Originally Posted By: meep
sorry-- I should have clarified--- I just pulled the 580/NAG as an example and the 545 as another strictly for gear ratio. Doesn't matter the unit--- the principle remains the same.
... and ... I am a huge NAG1 fan. it was probably the best part of the 08 WK I used to own. Fantastic trans.
My only gripe with the analogy is that the final O/D gearing in automatic trannies isn't just a big gear driving a small gear. Its a planetary set, with something like the planet-carrier driving the ring gear with the sun-gear held fixed... in which case the driven gear actually has the biggest tooth area of all (note there are other ways to step up ratios with planetary sets as well, so it may not be done the way I describe in a given tranny). Direct drive (highest non-O/D gear) in most rear-drive automatics doesn't actually involve any gears at all- the planetary sets are locked together so that the whole transmission becomes a solid shaft, so its clear why that's the preferred gear for sustained heavy loading.
Another issue is the TC lockup programming. Take your examples again: the NAG-1 is primarily a car transmission (though it is very tough and got put in WK and WK2s which are tow rated and moderately heavy), and uses the typical car-like variable-lockup schedule where the TC clutch can be deliberately slipping for a good bit of time. The clutch is made to take it with plenty of cooling available, but its really only supposed to do that under relatively light loading- the weight of the vehicle itself without a trailer. The 545RFE is a truck transmission, and when you put it in either "Tow/Haul" mode (which picks the lowest 5 gears out of the 6 available, including 1 of the 2 overdrives, instead of skipping "2-prime" like the regular mode does) or "OD Off" mode (which locks out both O/D gears), it changes up its TC lock schedule so that there's a whole lot less deliberate partial-lock time than when you're in normal mode. So even though "Tow/Haul" mode still includes an overdrive, its OK to use so long as the transmission isn't forced to do a lot of hunting, in which case its time to go to "O/D off."
But the take-home is the same: O/D should be used sparingly when towing, and only if its specifically allowed in the owners' manual.
sorry-- I should have clarified--- I just pulled the 580/NAG as an example and the 545 as another strictly for gear ratio. Doesn't matter the unit--- the principle remains the same.
... and ... I am a huge NAG1 fan. it was probably the best part of the 08 WK I used to own. Fantastic trans.
My only gripe with the analogy is that the final O/D gearing in automatic trannies isn't just a big gear driving a small gear. Its a planetary set, with something like the planet-carrier driving the ring gear with the sun-gear held fixed... in which case the driven gear actually has the biggest tooth area of all (note there are other ways to step up ratios with planetary sets as well, so it may not be done the way I describe in a given tranny). Direct drive (highest non-O/D gear) in most rear-drive automatics doesn't actually involve any gears at all- the planetary sets are locked together so that the whole transmission becomes a solid shaft, so its clear why that's the preferred gear for sustained heavy loading.
Another issue is the TC lockup programming. Take your examples again: the NAG-1 is primarily a car transmission (though it is very tough and got put in WK and WK2s which are tow rated and moderately heavy), and uses the typical car-like variable-lockup schedule where the TC clutch can be deliberately slipping for a good bit of time. The clutch is made to take it with plenty of cooling available, but its really only supposed to do that under relatively light loading- the weight of the vehicle itself without a trailer. The 545RFE is a truck transmission, and when you put it in either "Tow/Haul" mode (which picks the lowest 5 gears out of the 6 available, including 1 of the 2 overdrives, instead of skipping "2-prime" like the regular mode does) or "OD Off" mode (which locks out both O/D gears), it changes up its TC lock schedule so that there's a whole lot less deliberate partial-lock time than when you're in normal mode. So even though "Tow/Haul" mode still includes an overdrive, its OK to use so long as the transmission isn't forced to do a lot of hunting, in which case its time to go to "O/D off."
But the take-home is the same: O/D should be used sparingly when towing, and only if its specifically allowed in the owners' manual.
