Automatic Transmission - af40-6 - Description and Operation Cadillac SRX II

Scheme 73

For automatic transmission AF40, the identification plate is on the top of the transmission housing.

Transmission General Description

The AF 40-6 automatic transmission is an electronic 6-speed automatic transmission with lock-up. 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.

A special automatic transmission fluid must be used. The transmission does not have an oil dipstick. The drain plug has an extra, central check plug for checking the level that is connected to a rising tube that reaches up to the correct level in the box. Fill the oil through the conventional oil filler plug until the level is correct and oil runs out of the check plug hole in the rising tube. Measure the temperature when checking the level. The temperature is read using a scan tool.

The AF 40-6 transmission has planetary gears, disc clutches, brakes and torque converter with lock-up. The clutches and brakes are applied hydraulically. The hydraulic system is controlled by the Transmission Control Module (TCM) with the help of 8 solenoids, 2 shaft speed sensors and one oil temperature sensor. TCM communicates on the high speed bus.

The TCM is mounted directly on top of the transmission and has one connector to the car's electrical system and one directly to the transmission. The gear position sensor is a Hall sensor and is integrated in the control module. The TCM is mounted so that the selector lever shaft goes through the control module. The selector lever position can be calibrated with the scan tool, there is no mechanical adjustment.

The TCM controls the transmission according to its programming, the information from its own sensors and from other control modules. The control module adapts its control of the solenoids to compensate for normal wear in the transmission. The TCM is SPS programmable.

Shifting

The TCM can determine whether to leave a gear engaged or to change up or down. What it does is determined mainly by the gear that is currently engaged, the speed of the output shaft and the position of the accelerator pedal. The TCM changes gear by applying or releasing a friction element in the transmission. The application is regulated with PWM from the respective solenoid.

The gear can also be changed manually with the selector lever in D position if using the Tap Up/Tap Down switch. The switch is hardwired to the BCM which converts the information into a HSCAN message that is used by the TCM.

The maximum shift speed is at 7000 RPM.

Lock Up

The torque converter clutch is activated at a steady speed when there is no need of torque amplification. If the brakes are applied or acceleration increases, the function will be deactivated.

Safety Features

To protect the transmission there are temperature programs to prevent overheating. The TCM can choose to raise the shift points and activate lock up more often if the oil temperature is too high.

The reverse gear will not be activated if the car is travelling faster than 7 km/h (4 mph). If the power supply to the solenoids fails, it is still possible to drive the car. The gear position D/M gives 2nd gear and P-R-N will work as usual. In case of a major fault in the transmission, mechanical or electrical, the TCM will cut the supply to the solenoids in a controlled manner (limp home).

In case the TCM detects a failure it will go into emergency mode.

Special Gear Change Program

If an uphill gradient is detected, the gears will change at higher engine speeds and each gear will remain engaged for longer periods.

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.

Transmission Component and System Description

Gearbox functions can be divided into three parts

1. Mechanical Components
2. Hydraulic Components
3. Control System

Mechanical Components

1. Torque converter with lock-up clutch
2. 2 planetary gear units, of which the rear is double.
3. 3 disc clutches
4. 1 band type brake
5. 1 multi-disc brake
6. 1 free wheel
7. Final drive with differential

Scheme 74

Scheme 75

Control System

1. Linear solenoids
2. PWM solenoids
3. Speed sensors
4. Temperature sensor
5. Transmission Control Module including gear position sensor
6. Gear shift lever
7. Sport Mode Switch

General

The changing duration is calculated in real time by comparing the speeds of the input shaft and output shaft. The control module knows the gear ratio for the current gear and the target gear and can follow the change in gear ratio until the changing process is complete. The time it took to change gear is then compared with the desired value for the gear change and an adaptation is made.

The adaptation is done by correcting the requested pressure for the applied friction element solenoid and the correction is then used the next time a change is made between these gears. The adaptation value is saved in a non-volatile memory and must be zeroed using a scan tool each time the TCM, transmission or valve housing is changed and after SPS programming.

After zeroing with a scan tool, it will usually only require a road test, see below, for the TCM to be able to adapt itself to the new conditions.

Adaptation, Resetting

After changing or updating automatic transmission, TCM or software, the old adaptation data must be removed and a new adaptation performed as described below.

