FIGURE 2—HARDWIRED INPUTS FOR THE PCM
FIGURE 2—CVT PUMP
FIGURE 3—STEEL BELT
Continuously Variable Transaxles
A continuously variable transaxle, or CVT for short, was first introduced on the 2007 Dodge Caliber. It was mated with both the 2.0 and 2.4-liter engines. This transaxle is unique and operates much differently than a conventional automatic transaxle. Other than the fact that neither unit requires the driver to manually shift gears, there are few similarities between the two.
To bring you up to speed on this new technology, we are going to present an overview of the CVT. It will include identification, components and operation, as well as a summary of the driving experience. So, let's get started with the basics.
First of all, a CVT is easy to identify. The shape of the case is unique and it's unlikely that you will mistake it for a standard automatic transaxle. The standard FCA identification label is also located on the top of the transaxle with the serial number and other information.
The main advantages of the CVT compared to a conventional automatic transaxle are efficiency and smooth operation. The wide range of ratios (2.3-to-1 in low, to an overdrive ratio of about 0.4-to-1 in high) contributes to improved fuel economy.
The CVT is designed so that acceleration demands are met using continuously variable ratios and an engine speed that ramps up quickly to a constant level. This operation is in contrast to a standard automatic transaxle in which there are specific gears and ratios and a series of increases and decreases in engine speed. These two modes of operation are illustrated in Figure 1.
There are several benefits to meeting acceleration demands with continuously variable drive ratios. First of all, because the engine is operating in an efficient rpm range for a long period of time, fuel economy improves. Secondly, there is a smoother transition from low to high compared to a standard automatic transaxle. Thirdly, the torque converter clutch can engage earlier during acceleration and disengage later during braking. Fuel economy improves because the early engagement minimizes slippage in the torque converter and later disengagement allows the PCM to prolong decel fuel shutoff.
As you might expect, driving a vehicle with a CVT is going to be a different experience compared to a car with a conventional automatic transaxle. As a result of smooth transmission ratio and engine speed changes, the driver, under most circumstances, will not notice distinct gear shifts.
One exception to this shiftless experience is wide-open-throttle operation. When the driver pushes the pedal to the floor, there will be a kickdown-type change in transaxle ratio and engine speed. The other exception is AutoStick® mode in which six (6) distinct ratios are set.
As stated earlier, the conventional automatic transaxle and the CVT have little in common. While the torque converter and converter clutch are similar, the pump in the CVT is driven by a chain and maintains a much higher system pressure (as high as 750 psi) as opposed to its standard counterpart (Figure 2).
One critical difference between the two is the fluid. CVT automatic transmission fluid is unlike any other ATF. To emphasize the difference, a green color is used to identify it. Mopar® CVTF+4® has unique friction characteristics and using any other ATF will damage the transaxle.
Two filters are used to keep the fluid free of contaminants. A screen filter is used in the oil pan on the pump pickup. Also, there is a non-serviceable paper filter behind the onboard cooler. This cooler is located in the transaxle. Engine coolant circulates around the fluid passages to help warm the fluid in cold weather. Note: 2010 MY Caliber/Compass/Patroit CVT transmissions have a larger combo cooler (transmission oil cooler and A/C condenser) and a new bypass valve that replaces the built in cooler in 2007–2009 models.
But when we get to the actual guts of the transaxle, that’s where the big changes are seen in componentry. On conventional automatics, clutches drive or hold parts of planetary gearsets, which provide distinct gear ratios. On a CVT, a continuous range of ratios is provided by two pulleys and a special steel belt (Figure 3). This belt and pulley system is known as a variator.
The belt consists of approximately 400 segments held together by two sets of 12 steel bands. The bands fit into grooves in the segments, which have serrations to allow the belt to grip the pulleys. The primary, or input, pulley has a fixed side and a moving side which is positioned by hydraulic pressure. The secondary, or output, pulley has fixed and moving sides, too, but the positions are switched from the locations on the primary pulley.
In simplest terms, the CVT operates as follows: the input pulley drives the steel belt which then turns the output pulley; as the sides of one pulley move out and apart, the sides on the other pulley move in and closer, resulting in a change in the working diameter of both pulleys and the drive ratio. Let's take a closer look.
When the vehicle accelerates from a stop, the primary, or output, pulley sides are out (farther apart as opposed to being closer, you'll understand as we go along); conversely, the secondary, or output, pulley sides are in. In effect, what is happening at this point is the primary pulley is a small diameter pulley driving the steel belt and the steel belt is driving a large diameter, secondary pulley. The net result is a low drive ratio.
As the vehicle speed increases, the sides of the primary pulley move in (increasing the diameter), while the sides of the secondary pulley move out (decreasing the diameter). This change in effective pulley diameter allows the variator to change the drive ratio from a low ratio to high ratio. As the effective pulley diameters continue to increase with engine speed, the 1-to-1 ratio is reached, then the overdrive ratios.
Once past the secondary pulley, power flow is similar to that in a conventional automatic transaxle. The shaft that supports the secondary pulley has a final drive output gear and supports the “park” gear. The final drive in the CVT is also similar to that found in a conventional unit. But due to the extremely low overdrive ratio, the CVT final drive uses a high numerical ratio around 6-to-1.
CVT MAINTENANCE AND SERVICE
Scheduled maintenance for the CVT is simple. Under maintenance Schedule A, this transaxle is fill-for-life and requires no regular maintenance. When subjected to severe service, Schedule B applies, which requires fluid replacement at 100,000 km.
There are some precautions regarding towing of CVT-equipped vehicles. Never flat-tow any vehicle that is equipped with a CVT. Front wheel drive vehicles can be towed with the transaxle in Neutral and the rear wheels on the ground; however, all-wheel-drive vehicles should be transported on a flatbed.
Courtesy of Mopar Magazine