Photoillustration: Harold A. Perry;Photos: Jupiter Images & Wieck Media

50 June 2007

CVT DIAGNOSTICSDaVinci sketched the first CVT more than500 years ago, but CVT-equipped vehiclesremained a relative rarity until themid-1990s. Your CVT diagnosticsuccess will be ensured by anunderstanding of theirunique operatingstrategies.

BY WAYNE COLONNA

The positive effect continuously vari-able transmissions (CVTs) have onengines is what makes them such afavorite with a number of automobilemanufacturers. They also make aperfect companion to hybrid vehi-cles. But, like any transmission, CVTs do break downoccasionally, and diagnosing some of the problemsthat crop up is not always a piece of cake. This articleis designed to help you get those CVT vehicles thatdo end up in your service bays back on the road.

The idea behind a CVT is to optimize enginetorque and to have greater control over engine rpmalong with fuel economy. Its the same reason werebeginning to see more and more six-, seven- and

eight-speed transmissions, especially behind largerengines: With an increase in the number of ratiosfrom a low gear to Overdrive, engine rpm need notrun as high before the next shift occurs. This simu-lates the seamless ratios of a CVT.

Compare, for example, the vehicle speed to en-gine rpm of a 2006 Nissan Altima 3.5L using anRE4F04A four-speed automatic transmission to a2006 Nissan Murano 3.5L using a CVT during afull-throttle upshift (Figs. 1 and 2 on page 54). No-tice how engine rpm with the CVT is more linearthan that of the engine pushing the four-speedtransmission. This has a significant effect on thevolumetric efficiency (VE) of the engine, as youcan see comparing the graphs in Figs. 3 and 4.

51June 2007

Even though most CVTs use driveand driven pulleys with a push belt(photo 1 above) to provide seamlessratio changes, they each have theirown unique ways about them. Itsidentifying these differences that be-comes an essential element in the di-agnostic process.

One significant difference to beaware of is whether or not a CVT thatshows up at your shop uses a torqueconverter. This determines the strate-gy used when the vehicle is placed in-to gear as well as when it comes to astop while in gear. In most cases,when a conventional torque converteris used, a CVT will remain in gear in asimilar fashion to a conventional auto-matic transmission. The Nissan Mura-no RE0F09A (photo 2), Saturn VUEVT25E, Ford Five Hundred CFT30and Dodge Caliber CVT2 CVTs arejust a few examples of this combina-

tion of a CVT and a torque converter.These converters also have a clutch

that supplies engine torque directly tothe transmission when the clutch is ap-plied. Unique strategies are utilized tosafeguard against damage to the beltand pulleys should the converter clutchget stuck on due to a faulty solenoid,stuck valve, leaking sealing rings or adefective converter. In one such strate-gy, the computer compares enginespeed to the primary pulley shaft (tur-bine/input) speed after a start-up inPark. If theyre exactly the same orwithin a specific parameter, the com-puter will determine that the converterclutch is stuck on and will prevent aReverse or Drive engagement as a safe-guard for the belt and pulleys.

Another strategy allows for the con-verter clutch to be applied early in theratio up-change. If the converterclutch experiences a chatter problem,

the vibration could be misinterpretedas a belt-to-pulley problem. If thechatter is very slight, it could also bemisinterpreted as an engine miss. Thecause for this vibration could be some-thing as simple as deteriorated or in-correct fluid usage. This could be diffi-cult to diagnose without the propertest equipment. Test equipment willallow you to graph engine speed aswell as the drive and driven pulleyshaft speeds during the time of thechatter, and comparing these signalswould allow for greater diagnostic ac-curacy. If the converter clutch was thecause of the chatter problem, it couldbe seen with the rpm irregularities be-tween engine speed and drive pulleyspeed. If rpm irregularities are seenbetween the pulley speeds, theres abelt issue. If irregularities are not seenbetween engine speed and drive pul-ley or between the pulleys, an engine

52 June 2007

CVT DIAGNOSTICS

1 2 3

4 5

SecondaryDriven GearOn the FinalDriveshaft

SecondaryDriven GearOn the FinalDriveshaft

Start ClutchDrum SplinesOnto the EndOf the DrivenPulley Shaft

Start ClutchDrum SplinesOnto the EndOf the DrivenPulley Shaft

High-EfficiencyPump

Chain-DrivenBy the

Input Shaft

High-EfficiencyPump

Chain-DrivenBy the

Input Shaft

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miss should be considered as the cause.CVTs without a torque converter are

typically driven through a dual massflywheel or a torsional damper platethat connects the crankshaft to the in-put shaft. The BMW Mini CooperVT1F, the Audi A6/8 01J and HondaCivic (photo 3) CVTs are examples ofthis design. To prevent a stalling condi-tion when the transmission is placed ingear or when it comes to a stop in gear,a clutch inside the transmission mustrelease. The VT1F and 01J CVTs re-lease the forward or reverse clutch,while the Honda Civic CVT releases astart clutch in both Drive and Reverse.These clutches are pulsed on and offvery frequently and are susceptible tofailure. This can cause a variety ofcomplaints, such as chatter on takeoff,sudden slipping or neutralizing condi-tions, unusual idle fluctuations or a lossof hill-hold function.