1. Drive the vehicle slowly. Stop the vehicle by shifting the shift lever to P range and chock the vehicle. Stop the engine then turn ignition on again. NOTE: Check that the vehicle does not move before operation.
2. Release the shift lock, and shift the lever to N range.
3. Check that the N position mark of the TCM is correct.
4. Use a scan tool to input the command of the N position. NOTE: It is not possible to input the command, unless the ignition is on, the shift lever is in N range, the engine is stopped, and the vehicle speed is 0 km/h.
5. Shift the shift lever from P to D range, then check that the indicator displays the correct position. If the display is incorrect check DTCs.
6. Reset adaptation using a scan tool.
7. Operate the transmission to working temperature (65-110°C). Read from the scan tool.
8. Adaptation for shifting N-D and N-R is carried out with the handbrake applied. If the procedure is done in the workshop, connect an exhaust ventilator, start the car and run at idling speed. Shift from N to R. Keep R engaged for more than three seconds, then shift back to N. Repeat the procedure 5 times. Then shift from N to D the same as above, repeating the steps 5 times.
9. Callout Component Name 1 First Gear 2 Second Gear 3 Third Gear 4 Fourth Gear Adaptation for shifting up and down is done by driving on a road. Engage D and accelerate up to just over 50 km/h with the accelerator pedal at a constant position (approx. 15-20% depressed) until 4th gear has engaged. Brake until the car is stopped. Acceleration up to 50 km/h must take more than 30 sec. and braking must take more than 14 sec. Repeat the above 5 times.
10. Adaptation for manual shifting from 2nd to 1st gear is carried out on-road. Engage M2 with the selector lever. Accelerate to 25 km/h. Engage M1 and brake smoothly until the car is stationary. Repeat this procedure 10 times.
11. Callout Component Name 1 First Gear 2 Second Gear 3 Accelerator Pedal Position (Pedal Applied) Check that adaptation has had the desired effect and that shifting performance has improved.

Transmission Indicators and Messages

The following transmission-related indicators and messages may be displayed on the Driving Information Center (DIC). For a complete listing and description of all vehicle indicators and messages, refer to Indicator/Warning Message Description and Operation .

TRANSMISSION HOT IDLE ENGINE: This message is displayed if the temperature exceeds 150°C for longer than 2 seconds.

SERVICE TRANSMISSION: This message displays when there is a problem with the transmission.

Scheme 76

The TCM is mounted directly on top of the transmission. The selector lever is integrated in the control module. The TCM is mounted so that the selector lever shaft goes through the control module. The selector lever position is calibrated with a scan tool, there is no mechanical adjustment.

An exterior 16-pin connector connects to the car's electrical system. Underneath the control module is another connector (22-pin) that connects directly to the transmission. The TCM makes contact with all the transmission solenoids and sensors here.

Note. Magnetic fields from e.g. magnets and from high-current cables, such as starter cables and cables to auxiliary equipment, can interfere with the gear position sensor. A rule of thumb is max 1A per mm in distance from the control module. A starter cable carrying a current of 200A must therefore be kept at least 20 cm away from the control module.

The TCM has a microprocessor with clock, RAM memory and a programmable ROM. An internal bus connects the processor and memory with the I/O unit. The I/O unit reads values from the A/D converter for analogue inputs, digital inputs and bus, as well as activates the transistors' output stages.

Adaptive values are saved in a non-volatile memory (ROM). When changing transmission, these values must be zeroed using scan tool. After changing TCM they will be zeroed automatically after adding the control module using the scan tool.

Scheme 77

The normally open solenoid valve is mounted in the transmission valve housing and is used by TCM when activating C2. The pressure to SLC2 is then raised from system pressure to D-pressure. The valve is also used to apply pressure to brake B2 for engine braking in 1st gear.

The control module activates the valve by applying B+ to it. The valve is powered from control module pin 5 in the connector in direct contact with the transmission on the bottom of the control module. The valve is grounded in the valve housing.

Solenoid Valve S2

The normally closed solenoid valve is mounted in the transmission valve housing and is used together with S1 to apply pressure to brake B2 for engine braking in 1st gear.

The control module activates the valve by applying B+ to it. The valve is powered from control module pin 2 in the connector in direct contact with the transmission on the bottom of the control module. The valve is grounded in the valve housing.

