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Author Topic: tranny question..  (Read 2967 times)
travelingfools
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« on: September 28, 2008, 07:57:00 PM »

Ok..so when Im going down the road, tranny shifts smooth and quiet up and down till it drops into first.... some times.... Usually you dont even feel it slip into first, but every once in a while it bangs in so hard I can feel the drive train shudder. It was suggested in the chat room to change the tranny filter which I willl do this week. Any other thoughts ?? MC 9, 6v92, auto tranny 1,2,3,D which Im guessing would make it a 4 speed, DDEC pushbutton... Any thoughts ?
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John P, Lewiston NY   1987 MC 9 ...ex NJT
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« Reply #1 on: September 28, 2008, 08:09:32 PM »

I'm no expert! But I do not get alarmed at an auto trans that shifts hard. At least it is not slipping. I do worry about your statement about drive train shuddering. I have no way to know what you mean by this statement. My MC-9 6v92 with auto as yours is will shift hard from first to second I can't say I have seen it shift hard down to first.

John
« Last Edit: September 28, 2008, 08:14:15 PM by Jriddle » Logged

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John Riddle
Wells NV
1984 MC9
travelingfools
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« Reply #2 on: September 28, 2008, 08:13:29 PM »

John, when it shifts down into first, every onse in a while, it shakes the whole bus and you can almost picture the drive shaft shuttering...
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John P, Lewiston NY   1987 MC 9 ...ex NJT
makemineatwostroke
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« Reply #3 on: September 28, 2008, 08:25:35 PM »

TF;is your transmission a 741 0r 748 Transynd fluid will make a huge difference in the up and down shifting on the transmission even a C4 15/40 will help please post your model number for us it is on the drivers side about the middle and above the oil pan on the transmission , the first thing i would check would be the idle speed of your engine  have a great evening
« Last Edit: September 28, 2008, 08:42:36 PM by makemineatwostroke » Logged
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« Reply #4 on: September 28, 2008, 08:31:06 PM »

The filter change will not cost to much. But if it is shaking the whole bus that may be something much worse. I have to admit when my bus shifts up to second  it seems very harsh! I guess I can imagine drive shaft shuddering. I'm going to drive it and see what happens.

John
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John Riddle
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« Reply #5 on: September 28, 2008, 10:26:30 PM »

keep me posted....i have the same issue on my mc9 but was also told that it is normal.  depending on how i drive it....if i drive it like a car, the shift from first into 2nd is usually rough.  i've now managed to realize that if i back off the throttle as its about to shift, the shift goes really smooth.

though i do have this problem when climbing hills.  it literally shifts from one gear to the next and i just get thrown back into my seat.  not quite sure how normal/not good for the trans that whole process is.  again, i usually just have the accelerator floored when climbing hills, if i manage to let off as its about to shift, it doesnt really do it/do it as hard.
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« Reply #6 on: September 29, 2008, 03:39:47 AM »

Guy's,

It's an ALLISON.... They all shift hard when no throttle is applied.   Shocked  Ususlly staying in the throttle keeps them smooth.

Have a great day
Nick-
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« Reply #7 on: September 29, 2008, 04:51:06 AM »

Our coach with a 740 did this same thing until it was diagnosed with a faulty skinner valve. I think it was a skinner valve. It was a valve that was activated electronically and by air! Once this was replaced, it shifts very smooth! Now let me say, the only time it shifts hard is when I'm rolling to a stop and it downshifts to 1st. That shift will jar you but any other time, its really smooth and quick!

Ace
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johns4104s
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« Reply #8 on: September 29, 2008, 07:22:26 AM »

I reciently got a MCI 9/Alison 750. It also shifted very hard from 1st to 2nd and then back down. After asking lots of questions on several boards. I went out to our local Transit dept. I was told by them that if the fluid was high or low it would cause this problem. My transmission from new only has 6000 miles on it. I took a lot of time making sure the temperature was up and the ground was level and after several checks found out I was 1 gallon low. Since I got the level on the right mark ( its very difficult to read because of the multigrade Rottella and the position of the dip stick)then The heavy changing has very much improved.
I also was told that if I change the fluid to a synthetic transmission fluid called Hydrotech ( they are on the net) this would also improve the low gear changes. I have the fluid on order, cost $300.00/10 gallons. so I will let you know if this fluid helps. I am told by Hydrotech that there fluid exceeds Alisons specs and they have fleets running 500,000 to 750,000miles between changes. There claim is that you ship them a sample of the fluid every 50,000 miles and they tell you what,s going on and when it is time to change it.

