MMT6

Design

Transmission, general




MMT6 is a six shift manual transmission. All gears, including back-up, are synchronized. The synchronizing units, interlock ball, springs and sliding block make up a single unit. This design facilitates service work on the synchronizing unit.
The transmission is intended to be part of a transverse drive line with front wheel drive.
The transmission and final drive are constructed together in a common housing made of aluminum. The transmission is very compact. The transmission is a "light weight model" within its class and weighs 52.5 kg (115.7 lbs) without fluid.
The transmission has a maximum torque transfer of 350 Nm.
The transmission fluid is synthetic and does not normally need to be changed. The oil is heat resistant and tolerates high loads. Fluid is topped up via the filler pipe. The fluid is checked via the level plug. The fluid is drained through the drain plug in the base of the transmission.
The gear wheels are located on three shafts.
- One input shaft, the primary shaft
- Two output shafts, the upper and lower secondary shafts.
The transmission housing only has a sealing ring. The seal consists of a liquid gasket.

Transmission, internal components









All driven gear wheels in the transmission are on the input shaft. The input shaft is solid. The input shaft has a ball bearing and a roller bearing. This design solution does not require preloading or adjustment shims. The seal for the input shaft is pressed into the clutch cover.

Transmission input shaft









The gear wheel for 1st and 2nd gears are an integrated part of the input shaft. The gear wheel for 3rd and 5th gears and the gear wheel for 4th and 6th gears are pressed on the shaft.
Due to the extreme pressure used when the gear wheels are pressed on the shaft, they cannot be pressed loose during service work. A defective input shaft can only be replaced as a complete unit.
Defective bearings can be pressed loose and replaced.

The transmission output shaft, gears 1, 2, 3 and 4









The output shaft for 1st, 2nd, 3rd, and 4th gears is hollow. The internal oil ducts lubricate the needle bearings and the roller bearings and the synchronizing units. The output shaft for 1st, 2nd, 3rd, and 4th gears runs on the conical roller bearing. This bearing assembly operates on condition that the shaft is preloaded. Shims are used to achieve this preloading.
The output final drive gear on the output shaft for 1st, 2nd, 3rd and 4th gears is an integrated part of the shaft.

Caution! Note the position of the oil collection ring when installing. If the oil collection ring is incorrectly positioned, the transmission fluid pressure can drop in the output shaft resulting in insufficient lubrication.

Transmission output shaft, 5th, 6th and back-up gears









The output shaft for 5th, 6th and back-up gears is hollow. The internal oil ducts lubricate the needle bearings and the roller bearings and the synchronizing units.
The output shaft for 5th, 6th and back-up gears runs on conical roller bearings. This bearing assembly operates on condition that the shaft is preloaded. Shims are used to achieve this preloading.
The output final drive gear on the shaft for 5th and 6th and back-up gears is an integrated part of the shaft.
An oil collection ring has been installed on both output shafts.

Caution! Note the position of the oil collection ring when installing. If the oil collection ring is incorrectly positioned, the transmission fluid pressure can drop in the output shaft resulting in insufficient lubrication.

Gear selector, internal




The shift unit is in the transmission housing and is secured by four screws. The shift unit consists of:
- Aluminum housing with integrated bearing and integrated seal
- Support with integrated selector guide
- Bearing, gear selector gate and end stop for the lateral movement arm.
The shift unit is consolidated in a shift control unit assembly.
Back-up gear in the gear selector assembly is furthest to the left and forward. Neutral is between 3rd and 4th.
The reversing light switch is in the shift control unit and is actuated by the shaft for lateral movement. The reversing light switch is directly connected to the Central Electronic Module (CEM).
The shift control unit must not be taken apart. In the event of a fault, the entire control unit must be replaced.
Four gear selector forks transfer the movement from the selector unit to the relevant coupling sleeve.

