from the automatic transmission oil pump. This is itself a hydraulic transmission that amplifies the engine torque the more it slips. The greatest torque amplification is obtained when the engine stalls (car stationary and full throttle in Drive), as the engine torque is at a maximum while the transmission input shaft is stationary. There will not be any amplification if the engine speed is the same as the transmission input shaft speed.
The torque converter clutch (lock up) is activated at a steady speed when there is no need for torque amplification. The difference in speed between the engine and the transmission does give an amplification of torque even at steady speeds but also entails heat loss. These losses result in unnecessary fuel consumption.
(1) Torque converter
(2) Transmission's output gear
(3) Intermediate shaft
(4) Differential
(b1) Brake
(b2) Brake
(c1) Clutch
(c2) Clutch
(c3) Clutch
(f1) Free-wheel
The transmission has 2 planetary gear units; front and rear. The front one is a single planetary gear and the rear one is double. The rear unit is Ravigneaux type with two planet gears between the rear sun gear and the only ring gear, which is the transmission output shaft. The two planet gears are fitted on a common planet carrier.
The front planetary gear sun gear is fixed and the power from the torque converter enters via its ring gear. This entails a reduction of the planet carrier speed used for gear 1-5 and R. The power from the planet carrier is conveyed to the rear planetary gear unit via clutches C1 and C3.
The rear planetary gear unit receives power either directly from the torque converter via clutch C2 or from the front planetary gear via clutches C1 and C3. The 2 brakes, B1 and B2, are used to hold firm the components in the rear planetary gear unit. B2 is combined with a free wheel (F1).
The activation of one or more friction elements results in a gear ratio. Brakes and clutches are activated hydraulically using solenoids controlled by the transmission control module (TCM).
The table shows the active elements for each of the gears.
Operation of Clutches and Brakes
Overview of Clutches and Brakes
(1) Output shaft gear
(2) Intermediate shaft
(3) Location of oil pump
(4) Input shaft
(5) Differential gear
(6) Rear planetary gear unit:
(6a) Planet gear
(6b) Sun gear
(6c) Ring gear
(7) Front planetary gear:
(7d) Planet gear
(7e) Sun gear
(7f) Ring gear
(7g) Lock
(8) Planet carrier
(9) Front sun gear in rear planetary gear unit
(10) Rear sun gear
(11) Front planet carrier
(12) Front sun gear
(b1) Brake
(b2) Brake
(c1) Clutch
(c2) Clutch
(c3) Clutch
(f1) Free-wheel
Operation of Front and Rear Planetary Gear Units
Gear changing performance is controlled by the applied element. When changing gear, it is not possible to apply more than one friction element for consistent behavior. In the same way, it is not possible to release more than one friction element when changing gear. This means it is not possible to change from one gear to an optional gear. The following shows the normal gear changing sequence from standstill:
In this sequence you can clearly see that no more than 1 element is released or applied at one time. If you suddenly release the accelerator in 3rd gear after accelerating at full throttle, it is logical to change up to 6th gear.
Changing directly from 3 to 6, however, is not suitable because C1 and C3 must then be released and C2 and B1 applied. Changing 3 to 5 and 5 to 6 must be done instead. The need of these intermediate changes is especially noticeable during kickdown at various speeds. The permitted changing sequences are determined by a program in TCM.
The transmission is blocked mechanically in selector lever position P.
(1) Torque converter
(2) Valve housing:
(2a) Solenoid, solenoid valve
(2b) Linear solenoid
(2c) Control valve
(3) Clutch, brake
(4) Oil cooler
(5) Oil pump
(6) Planetary gear
(7) Oil pressure
(8) Transmission oil, inlet
(9) Transmission oil, return
(10) Operating pressure
(11) Lubrication
(12) Oil flow to pump
A special automatic transmission fluid must be used. The transmission does not have an oil dipstick. The drain plug has an extra central plug for checking the level. It is connected to a rising tube that reaches up to the correct level in the box. Fill the oil through the conventional filler plug until the level is correct and oil runs out of the level hole.
The temperature of the oil is important when checking the level. The temperature can be read with a scan tool.
All the friction elements are applied hydraulically. The hydraulic pressure is generated by a pump driven by the engine. The pump is mounted in the transmission behind the torque converter. The drive is direct from the torque converter.
Oil passes to the valve housing where the manual valve is integrated. The valve is affected by the gear selector wire, which in turn is operated by the selector lever. The valve has 3 positions, P/N, R and D/M. In position P/N, the oil will drain directly back to the transmission sump. In position R, the oil will pass directly to brake B2, which is used for reverse gear. In position D/M, oil will pass to the circuits used for the forward gears.
To control the flow of oil to and from the friction elements, 6 stepless (linear) solenoids are used. With them, TCM can regulate the pressure precisely. Each solenoid is connected to a mechanical control valve. They work as variable relay valves and are needed to control the relatively high flow of oil that is required. The solenoids do not possess the flow capacity needed to feed the oil directly to each friction element.
The table below shows the linear solenoids and their task.
(1) On
(2) Off
(3) Ex
(slc1) Solenoid valve SLC1
(slc2) Solenoid valve SLC2
(slc3) Solenoid valve SLC3
(slb1) Solenoid valve SLB1
SLB1, SLC1, SLC2, and SLC3 Solenoids
(1) On
(2) Off
(3) Ex
(slu) Solenoid valve SLU
(slt) Solenoid valve SLT
SLT and SLU Solenoids
(1) On
(2) Off
(s1) Solenoid valve S1
(s2) Solenoid valve S2
There are also 2 on/off-type solenoids. The following table shows these digital solenoids and their duties.