John
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makemineatwostroke
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« Reply #9 on: September 29, 2008, 07:45:47 AM »

Ace; those are called"trimmer" valves adjusting the the trimmer spring will give you a nice smooth shift on a 740 but harsh down shifts are usually caused by idle rpm the engine needs to idle at 650 rpm   have a great day
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travelingfools
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« Reply #10 on: September 29, 2008, 06:20:44 PM »

Thanks for all the info..Im off from work on thursday and will change the filter and check the level. Crossing my fingers that will help it..As I said, it only does it once in a while just befor I come to a stop..Ill keep you'all posted..
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John P, Lewiston NY   1987 MC 9 ...ex NJT
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« Reply #11 on: September 29, 2008, 06:44:23 PM »

dont know if the cause is related but I drove mine for the first time in two years and noticed that at the top of first it seems like it slips before shifting. if I lift on the throttle just as or before the point it seems to slip it will go right to 2nd. It does seem to shift back to 1st some what harshly but I dont have a reference for it. the down shift seems more normal than the upshift. It has new filter and fluid. is this normal or related to some issue?
thanks
steve
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« Reply #12 on: September 29, 2008, 07:17:32 PM »

My HTB748R was doing all the same things you guys have mentioned.

I put a quart of a product made by Lucas just for automatics in it and it has worked to perfection since.

Shop mechanics have mentioned a couple things to me.

(1) hard shifts may mean broken springs (should be all the time) and (2) the transmission has a pickup tube on it that is O ringed at the top of the pickup. This O ring must be changed by removing the pan and pickup each time the fluid is changed or it will dry out and suck air and air bubbles in your fluid will make your transmission do all kinds of things. Cheap fix though.

If I try to preselect with this transmission it fights back and makes me look bad. So I just let it do its thing.
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« Reply #13 on: September 29, 2008, 07:18:34 PM »

As you are planning, do the simple and cheap first, change the fluid and filter, and be sure the level is correct, fine tune it after running.

If the fluid is a distinct flavour darker than the new stuff, you want to be thinking hard about the dilution of what's left in there, and whether you will be wanting to do it again, as well as an indication of problems. Same as a car, dark tranny juice is NOT good...

I'm no fan of throwing good synthetic fluid (expensive experiment) into a tranny with problems.

Synthetic fluids don't come from the witchdoctor... how does one know whether a fluid change of dino juice would have done the same?...call me a cynic...

An occasional downshift shudder into 1st has me wondering if the lock up in the torque converter is sticking intermittantly... a fluid and filter change might help.

Take that coach out and put 1000 miles on it and see if it smooths out? It juist might be P.O.'d at sitting around...

Does this happen somewhere below 20 mph as you are slowing? The torque converter popularly unlocks as you pass through 20 mph when slowing on a mechanical 740.

happy coaching!
buswarrior
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« Reply #14 on: September 29, 2008, 07:32:29 PM »

I can say for certain that the fluid in it was not pretty. I changed it and the filters using dino as I plan on changing it at least a couple of times to try and dilute out the old. I wish I could put some miles on her but right now it has no upper sides, windows or floors. I only drove it about 12 miles as I had lost the use of where I was working on it and had to relocate. I may not worry about it till I can put some miles on it and change the fluid again. as for the speed that is mine is worst on downshift? hard to say, the speedo was in a box in the bay so it was hard to read.
steve
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luvrbus
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« Reply #15 on: September 29, 2008, 07:42:23 PM »

I use Cat TDTO 10w (45.00 for 5 gals) fluid in my 740 and it shifts smooth as any B500 I was told to stay away from the new Dexron has to many additives for older transmissions this was from a large Allision dealer, C4-15/40 is a choice also I know a lot of people that use it and have smoother shifts     good luck
« Last Edit: September 29, 2008, 07:47:36 PM by luvrbus » Logged
johns4104s
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« Reply #16 on: September 30, 2008, 05:36:55 AM »

The shudder happens as you go up or down in 1st 2nd at 13/15 mph. But mine is a 750. I believe the 1st gear is lower than the 740?

John
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travelingfools
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« Reply #17 on: September 30, 2008, 05:45:12 AM »

The shudder happens as you go up or down in 1st 2nd at 13/15 mph. But mine is a 750. I believe the 1st gear is lower than the 740?

John
It happens going down
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John P, Lewiston NY   1987 MC 9 ...ex NJT
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