Understanding a shifting problem with CVTs

Our tech guru has the answer
by Ferman Lao | Sep 12, 2011

Hi, Ferman!

Good day to the TGP community.

Our City is six years old (she grows up so fast) and she's got a new problem. When shifting in reverse, there are times when it seems to slip out of "gear," stop momentarily then re-engages. The fluids have been replaced but that didn't seem to do the trick. I can only guess that the CVT belts or chains or whatever have worn out. I think the CVTs are better suited to more developed countries, hence Honda's move to bring out the more conventional automatic in the newer City and Jazz.

What's the problem and what's the possible solution?


Aloha Milkyway

Hi, Aloha Milkway!

To better understand your problem let's take a quick look at the CVT transmission found in the 2003-2009 City. It uses a metal belt running across two conical pairs of a variable diameter pulley. When either of the conical pair of the variable pulley changes diameter, the opposite variable pulley changes in size as well. It changes at exactly the same rate in relation to each in order to keep the metal chain link belt taut at all times. By varying the diameter of each of the pulley the transmission effectively changes gear ratios from a 1:1 ratio to less than 1:1 in favor of the drive side or more than 1:1.

Continue reading below ↓
Continue reading below ↓
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A large input to the throttle pedal will usually cause a downshift (engine rpm goes up along with vehicle speed) of the transmission, while a releasing the pedal slightly followed by any input large or small will result in an upshift (engine rpm goes down with increasing vehicle speed). Keeping the pedal steady will also result in a steady increase in vehicle speed without a change in engine rpm, in this instance the transmission is also upshifting.

I've found that under most conditions the latter two methods of throttle application is all you need. Fuel economy also is better using the two methods. Keeping the shift lever in "D" instead of using the shift buttons also result in a lower engine rpm for a given vehicle speed. However, that's just how it shifts. Power has to be transferred somehow from the engine to the transmission and on conventional fixed-ratio-per-gear transmissions the job is done by a torque converter.

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Some car manufacturers also choose to use a torque converter for their CVTs. In fact Honda has chosen to go this route as well for their 2010 CVT-equipped model Jazz units, unlike the first generation Jazz that used a clutch much like the one you find in manual-transmission-equipped cars. Using a clutch has the advantage of less power loss and better efficiency vis a vis a torque converter. Over time the clutch wears down and slippage occurs, resulting in poor or inconsistent power transfer--this is probably what your encountering now. This wear is accelerated when the transmission is subjected to repeated torque or power inputs wherein more than the required amount to move the car forward is applied, as the transmission will let the clutch "slip" when the situation happens.

Once the clutch starts slipping often like yours does, the end of the line will be the car won't be able to move forward anymore when the clutch finally wears out. Replacement of the transmission is the only recourse--a rather expensive one as a new one will cost in the ballpark of P300,000. The cheaper alternative is getting a surplus unit for about P20,000. Be aware though that a surplus unit will be a gamble. Generally they will have quite a bit of usable life in them, but as always you run the risk of getting one that’s as bad as what you already have as well.

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You can try this experiment. On an incline try giving a large input (half or more of full pedal travel) to the throttle, see how much time it takes for your car to move up. After that, try applying a smaller throttle input (a quarter or less of total). You may be surprised to find out that there is very little difference in the time in takes to move forward.


Ferman Lao
Technical editor

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