Quote:
Originally Posted by Mountain Gnome
By not having a lockdown torque converter, the bus starts and stops smoother, as this is what it is doing quite a bit as it picks up and drops off kids. It is also designed to stay in a lower gear longer, again to make it smooth.
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the AT545 is an excellent route transmission and its simple inside as well.. I can rebuild one in a matter of hours on a bench esp since usually a failurei s a stuck valve or just worn out clutch packs.. the 545 has no bands, no lock up valves, a rather simple set up. the number of special tools to tear them apart and put them back together is minimal, clearancing the clutch packs is easy, a halfway competent fleet mechanic can learn to put them together and keep a spare or two.. they are relatively lightweight at 350 lbs so doing a remove and Replace is a matter of a couple hours for 2 mechanics in an equipped fleet garage.
as mentioned lockup can be a bit of a pain in route service.. however the MT643 was designed to only lockup in 'D' at about halfway through 3rd gear.. a bus geared for normal city with the ability to do some highway travel.. (11R22.5 tires and a 4.78 gear was a very common combo).. means that in neighborhoods doing stop to stop you would never reach lockup engagement speed on a 643.. the 643 will lockup in 2nd on Decel when you use the manual selector level and run it in '3' or '2' which makes it extremely nice for panic stops or steep hill descent.
in the modern busses lockup is controlled by the co,bo of the engine computer and the TCM.. school busses equipped with allison 2000 series transmissions are often programmed so when the Warning light 'Master' is set to on that lockup will be inhibited until you reach a higher speed and gear.. so that while running your route lockup stays off and when you turn the Master off then the trans will operate with lockup either beginning in high 2nd or in 3rd gear of the range. .
on either the MT643 or the AT545 making sure your shift modulator works is paramount to transmission longevity.. on mechanical engines its a cable that pulls in and out as you use the throttle.. this sets the throttle Valve pressure which not only determines when the unit shifts to the next gear but also how much pressure is applied to the clutch. a transmission shifting under full engine power needs more pressure.. or the shifts are early and slow which heats up and burns up the clutches..
on the 'E' engines that have fly-by wire.. (T444E,DT466E,cat 3126, C7) and others they use an electric modulator that the computer turns on and off based on how much load the engine is enduring..
rarely so the cable moidulators fail.. OFTEN the electric modulators fail and with it will soon go the transmission.. and if the modulator isnt tested and working your new transmission will not be long for the world either..
the 545 and 643 use the same modulator
testing them is rather easy.. if you drive at light throttle and shift from 3 to 4.. the nare able to punch it to the floor and have it downshift from 4 back to 3 your modulator works... again you drive at light throttle. . another way is if you get up high into third gear at mid throttle and then left off the peddle all the way and it goes up into 4 then your modulator works..
if it shifts the same time all the time regardless of light throttle or floored then your modulator is inoperative and needs fixed..
what do you guys think is a fair charge for labor?
Ordered the transmission!!
Linear Mode
