Thank You Scott,
Very Well Done! but,,, what is(or are?) WOT?(...high load/WOT
conditions...)
Tim Timothy
73 Glacier
NW FL
> mypentium wrote in reference to the THM425 transmission:
> ...it suffers from the lack of
> a lock up converter; a design innovation that began in the early 80's
> when downsizing of power trains began. With an overdrive lockup
> convertor and overall 3.66 gearing, there would be plenty of power in
> all three gears, and with the .75 transmission gearing O.D. would offer
> as opposed to the 1.00 of third gear...
> In reference to Bill's post above, I wanted to address some
> mis-information regarding lock-up torque converters. Contrary to the
> beliefs of some, the terms "lock-up torque converter" and "overdrive"
> are NOT synonymous. They are two very different and independent
> features of transmissions so equipped (and the THM425 is not one such
> transmission).
>
> The function of a lock-up torque converter is simply to provide a direct
> mechanical coupling from the engine to the input shaft of the
> transmission (bypassing the traditional fluid coupling). This is
> desirable because it increases the efficiency of power delivery to the
> trans by eliminating slippage. The lock-up portion of the converter is
> nothing more than the addition of a clutch (similar to that of a manual
> transmission) in series with the torque converter and a means to control
> it's engagement (computer or vacuum modulation).
>
> Most transmissions are "programmed" to engage the torque converter
> clutch (TCC) during light to moderate, relatively steady state driving
> conditions above a predetermined speed ( gear. More sophisicated transmissions will engage the TCC in lower
> gears as well. However, because of their design intent (to function at
> light/moderate loads), most TCCs are not robust enough to withstand high
> load/WOT conditions...and, as such, late model "consumer duty"
> transmissions are designed to avoid TCC engagement in these situations.
>
> Not to be confused with a lock-up torque converter, the term "overdrive"
> refers to an actual gearset(s) within the transmission (or within an
> auxiliary unit) which allows the transmission's output shaft to rotate
> at a FASTER rate than its input shaft.
>
> In general, automatic transmissions which incorporate an overdriven
> gearset are either 4- or 5-speed units. In contrast, because of their
> internal architecture, all 3-speed autos have a direct drive (1.00 to 1)
> "high gear" and are not set-up to accommodate an overdriven gearset.
> Also, keep in mind that a lock-up torque converter does NOT function as
> an "overdrive", but when engaged, mearly provides a direct mechanical
> link between the engine and transmission.
>
> Hope this helps to dispel some of the mystery about lock-up torque
> converters and overdrive transmissions.
>
> -Scott Widmer
> GCM-less in
> Cleveland, OH
atri63
