visionseeming
04-05-2009, 10:05 PM
According to the EPA's Fuel Economy website, my car's transmission has Computer Controlled Continuously Variable Lockup(CLKUP). I was wondering if anyone here knew in any detail what that means. I always assumed that TC lockup was an "all or none" event. Either it could be slipping, or not. If the lockup is indeed continuously variable, that would mean that there could be an infinitely large RPM range for a given vehicle speed and transmission gear, couldn't there?
MaxxMPG
04-05-2009, 10:23 PM
The transmission has a computer controlled pulse-width-modulated lockup converter. It is designed to allow some slippage under certain conditions in order to smooth the lockup sequence and also to allow the converter to remain partially locked at a lower engine or road speed.
This type of converter clutch is the evolution of the early on/off converter clutches that were actuated by electrical pressure switches in the valve body, and that caused what people referred to as the "chuggle-n-bump" feeling, especially in the overdrive transmissions.
The partial lockup mode allows the converter to remain partially locked at low speeds where it would otherwise have to unlock to avoid stalling the engine or needing to downshift. My Chevy can hold onto overdrive down to about 35mph, where the engine speed is just under 1000rpm, because the converter slips just enough to prevent the shuddering that would otherwise occur.
While the lockup is "continuously variable", the range is really only between "fully unlocked" and "fully locked". And since converter slippage is minimal once the car is moving above 10mph or so, there is no infinitely large engine rpm range, as that is dictated by which gear the transmission has engaged. The converter in your car probably has a stall speed around 2000rpm, so even at 'infinite slippage' - such as stalling the converter - the engine speed is still not going to go above that level. And for the record, don't stall the converter - it serves no purpose other than to make the transaxle fluid very hot in a very short time.