Gear selector assembly and mechanical transmission cables




1. Ventilation
2. Adjustment mechanism for mechanical gear selector cables
3. Gear shift lever
4. Mechanical transmission cable for lateral travel
5. Gear selector assembly
6. Gear selector lever and gear knob
7. Ball and lever for mechanical gear selector cables
8. Mechanical gear shift cable
9. Body lead-in (seal in the cowl, made of rubber)
10. Mechanical cable bracket
11. Ball and lever for mechanical gear shift cables
12. Reversing light switch
13. Shift unit.
The movements of the gear selector lever are transferred to the transmission via the two mechanical cables. This requires limited force for gear shifts and ensures that each gear selection is distinct. Vibration and other movement from the transmission is not felt through the gear selector lever.
The mechanical gear shift cables are color marked to reduce the risk of mixing up the cables.
- The end of the mechanical transmission cable for lateral travel is gray
- The end of the mechanical shift cable is black.
The mechanical transmission cable for lateral travel crosses the shift cable in the engine compartment.
Only the mechanical transmission cable for lateral travel can be adjusted. The mechanical transmission cable for lateral travel is adjustable lengthwise. The adjuster is positioned on the mounting towards the lever on the transmission (1). When adjusting, the spring in the gear selector assembly pulls the mechanical cable and then the mechanical cable is locked by hand using the catch. When locking, the gear selector lever must be in the neutral position and the components in the transmission must be in shift position 3 or 4.
The mechanical cables sleeves are secured in the gear selector assembly using quick-release connectors and to cable bracket on the transmission. The mechanical cables ends are secured with ball joints to the gear selector assembly and to the levers in the transmission.
In the event of a faults or damage with any of the mechanical cables, both mechanical cables must be replaced at the same time.
There is a return spring on the lateral movement lever. The shift control unit in the transmission also has a return spring.

Spring-loaded reverse inhibitor (- 2005)
The reverse inhibitor consists of a spring-loaded detent in the gear selector assembly.

Mechanical reverse inhibitor (2006 -)




1. Pin
2. Stop
To prevent the accidental engagement of reverse gear, the gear selector assembly has a mechanical reverse inhibitor. The inhibitor consists of a pin (1) that sits on the gear selector lever and a stop (2) that sits on the gear selector assembly. When the gear selector lever is moved to the left, the pin is pressed against the stop. This prevents the lever from moving farther to the left than the position between 1st and 2nd gear.
To engage reverse, the lever must be pressed down so that the pin goes under the stop.

Differential




1. Conical roller bearing, transmission housing
2. Differential unit
3. Conical roller bearing, clutch housing
4. Bearing shell, conical roller bearing
5. Crown wheel bolts
6. Crown wheel
7. Bearing shell, conical roller bearing
The differential distributes power equally between the drive wheels, even if the drive wheels are rotating at different speeds.

Synchronization




1. Companion flange section
2. Synchronizing hub
3. Coupling sleeve.
The synchronizing units, interlock ball, springs and companion flange section make up a single unit. The synchronizing rings are made of brass. 1st, 2nd, 3rd and 4th gears have double synchronizing rings. 5th, 6th and back-up gears have a single synchronizing ring.
The synchronizing units for 1st, 2nd, 3rd and 4th gears are on the output shaft for 1st, 2nd, 3rd and 4th gears.
The synchronizing units for 5th, 6th gear and back-up gears are on the output shaft for 5th, 6th and back-up gears.

Clutch, general




The clutch is between the engine and transmission. It consists mainly of a pressure plate and a clutch driven plate. The clutch transfers torque to and from the transmission and then on to drive the wheels.
The clutch is a single disc dry clutch. The clutch disc is connected to the transmission input shaft.
On vehicles with more powerful engines, self-adjusting clutches are used to lower the engagement forces.
MMT6 has self-adjusting clutch of the XTend type.

Hydraulic clutch control mechanism




1. Dual mass flywheel
2. Air-bleed nipple
3. Hydraulic hose
4. Master cylinder
5. Clutch slave cylinder
6. Pressure plate (self-adjusting).
Clutch control is based on a well proven system with hydraulic power transmission.
There are ratios built-in to the system to reduce the force required to transfer torque:
- pedal ratio
- hydraulic ratio
- clutch pressure springs ratio.
The force is reduced from approximately 8600 N at the clutch to approximately 100 N at the pedal.