Audi offers an update package thatincreases the forward clutch from sixfrictions to a seven-friction stack-up(photo 4), but it must be accompaniedby a computer reflash that changespressure and apply strategies. If thereflash is not performed, the updatedclutch pack will be damaged in fairlyshort order.

Honda also offers a replacementstart clutch drum (photo 5). In fact,theres a specific TSB (02-003) for1996-98 Honda Civic HX models for acomplaint of shudder, judder, surging,shock, noise or idle fluctuation. Thebulletin calls for replacement of theTCM and start clutch as a set. TSB 02-084 describes the fix for similar symp-toms on later Honda models.

Relearn procedures are very impor-tant on CVTs that substitute a clutchinside the transmission for a torqueconverter. If relearn procedures are

not performed after any repairs, drive-ability problems and transmission fail-ures will absolutely occur. Be awarethat these procedures exist and thatthey must be performed.

Audi not only has a clutch relearnprocedure, but also a special proce-dure for changing the fluid. All fourwheels must be off the ground a mini-mum of 8 in., and each wheel must beable to be rotated freely by hand.While observing the Tiptronic indica-tor, step-shift the CVT from its lowestratio to its highest (six or sevenspeeds), accelerating moderately aftereach shift but never exceeding 35 mph.Shift the transmission back down to 1stgear and gently apply the brake untilthe wheels come to a stop. Once thewheels are stopped, place the trans-mission in Reverse, release the brakeand moderately depress the accelera-tor to a Reverse gear speed of 12 mph.

54 June 2007

CVT DIAGNOSTICS

Fig.1: 2006 Altima with 4-Speed Automatic Transmission Fig.2: 2006 Murano with CVT

Fig.3: 2006 Altima with 4-Speed Automatic Transmission Fig.4: 2006 Murano with CVT

RPM

VSS

RPM

VSS

RPM

VSS

VE

MAF

TP

TAF

RPM

VSS

VE

MAF

TP

TAF

Once again, gently apply the brakesuntil the wheels come to a stop. Re-turn the selector lever to Drive and re-peat these steps five more times.When completed, place the selectorlever in Park and turn the engine off.Now you can change the transmissionfluid. When youre done, repeat theseprocedures before placing the vehicleon the ground and doing a road test.

Understanding the internal differ-

ences among CVTs will aid diagnosis.As previously stated, the Honda CVTuses a start clutch in place of a torqueconverter. The start clutch is located be-tween the output secondary pulley andthe differential (photo 6). This meansthat the forward clutch and the reverseclutch are not pulsed on and off. Whenthey apply, they stay applied. The startclutch is the clutch thats pulsed on andoff, disconnecting the torque flow to the

56 June 2007

CVT DIAGNOSTICS

8

9

6 7

StartClutchDrum

PH/PL SolenoidPH/PL Solenoid Start ClutchControl Solenoid

Start ClutchControl Solenoid

Start ClutchControl Solenoid

Start ClutchControl Solenoid

Reverse InhibitorSolenoid

Reverse InhibitorSolenoid

Reverse InhibitorSolenoid

Reverse InhibitorSolenoid

Shift Control SolenoidShift Control SolenoidShift Control SolenoidShift Control Solenoid

Drive Pulley Control SolenoidDrive Pulley Control Solenoid

DrivenPulley

ControlSolenoid

DrivenPulley

ControlSolenoid

Wires Are Cross ConnectedWires Are Cross Connected

CorrectConnection

CorrectConnection

differential and rendering the transmis-sion in a neutral state. This means thatwhen the clutch fails, it affects both for-ward and reverse operation.

Compare this to the Audi 01J unit.The Audi transmission pulses the for-ward and reverse clutches on and off.If the forward clutch fails, it affectsforward only, and reverse operationremains good. Its worth mentioningat this time that the TCM is insidethe transmission on the Audi CVT(photo 7), so should you diagnose afaulty TCM, youre not going to find

it under the hood, seat or dashboard.Honda Civic CVTs have been on the

road since 1996, and have been show-ing up more frequently in shops tohave the start clutch replaced. After thejob is completed, stalling when thetransmission is placed in gear may oc-cur. This is usually caused by cross-con-necting the internal wiring harness forthe start clutch solenoid and the shiftcontrol solenoid (photos 8 and 9).

From these examples, its obviousthat there are finer points in diagnos-ing specific model CVTs. A good start

is to know which use a torque convert-er and which do not, and that relearnprocedures are imperative.

I believe well be seeing greater useof continuously variable transmissionson more and more vehicle models. Thechallenge will be to stay informed abouttheir individual unique strategies. If wedo that, our chances for successful CVTdiagnosis will be enhanced.

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CVT DIAGNOSTICS

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