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what you're doing should determine what schedule you should use... I reckon it could come down to a preferential thing, but the only time that is reasonable, in my opinion, is if you're using your rig right at the 'swing' point between the two major functions...
the torque converter, while LOCKED, physically connects the input shaft and the engines crank.. the workload is directly on the engine...
the torque converter, while UNLOCKED, allows the crank to spin faster than the input shaft of the transmission is spinning- some people use the term 'torque amplification', though that isn't exactly what's happening.. it's more along the lines of 'inertia amplification'....
think of the UNLOCKED t/c as an arrow and the LOCKED t/c like a bullet.. the bullet strikes a target and the target instantly negotiates and absorbs the energy in whole.. an arrow strikes a target and the target absorbs the energy from the tip, just like a bullet, but then it has to absorb the energy from the shaft, and then the end- which means the arrow has more penetrating power, and also (more in line with why I present it as an illustration), it has more INERTIA behind it- it's harder to stop..
put that in drive terms:
if you're approaching a hill with the converter locked, the EGT's are going to climb as the engine alone has to produce the need for added torque as that hill's incline depletes the torque it's already producing.. it may or may not be capable of doing so.. with the extra leverage provided by an unlocked t/c, that grade may not be able to deplete all the inertia stored up in the t/c..
put that in real world terms:
the t/c UNLOCKED produces energy you may not be needing, which is expelled in terms of heat.. if you don't need the energy/inertia, you're going to be making excessive heat- heat is enemy #1 for transmissions, but it will make the engine happier.. The t/c LOCKED reduces the transmissions heat, and will make a transmission last longer- but will transfer the workload more towards the engine and it's components, which is much harder on engines, but will make a transmission happier..
you gotta strike the balance based on what you are doing with your rig.. select the higher lock higher shift schedule when racing about, or pulling loads/hauling cargo.. use the lower shift lower lock when daily driving loads the engine can handle.. basically, if you find yourself using the tow/haul feature often, then you are a likely candidate for using the higher shift/lock schedule.. if you rarely use it, then you (and your transmission and engine) are going to be better off with the low shift/low lock schedule..
one more thing: monitoring the EGT's is the tell tale sign.. just for fun, and to prove my point, next time you are driving unloaded and at a steady speed of say, 55mph, locked t/c and 5th gear, reach up and pop off the o/d.. you'll watch your RPM's kick up a few hundred digits, but you'll watch your EGT's lower... why? less strain on the engine using the better leverage of the lower gear... that is a perfect example on a more visible scale of the balancing act between locked and unlocked and how the work load shifts between the transmission and the engine..
edited to add: our rigs, as delivered from the factory, have a hugely wide torque band (over 1k RPM's wide).. unless you want to race around, or unless you are pulling/hauling weight in excess of 1/4 the rigs capacity, your engine and trans are going to appreciate the shifts and the t/c locking up sooner.. your wallet in terms of fuel costs are going to like you better too.. if you are all about getting maximum torque to the terra as you would be in racing about, or truly working your rig- you're going to benefit from late shift/locks..
Last edited by drewactual; 06-14-2013 at 09:50 AM.