S1 and S2 Solenoids
Cooling System
The automatic transmission (2) has an oil cooler integrated in the engine radiator (1) and constitutes one unit together with the transmission. Fluid is circulated by the transmission fluid pump and carried via hydraulic hoses (3) to the radiator and back to the transmission.
Control System
General
The transmission is electronically controlled by a control unit (TCM, Transmission Control Module). The TCM (1) is mounted directly on top of the transmission (2) and has one connector to the car's electrical system and one to the transmission. The gear position sensor is of Hall type and is integrated in the control module. The TCM is mounted so that the selector lever shaft goes through the control module. The gear lever sensor position is calibrated in position N with a scan tool, there is no mechanical adjustment. TCM controls the solenoids to maintain or change gear. The solenoids are controlled with PWM (Pulse Width Modulation) to achieve a variable flow of oil.
Shifting
The determining factors for changing gear are two values: Current gear and target gear. Current gear is the one engaged at the moment and the target gear is the one TCM wants to change to. As long as these to values are the same there will be no gear change. Gear changing is initiated only when the target gear differs from the current gear. It is mainly the vehicle speed and acceleration that determines the target gear in automatic transmissions. The vehicle speed is taken from a special sensor, output speed, while the acceleration is taken from the engine management system (ECM) via the high speed bus.
Gear changing must also follow a permitted schedule. If a direct change to the target gear is not present in the schedule, there will be an intermediate gear change. If, for example, the car is in 5th gear and the accelerator suddenly indicates a target gear of 2 (kick-down), the gear change will be 5 to 3 and 3 to 2. This is because 5 to 2 is not a valid gear change.
When a gear change is carried out, TCM will calculate a requested pressure for each solenoid circuit. The pressure is converted to a requested current that later results in a PWM ratio. The requested pressure is modulated according to a predetermined procedure so that the gear change is made comfortable without unnecessary wear. Current engine torque taken from ECM via the high speed bus is used for this purpose. A higher pressure is required for higher engine torque.
TCM reads the resulting current to each solenoid and can finely adjust the PWM ratio so that the requested pressure (current) agrees with the actual current. The transmission does not have a pressure sensor so current pressure cannot be sent back to the control system. For this reason, the solenoids are calibrated very precisely.
Manual gear changing can be accomplished with the selector lever in position D using the up/down switch in the lever. A manual gear change must be fully completed before TCM will accept a new change command. The current gear is shown in the driver information center DIC. The highest permitted gear for pulling away is 3rd. For safety's sake, there will be a change down from gear 6, 5, and 4 automatically after kick-down if engine speed is below 2,000 rpm.
TCM sends continuous information on the high speed bus concerning the engine torque allowed by the transmission. When changing gear this value drops and ECM limits engine torque if it exceeds this value.
Lock Up
The torque converter clutch is activated at steady speed in gears 3-6 when there is no need for any torque amplification. The slipping in procedure is regulated by comparing the speed on the input shaft with the engine speed obtained from ECM via the high speed bus. Lock up will not work at temperatures below 15° C (56° F). If the brake is depressed or acceleration increases, the function will be activated.
Safety Features
A reverse detent function prevents reverse gear being engaged if the car is travelling faster than 7 km/h when the selector lever is moved from N to R. Overrevving in gear position M will be prevented by not allowing changing down to take place if the speed is too high.
Special Gear Change Program
For high engine loads without a corresponding increase in speed one of two hill-climbing programs will be chosen. Gear changing is done at higher engine speeds and the gear will remain engaged for longer periods until the extra load ceases. These gearchange programs are used to avoid changing up and down unnecessarily, e.g. when driving up long inclines with trailer and to avoid overheating the transmission oil. The Sport Mode can be engaged by a switch. Upon detecting the sport mode request TCM adapts the shift lines in order to enable a more progressive driving. Special gear change programs are not activated at speeds above 150 km/h (93 mph). If a special gear change program is active when the speed increases to over 150 km/h (93 mph), the program will continue, however, until the load has normalized.
Diagnostics
The control module has diagnosis for internal faults. The solenoids are diagnosed with respect to open circuits and short circuits. They are also pulsed and the inductive voltage peak is analyzed to make sure the coil is not partially shorted. The speed sensor and temperature sensor circuits and plausibility are checked. High speed bus communication is checked to make sure no necessary messages are missing. Mechanical faults (slipping or wrong gear) are detected by comparing speed sensor values with the engaged gear.
Limp Home
If the power supply to TCM or to the solenoids is cut, the car can still be driven. Gear position D/M will give 2nd gear and P-R-N will work as normal but the gear will be engaged roughly as there is no regulation.
In case of a major fault in the transmission, mechanical or electrical, TCM will cut the supply to the solenoids in a controlled manner. In most cases, the control module will attempt to maintain the current gear until the car has stopped. Subsequently, the control module will attempt to engage 2nd gear if D/M is selected. After this, the supply to the solenoids will be cut and the transmission will lose all electrical control.
If an invalid gear ratio or the wrong gear is detected, the engine torque will be limited to 170Nm in reverse and 150 Nm forward. For other faults, the torque will be limited to higher values depending on failure.