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Author Topic: Driving a bus automatic  (Read 2917 times)
skipn
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« on: July 22, 2008, 09:01:08 AM »


 In another thread a comment was made about not driving a bus automatic like a
 passenger vehicle........

  Of coarse it got me thinking.....ok what am I missing (or doing wrong)

 intown I set mine at 3 or 2 and just drive
 2 lane highways in 3 on roads up to 55
 interstate 4

Opinions or facts either will do... Smiley
  657 scraper     16th gear and pedal to the metal  Shocked
   I miss those automatics running through 16 speeds........(well not that much)

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TomC
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« Reply #1 on: July 22, 2008, 10:20:20 AM »

You can do what you want with an automatic, and you won't hurt anything.  All truck/bus automatics start in 1st, no matter what you put the shifter in.  All you're doing by putting it in 2, 3, etc is limiting the highest gear that the transmission will shift into.  Just to warn you, most all automatics will also up shift to the next gear if over revved.  For instance, if you're in 2nd coming down a grade with a 2100 rpm engine and the engine gets up towards 2400rpm, the transmission will up shift to save the engine-which can be disturbing if you're coming down a steep, long grade.  Shift as you like, but keeping it in D is alright too.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
kyle4501
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« Reply #2 on: July 22, 2008, 10:40:22 AM »

? ? ?
On another thread I mentioned that the automatics shifted too hard for my liking & was told they weren't being driven properly. It seemed to me they were being driven like a car.  Huh

Dad has a 330 CAT & a 6spd Allison (Freightliner chasis MH) & if he drives it like a car, it will overheat on long pulls. The engine isn't at max power when this happens either. If dad manually drops a gear & reduces speed to normal engine cruise rpms, the temp drops right away. The shifts are quite firm on his too.


So, how do you drive an automatic to get smooth shifts like a manual?
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skipn
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« Reply #3 on: July 22, 2008, 10:47:06 AM »


 I cheat........well one might think so.

  RPM and MPH before the shift I back off on the throttle so I am not full RPM
 on the shift. I have a big jump between 2 and 3 so full throttle during this
 shift produces a neck whipping clunk....not good in my opinion

  YMMV

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junkman42
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« Reply #4 on: July 22, 2008, 11:35:17 AM »

I have a mc7, 8v71 with a allison ht70.  If I keep My foot full in it there is rarely a harsh shift.  However if I back off it will shift into 6th with a earth shattering slam.  I have jakes and a retarder and have never had a problem on down hills.  The only problem is on hot days if You use the retarder the tranny temp and engine temp climbs quite rapidly.  With jakes and retarder at the sme time You can stop on any down hill with no effort.  Sorry for the rambling, John.
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Hartley
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« Reply #5 on: July 22, 2008, 12:26:46 PM »

Soft Shift can mean either perfect throttle action or slipping. Slipping causes heat and wear...

Hard shift means immediate change with minimal slip. Better because less heat and wear on the clutches.

Auto Shift.. Clutch Once and let the system sort it all out the most efficient way.

If we only had a CVT for buses.. That would be cool ! (also for the shiftless crew!)

Dave...
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Eagle
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« Reply #6 on: July 22, 2008, 01:23:54 PM »

I have a 6V92TA with a Allison HT740.  I changed the fluid in the tranny to synthetic and the shift is smooth no matter what, plus the temp. is about 15* cooler.  I drive mine as you would a stick.  I down shift when the RPM's get below a certain point going up a hill.  If you wait for the tranny to down shift like a car you are way below the desired RPM,s before it will down shift.  I down shift before going down a hill to maintain a safe speed descending the grade.
« Last Edit: July 22, 2008, 01:27:19 PM by Eagle » Logged
Tom Y
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« Reply #7 on: July 22, 2008, 02:11:22 PM »

Eagle, So is it better with the new oil? No change? Did you run it with regular tranny fluid?  Thanks Tom Y
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Tom Yaegle
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« Reply #8 on: July 22, 2008, 02:17:02 PM »

The older Allision 6-speeds of years past could be "fooled" into shifting smoother by backing out of the throttle just before the shift.  You could also get soft downshifts by "bipping" the throttle just when you down shifted.  Of course this technique was employeed manually shifting the Allision.

I suppose todays electronic trannys do/could shift early like a car auto.  All of the heavy Allisions I drove were "programmed"--er "set up" to stay in a partucular gear within a certain road speed.  An example would be staying in 3rd or 4th instead of auto upshifting to 5th.  Shifter was in "D".

Could this have been done at the dealership?  Fire apparatus.  It helped coming down steep grades.  I do not remember if the Allison had a retarder or not, or if it would make any difference as to how the tranny modulated.  Perhaps the transmisions could be set up any way you wanted?  Smiley Smiley Smiley
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H3Jim
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« Reply #9 on: July 22, 2008, 02:35:35 PM »

I have a B500 world with 430,000 miles on it.  Most of the time its a smooth shifter, but occasionally, especially when warm and I am babying it, it will SLAM downshift.  Makes me think its going to drop out of the bus and drag along the road.  Its had Transynd in it for all of its life but 20,000 miles.
I'm hoping to get several hundred thou more miles out of it.
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Jim Stewart
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« Reply #10 on: July 23, 2008, 05:21:08 AM »

There is a "trimer" value in most Allisons that can be adjusted to make the shift much smoother. We are using transmissions meant for much heavier units that our conversions and not making any adjustments - talk to your allison dealer to see how much of an effort it is to adjust. On electronic transmissions it is very easy to adjust. Don't know about hydro shift but I think it will be easy also.
JimH
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Eagle
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« Reply #11 on: July 23, 2008, 05:28:39 AM »

Tom-Y

It is much better with the synthetic oil.  It had 103000 miles on it after rebuild when I purchased the coach and shifted hard like most HT740's do.  Since changing the fluid it shifts very smooth and the Temperature is about 15* cooler.
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Tom Y
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« Reply #12 on: July 23, 2008, 09:14:24 AM »

Eagle, Thanks.  Tom Y
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Tom Yaegle
Tim Strommen
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« Reply #13 on: July 24, 2008, 12:20:28 PM »

I'll try not to say anything stupid here...

My HT740 (attached to a 6V92), when I'm in the city I put it in "3".  This allows the tranny to go from 1 to 2 to 3 (but not into over-drive).  A side effect of this is that it supports engine braking better in this setting (the torque converter stays locked up until it drops below 25MPH road speed - so the engine stays "attached" to the wheels).  In "D" range, the tranny seems to allow drive shaft over-speed, so the bus will pick up speed going down hill (even if you let off the throttle).

When I put the tranny in "D" range, I get the over-drive gear for flat roads - but on hills, the tranny tends to "hunt" between for example 2nd and 3rd (on a hill climb, you want to keep a v92 running fast - or you might lug it...).

Thus, I have a basic method of "D" for flat freeways, "3" or "2" for hills (both climbing and decent), and "3" for city driving.  Hasn't failed me yet...

-Tim
« Last Edit: July 24, 2008, 12:23:31 PM by Tim Strommen » Logged

Fremont, CA
1984 Gillig Phantom 40/102
DD 6V92TA (MUI, 275HP) - Allison HT740
Conversion Progress: 10% (9-years invested, 30 to go Smiley)
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« Reply #14 on: July 25, 2008, 05:33:14 AM »

I use synthetic in almost everything I own, but I haven't used it in any transmissions on which I can't drain the converter- I have been repeatedly warned not to mix synthetic and regular transmission fluid. Does the Allison converter have a drain? Did you mix the fluids when you changed?
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