Multitronic continuously variable transmissions – operation and servicing
Introduction
Continuously variable automatic transmissions are a popular market choice and have many advocates. These transmissions are known by various names, depending on the manufacturer. As an example, when installed in Audi cars between 2000 and 2016, they were called “multitronic”. A multitronic transmission does not differ from a typical CVT transmission designed by other manufacturers. Generally speaking, the continuously variable automatic transmission uses a variable-speed mechanism, with multitronic having a number of specific details in its design.
There are two generations of multitronic transmissions on the market. The first generation was installed in 2000–2007 and marked as 01J (VL300-7F): the next one was specific to more powerful engines (VL380-7F). A changed version, marked as 0AW (VL381-8F), premiered in 2008. Both generations are very similar in terms of their design.
A continuously variable transmission operates based on changes to the effective diameter of the metal belt or roller chain at the variators (Photograph 1). In the transmission there are two variators with conical, smooth surfaces. Moving the variator surfaces closer to or further away from each other changes the effective diameter of the belt or chain. When the effective diameter increases at one variator, the corresponding diameter decreases at the other. Such operation of the transmission allows the vehicle engine to continuously run at the optimum range of efficiency and useful torque.
Fig. 1. Drive variator with roller chain
As opposed to other designs using a converter, multitronic transmissions use multi-plate wet clutches to transmit the drive from the engine to the input shaft (Photograph 2). The larger external clutch disk is for driving backwards and the smaller one for driving forwards. This type of clutch can transmit significant torque and is highly durable. Their shortcoming, however, is rapid oil contamination from the dust generated by material abrased from the friction discs. The resulting contaminants get into the mechatronic module channels, causing jamming of the internal hydraulic valves. The initial symptom of this, which is perceivable during operation, is the transmission jerking during acceleration.
Clutch engagement and operation also create temporary but significant oil temperature hotspots of up to 400°C. Such high oil temperatures have a negative impact on ATF oil, causing, for example, faster degradation of a number of oil additives, including dispersants and antioxidants. They are responsible for dissolving small particles of carbon generated at high temperature. If the oil becomes degraded or is not changed on time, or if a poor-quality product is used, the carbon particles do not dissolve but collect inside the transmission and clog up filters and the mechatronic module hydraulic plate channels.
Photograph 2. The friction plate and the forward clutch shim
a) Transmission mechanism
The most sensitive components of a multitronic transmission are the variators – more specifically, their conical surfaces that the roller chain transfers the drive to. The chain is designed to be in contact with the variators and transfer the drive onto their surfaces with its pins only (Photograph 3). In other transmissions a metal belt is used, transferring the drive with its whole side surface (Photograph. 4).
Photograph 3. Roller chain – view of the drive transfer surface of the pins
Photograph 4. View of the drive transferring surface of the metal belt
The design allows the shortening of the variator’s effective diameter, thus increasing the available gear ratios in comparison with designs of the same size that use a metal belt. Additionally the chain can transmit more torque. For instance, this allowed Audi to use a CVT transmission in its 3.0 TDI engine configuration, offering significant torque.
Due to the high torque being transferred by the pins through a single point of contact, the surface of the variators is subject to their aggressive impact, resulting in intense wear of the conical surfaces (Photograph 5) where they should normally be smooth. If the surface of the variators continues to degrade, pitting will occur, the transmission will become jerky and the overall condition will rapidly deteriorate, becoming more and more noticeable. Analysis of the useful life of the transmission unit shows that the wear gradually intensifies. If the chain scratches the variator surface, it will create a metal particle. If a particle gets between the chain and the variator, the chain can slip again, creating another particle. Metal particles are generated at staggering speed, leading to rapid wear of the variators and subsequent jamming of the transmission. Changing the transmission oil regularly allows the removal of particles, thus significantly prolonging the life of the transmission.
Photograph 5. Damaged variator drive transfer surface
Photograph 6. Magnetic variator rotational speed sensor transmitter
b) Mechatronic unit
Another multitronic transmission component that is very sensitive to contamination is the mechatronic module. It consists of electromagnetic valves (Photograph 7) and hydraulic valves. Contaminants getting into the module via the oil not only clog the electromagnetic valve channels but even worse, they also cause the hydraulic valve pistons to jam. One of the components of the mechatronic module is the hydraulic plate, which contains a number of bored oil channels (Photograph 8). The oil pressure is directed to the appropriate channel by pistons moving in their respective sockets (Photograph 8). Contaminated oil prevents free movement of the pistons, eventually leading to complete blockage. The backing springs cannot return the hydraulic valve piston to its initial position, which may ultimately stop the transmission from operating.
Photograph 7. Solenoid valve oil channels
Photograph 8. Solenoid valve piston with return spring. The hydraulic plate’s internal oil channels are visible.
Failure to change the oil causes metal particles to scratch the surface of the hydraulic valve pistons, damaging them and causing the valve plate to lose its tightness. Subsequent oil changes are no longer sufficient and a costly transmission overhaul is required.
The hydraulic module also has small internal filters to remove contamination and particles, which may get blocked completely and prevent the passage of oil. After disassembling the hydraulic plate, a significant amount of particles can frequently be seen in the filters, characteristic of this type of transmission.
Photograph 9. Internal hydraulic module filter – large amounts of metal particles can be seen
Due to their design and method of operation, CVT transmissions are the most sensitive of all types of transmission to a decline in gearbox oil quality. Even a slight degradation of oil quality can immediately precipitate irreversible wear in the transmission components. Current service experience shows that failure to change the oil can result in damage to multitronic transmissions from as early as 150,000–200,000 kilometres, depending on the power of the engine; while an oil change within 60,000 kilometres can add at least a twofold increase in useful life.
When selecting a product for a multitronic transmission oil change, remember that it must fulfil a number of criteria that often seem mutually contradictory. On the one hand the oil is supposed to protect against excessive friction and wear to variator surfaces and the roller chain; on the other hand it must ensure proper protection and sufficient friction between the large plate clutches. This type of transmission contains a lot of bearings and has a planetary gearset to allow for reverse, which also needs to be protected against wear, not to mention the flexible internal sealing components.
Castrol Transmax CVT (product number 467204) is an oil product that perfectly matches all of the above criteria, including Audi specifications, and has been created specifically for CVT transmissions.
NOTE: multitronic transmissions should only use oil that has been specifically developed for continuously variable CVT transmissions and that fulfils Audi specifications. A mistake could cause immediate and irreversible damage to the transmission!
First-generation multitronic transmissions (01J, 0AN) are factory-filled with approx. 7.5 litres of oil. For a standard oil change we need approx. 5 litres of oil. Second-generation multitronic transmissions (0AW) are factory-filled with approx. 8 litres of oil, while approximately 6–7 litres should be used for a standard oil change.
When carrying out maintenance on this type of transmission, do not forget the oil filters. Multitronic transmissions have two oil filters:
- Internal filter – changing this requires opening the transmission, so it is only changed at transmission overhaul.
- External filter – to be changed during servicing and oil changes.
In first-generation multitronic transmissions (01J, 0AN), the internal oil filter is located in the oil pipe leading to the cooler. In second-generation multitronic transmissions (0AW), the filter is installed directly on the left side of the transmission, bolted on with three bolts and has connectors for oil ducts leading to the transmission oil cooler.
Due to the characteristics of continuously variable transmissions (specifically the multitronic transmissions described in this article) and their tendency to generate multiple metal particles during operation, it is recommended to have them regularly serviced – at least every 60,000 kilometres.