The ABCs of CVTs

Posted 4/1/2009
By Tom Nash, AAM

Sooner or later, a vehicle with a continuously variable transmission (CVT) will limp - or be towed - into your shop needing service. What will you do? Will you tell the owner you can't fix his "belt box?"

In doing so, you will also be saying good-bye to any other vehicles in his family - and maybe any future business from his family's vehicles. Or, you can be prepared to inspect, diagnose and service his vehicle, make a profit, satisfy a customer - and maybe add another name to your customer base. The choice is yours.

CVTs are here to stay. Because of their smooth, fuel-efficient operation - especially on smaller vehicles with smaller engines - CVTs have grown in popularity over the last several years. There are more than 1 million vehicles with CVTs on the road in the United States and hundreds of thousands are being sold each year. Due to the current trend of downsizing to smaller vehicles with lower fuel costs, CVTs are likely to grow in numbers for at least the next decade or so.

Currently, original equipment manufacturer (OEM) dealers are performing the lion's share of CVT service, but savvy aftermarket service facilities are beginning to tap into the market.

The Rise of CVTs

The DAF 600 was the first European car to utilize a CVT in the late '50s.

The modern CVT era was born in 1958, when the Dutch company, Van Doorne, created the first viable automotive CVT (called Variomatic). It was first marketed in the small DAF 600 "Daffodil," which was mildly popular in Western Europe and the United Kingdom but never imported to North America.

The drive system consisted of a differential/transmission with two conical plates, actuated to move toward each other or apart to change the output ratio to a similar set of plates on the axle. The system used reinforced rubber belts, which weren't very durable and were exposed to the weather, further minimizing their life span.

The 1989-1993 Subaru Justy was the first mass-produced vehicle with a CVT to be sold in the United States.

The first notable vehicle to use a CVT in the United States was the 1989 Subaru Justy. The inexpensive, entry-level, rear-wheel-drive hatchback did not gain popularity with American drivers and, by 1994, was only available with a five-speed manual gearbox. Competition from low-cost vehicles by Honda, Toyota and others eventually spelled the end for the Justy as Subaru began to focus on its niche: all-wheel drive (AWD) models.

Honda put the CVT-driven Civic HX Coupe on the road in 1996 and kept it in the line until 2005. It sold well in California, but not as well in other areas of the
country.

Saturn Vue and Ion Quad Coupe used this Fiat-made CVT.

In 2002, Saturn launched the Vue, with an electronically controlled, continuously variable transmission made in Hungary under a General Motors Corp. joint operation with Fiat. The VTi, as it was called, was also used in the 2003-2004 Ion Quad Coupe, but was phased out in 2005 and replaced by a four-speed automatic.

Also in 2002, Audi began importing its 3.0L A4 model with a unique chain-driven CVT for front-wheel drive versions. Nissan introduced CVTs in 2003 and still uses them. Since then, other manufacturers have followed.

Not a Belt-Driven CVT?

It must be noted that Toyota and Lexus hybrid vehicles use a constantly variable form of transmission, called E-CVT. However, they are not belt-driven. These units incorporate a planetary gear and chain-drive system, and can most accurately be described as "torque blending devices." The Ford Escape hybrid and Mercury Mariner hybrids also use this technology, licensed from Toyota. For the sake of avoiding confusion, those types of transmissions will not be detailed here.

How CVTs Function

There are different forms and styles of CVTs used on equipment ranging from lawnmowers to mining, construction and agriculture machinery, but we'll stick to the type used in automobile transmissions. This type of CVT operates on the "two pulleys and a belt" principle.

When the drive (driving) pulley actuates apart and the driven pulley actuates closer together, the low gear ratio results.

Each pulley consists of two conical discs, facing each other, mounted on a single axle. A transmission control unit responds to sensors and other input commands to activate an actuator system, which moves the discs closer together or farther apart.

The system contains two pulleys. One pulley is the input or "driving" pulley, powered by the engine; the other pulley is the output or "driven" pulley that powers the wheels. The torque transfer belt rides in the V-notch of both pulleys.

The two pulleys are in a constant state of changing from one ratio to another, actuated by the vehicle's needs. As one pulley narrows, the other conversely widens, changing the ratio - thus the name: continuously variable transmission.

The CVT drive belt consists of small plates held in place by steel bands.

If the driving pulley discs move apart, the belt drops lower in the pulley notch (and higher in the driven pulley notch as the pulley discs move closer together). The result is a lower "gear" with more power or torque. Conversely, if the driving pulley discs are drawn closer together, the belt rides higher in the pulley notch (and lower in the driven pulley notch as the pulley discs move apart). The result is a higher "gear" with more speed. To better understand this operation, think of the gearing on a 10-speed bicycle.

The robust belt that transfers the power is quite unusual. It consists of hundreds of tiny metal plates, held together by high-strength steel bands. The key to its strength and durability is that it pushes, rather than pulls, unlike rubber-based automotive accessory drive belts. Some CVTs, however, use a fortified chain-link belt.

Early versions of CVTs commonly experienced belt failure. The lack of durability and strength limited the torque CVTs could handle. However, thanks to improved technology and metallurgy, they now can be used on vehicles up to 3.5L and upward of 250 hp.

Power is smoothly transferred from the driving pulley to the driven pulley.

Transmission operation is monitored by sensors and controlled by an electronic powertrain module to perform at optimum efficiency. Whether the vehicle needs speed for passing, or power for climbing a hill, the CVT delivers the correct ratio smoothly. In fact, some manufacturers have built in artificial "shift points" to comfort drivers who expect the feel of a shifting transmission. Some units even offer simulated manual shifting.

Driving a vehicle with a CVT takes a little getting used to. Most units on the road today still experience a little of what engineers call the "rubber band effect." When the accelerator is pressed hard, the engine revs climb quickly, but the vehicle speed lags a bit behind the revs and catches up slower than you might expect. It feels a little like a slipping clutch. So, when diagnosing a CVT, don't automatically assume the clutch is slipping until you've consulted the correct service information and diagnosed further. Manufacturers say this effect will be lessened or eliminated in the next generation of CVTs.

Tom Nash, AAM, a veteran automotive industry writer, is a freelancer whose firm, Blue Car Media, is in Sterling Heights, Mich. He is a 2005 graduate of the Automotive Management Institute, from which he received his Accredited Automotive Manager designation. His e-mail address is tom.nash@hotmail.com.