Shock load limiter




If the clutch pedal returns too quickly, the drive line could be damaged by the high torque. There is a shock load limiter in the clutch to avoid this. The shock load limiter chokes the flow between the clutch slave cylinder and the master cylinder if there is a danger of excessive torque. The shock load limiter is in the air bleed adapter on the clutch cable.

Clutch slave cylinder




1. Throwout bearing
2. Protective cap
3. Pressure spring
4. Hydraulic duct
5. Radial seal
6. Plastic housing.
During unaffected conditions the clutch slave cylinder applies a pressure of 100 N on the throwout bearing on the diaphragm spring to the clutch pressure plate. This reduces wear on the throwout bearing. The throwout bearing does not need to accelerate to engine speed when the clutch is engaged.
The hydraulic clutch system is bled via the bleed valve. The bleed valve is in the hydraulic line of the feed and return lines.
The self-adjusting clutch mechanism reduces the operating range of the clutch slave cylinder, which protects the radial seal. The clutch slave cylinder therefore only requires replacement if leakage occurs and if the transmission has been removed. This is because dirt and corrosion can cause damage to the seal after the system has been removed.
To achieve a better seal on the transmission, the radial oil seal for the input shaft is not integrated in the clutch slave cylinder.

Self-adjusting clutch ( XTend)




1. Spring
2. Adjustment ring
3. Wear sensor.
A self-adjusting clutch XTend detects the wear on the clutch plate and adjusts the wear using a adjustment mechanism between the sun spring and the clutch housing. The adjustment mechanism maintains the position of the sun spring and ensures that the pedal effort required remains constant throughout the service life of the vehicle.
The advantages of the self-adjusting clutch are:
- Constant load during the entire service life of the clutch
- Increased service life for the clutch driven plate
- Reduced length of stroke for the throw out bearing.

Pressure plate




1. Pressure springs
2. Pressure plate (housing)
3. Ramp
4. Ramp ring.
The clutch is a single disc dry clutch. Pre-tensioned lifting springs connect the clutch housing and the pressure plate.
As self-adjusting clutches cannot be reset when the clutch driven plate has been replaced, the pressure plate must be replaced.
The pressure plate is the diaphragm spring type. Unlike a traditional type, the pressure plate contains an adjustment ring made of steel or plastic. If there is wear, the adjustment ring moves slightly to maintain the engagement position of the clutch.
The fact that the springs are unable to move backwards has the following advantages:
- increased wear capacity
- the clutch cover can be made shorter because space is not required for rearward movement of the diaphragm spring.

Clutch driven plate




The clutch driven plate transfers engine torque to the transmission input shaft. The plate has a damper to reduce idle rattling. The inside of the plate center is lubricated when the plate is mounted at the factory. This reduces friction between the clutch driven plate and the input shaft, which makes shifting easier.
When replacing the clutch driven plate, the input shaft must be lubricated. See the instructions for replacing the clutch.
It is not possible to replace only the clutch driven plate with these types of clutch. The pressure plate and clutch driven plate must be replaced at the same time.

Dual mass flywheel




The dual mass flywheel reduces noise from the transmission and improves drivability.
The dual mass flywheel helps reduce the load on the crankshaft bearings. It also improves shift quality because the flywheel mass on the clutch driven plate is less.
The component parts of the dual mass flywheel can turn in relation to each other. The amount by which they turn depends on the engine load. On the dual mass flywheel the component parts can turn by up to 60 degrees in both directions. A certain amount of turning can be felt between the component parts if a dual mass flywheel is turned.

Clutch pedal




1. Clutch pedal
2. Clutch pedal support spring
3. Body seal
4. Clutch return spring.
The clutch pedal on vehicles with MMT6 transmissions, has a support spring which reduces the load force on the clutch. The support spring is angled and therefore functions as a pressure spring in a certain clutch pedal position, thus supporting the pedal force.