Scheme 78

The PWM controlled solenoid valve is mounted in the transmission valve housing and controls the pressure in brake B1. The valve is active in gear positions P, R, N, D (gears 1, 3, 4 and 5).

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 21 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 16. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLC1

The PWM controlled solenoid valve is mounted in the transmission valve housing and controls the pressure in clutch C1. The valve is active in gear positions P, R, N, D (gears 5 and 6).

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 11 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 10. The current varies between 0-1000 mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLC2

The PWM controlled solenoid valve is mounted in the transmission valve housing and controls the pressure in clutch C2. The valve is active in gear positions P, R, N, D (gears 1, 2 and 3).

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 17 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 18. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLC3

The PWM controlled solenoid valve is mounted in the transmission valve housing and controls the pressure in clutch C3. The valve is active in gear positions P, N, D (gears 1, 2, 4 and 6).

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 14 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 22. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Scheme 79

The PWM controlled solenoid valve is mounted in the transmission valve housing and controls the transmission system pressure. The value is controlled so that it is not higher than what is currently needed for any friction element. Brake B2 does not have its own pressure control valve and is therefore supplied by SLT.

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 3 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 1. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLU

The solenoid valve is mounted in the transmission valve housing.

The valve controls the pressure used for the torque converter clutch. The pressure is modulated so that full, partial or no engagement is achieved.

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 9 in the connector that makes contact directly with the transmission on the bottom of the control module. The valve is grounded from pin 4. The current varies between 0-1000mA. The solenoid is normally closed and the hydraulic pressure rises as the current increases.

Scheme 80

The sensor is mounted inside the valve housing in the front part of the transmission and measures the speed of the transmission input shaft. This value is used by TCM to calculate the actual gearchange time and to regulate lock up. The Hall sensor measures a toothed wheel with 36 teeth.

The sensor is powered from TCM pin 12 and is grounded through pin 13.

Output Shaft Speed Sensor

The sensor is mounted inside the valve housing in the rear part of the transmission and measures the transmission output shaft speed. This value is used by TCM mainly to determine the gearchange points. The hall sensor measures a toothed wheel with 56 teeth.

The sensor is powered from TCM pin 19 and is grounded through pin 20.

Transmission Oil Temperature Sensor

The temperature sensor is mounted in the transmission valve housing and informs the control module about the current temperature. The value is used to correct the PWM ratio to the solenoids so that the gear changes are not affected by the viscosity of the oil. It is also used to activate the special gearchange program in case of overheating.

Scheme 81

The sensor comprises an NTC resistor built into a plastic casing. The NTC resistor characteristic means that the resistance drops as the temperature rises. The NTC resistor is supplied with 5V through a pull up from control module pin 8 and is grounded through pin 7.

Scheme 82

The AF 40-6 automatic transmission (1) is an electronic 6-speed automatic transmission with lock-up. 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

1. Mechanical
2. Hydraulic
3. Control system

Mechanical

The main mechanical components of the AF40-6 automatic transmission

1. Torque converter with lock-up clutch
2. 2 planetary gear units, of which the rear is double
3. 3 disc clutches
4. 1 band type brake
5. 1 multi-disc brake
6. 1 free wheel
7. 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.

Scheme 83

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

Scheme 84

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.

Hydraulic

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 that is connected to a pipe 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.

There are also 2 on/off-type solenoids. The following table shows these digital solenoids and their duties.

Scheme 85

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.

Scheme 86

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 Hall type and 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.

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 (kickdown), 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 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 kickdown and engine speed below 2000 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.

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.

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.

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 170 N.m in reverse and 150 N.m forward. For other faults, the torque will be limited to higher values depending on failure.

Towing

The car may only be towed with the front pointing in the direction of travel. The selector lever must be in position N. National regulations concerning towing speeds must be adhered to. If legislation allows, the highest permitted towing speed is 50 km/h (30 mph) with all four wheels on the ground. The longest permitted towing distance is 50 km (30 miles). If the car should require towing for longer distances as 50 km (30 miles), the front wheels must be raised from the ground. If the front wheels has been raised, the highest permitted towing speed is 50 km/h (30 mph).

The engine cannot be started by pushing or towing the car.

See also:
• Indicator/Warning Message Description and Operation