Downshift thump in Allison 647

[Lin]

I am still enjoying the Allison we put in, but wanted to ask about one thing.  The transmission shifts very smoothly going up.  Going down though, it gives a good thump as it shifts from 2nd to first just before coming to a stop.  Is that normal?

[travelingfools]

mine was doing the same thing, but it seems to happen a lot less with the more miles I put on her...

[Busted Knuckle]

Lin,
I already know I'm going to get flamed for this, but here goes anyway!
Over the yrs I've been around many allisons that did the same exact thing. Some people will tell you it's an easy thing to adjust out, but I've not yet ever been able to get any solid info on doing it, or anyone to show me how to do.

But what I did learn and has helped tremendously is to switch from AFT to 15/40 Rotella in the trans! Works like a charm takes away the hard shifts, or on the ones that act like they are slipping makes them shift firmer! I've been flamed for suggesting this before and will be again, I'm sure! But I seriously found it to work great in all the old style allisons! (But don't do it to the B500's unless you want to be replacing it @ 100,000 miles! { and that last tip is from the **Late, Great - Sammy! may he rest in peace, and watch over us with a smile!)

Only issue with the 15/40 is you really have to let it warm up when it's really cold before using it, but other than that it works wonders!

Now when I was first told about using 15/40 in one I thought you gotta be kid'n me. So I asked and trusted hand I knew down in Memphis and he said "sure we do it all the time! But don't just take my word for it call so & so that builds them over @ blah, blah place and ask him!"
So I got the # and called him and asked and he said "well sure, I haven't done it in any buses but we use it in all the trans we built for the heavy duty/extreme users like fire, & garbage trucks, and construction equipment!"
I asked if he ever saw how a hired driver drives a bus, and he said "Huh?"
I told him "hired bus drivers always have their foot on the floor! It's either floored on the gas or floored on the brake and right back to the gas! Ain't no in between, now how much more extreme can ya get?"
He laughed a little bit and said "well I never looked at it like that, but I'd say you got a real good point and a reason to run 15/40 in yer buses too!"
I started doing then and still do! (other than having to warm it up some before putting it to hard use in cold weather) I can't say I've seen any problems or issues to date! FWIW
  BK 

[Lin]

BK,

It doesn't surprise me.  When we installed the transmission, there were several that told me to use straight 30 wt. saying it smoothed out the shifts.  I chose Valvoline synthetic that supposedly would do the same thing.  Anyway, I am happy with all the other shifts; it's just this 2nd to 1st that isn't smooth.  I don't really mind it if it is not destructive.  If it is within the normal range of Allison behavior, I won't try to mess with it until it is due for an oil change.

[RJ]

Lin -

Does the transmission clunk down into first when you've got the shifter in what would normally be called "drive?"  Or are you shifting it manually?

Remember that there's quite a gap in the gear ratios between first and second, much more so than the 2nd - 3rd and 3rd - 4th splits.  This, too, could be contributing to your clunk.

You might try keeping the gearbox in 2nd for some around-town experimentation, and see if that helps control and/or eliminate the drop to first as you come to a stop.  If it does, then you've found a work-around you can use to help keep from spilling the red wine.

FWIW & HTH. . .

[oldmansax]

My Wanderlodge is doing the same thing. I'm not sure what tranny it has & it's too cotton-pickin' cold to go look right now but it is a 1982 with a 6V92T & a 4 speed Allison.

It up-shifts fine. It downshifts fine 4 to 3, 3 to 2. It stays in 2nd it until it hits 18MPH. It then goes into Neutral until 10MPH. Then it shifts into 1st, giving the "bump" that's being talked about.

Is this what's happening to you guys? Will BK's fix fix it?

If so, that will be the easiest thing I've had to fix on the Bluebird so far.

TOM 

[Joe Camper]

When we first got ours I immediately perceived what I thought was an unusually hard first shift up and last downshift. I also noticed 5 distinct shifts (counting the hard one) making 6 forward speeds and I am sure our bus has a 5 Speed

After some investigation I was told what I perceived as a first shift ( and last downshift) is actually the converter going in and out of lockup.

Is this correct can someone confirm or correct this info?

Anyway with that info I put it behind me. Until I had the opportunity of driving more than a few other identically powered buses and realizing that comparatively ours still does it no where near as smoothly as others.

So this thread caught my eye. I am in Chi and don't think I am a candidate for the 10-40 although I would love to try , we store outside. If you ever find that "simple adjustment" Busted Knuckle do not hesitate to share that.

Just for the record I would find it hard to believe any shifting harder than ours. I constantly monitor things and so far accept the annoyance it does not seem to be doing any damage although that shift is for sure hard enough to make someone think otherwise. We have about 30000 on it since we bought it.

As is warms up it gets smoother but not ever entirely going away.

[TomC]

The cheapest thing to do, if you know you're stopping-like at a stop light or stop sign, just shift into neutral until stopped, then shift back into drive.  Won't cost you a thing.  Good Luck, TomC

[Jriddle]

MY OPIONION

Don't worry about hard shifting. Mine does it, his does it, yours does it sounds normal to me. You can control how hard it hits if you get your foot out of it when it is ready to shift. On the down shift you may go to neutral but sound like that would be unsafe to me. Not only do you loose the transmition for braking power you are turning your attention away from the road when you need to be paying attention the most.


John

[luvrbus]

Lin, the backoff spring is to tight but check the linkage 1st pay no attention to the shifter check it on the transmission making sure it clicks when shifting from the tower  


good luck

[belfert]

May not be relevant in any way, but my B500 really slams hard when downshifting into 1st, but only when cold.  As soon as the tranny warms up it shifts real nice.  I put Transynd in the bus and it didn't make any difference.  (I didn't install Transynd for this.)

[Lin]

Clifford,

I'm pretty sure the linkage is right.  I went through checking that a lot when I changed the shifter.  Bobby at Az Transmission suggested that the modulator cable is too tight.  Is that the same thing as the back off spring.  He said to disconnect the modulator cable to test what happens.  If the thump goes away, it is a modulator cable adjustment.  If is does not go away, it is something else--possibly a sticking governor.  What do you think?

[Rick 74 MC-8]

I'm with BK I changed to 15/40 right after I got the bus. It was recommended by an oldtimer from Allison If I remember I think cf4 is Allisons recommenced rating. He also told me to change it after sitting over night because the tork converter will drain back into the pan so you can drain all the fluid. About the hard downshifts mine does NOT do this but if just left in drive it shifts way to soon as I have heard many others also complain of. I'm thinking if yours up shifts right maybe the detent cable if set to ageseve. Not an expert by any means just a thought


                                                                                            Rick 74 MC-8

[buswarrior]

I wonder if the torque converter is unlocking properly at the proper speed?

Yes, Joe, the torque converter lock-up feels like a gear shift.

As the story goes, there are different strengths of springs for use in the valve body.

Different springs change the shift characteristics.

Whether this is related, I have no idea...

happy coaching!
buswarrior

[Joe Camper]

Given some more thought  I wonder just how much of that hard "shift" was changed with the introduction of the D-deck?  Ours is mechanical and I bet that was a major improvment on the shifts when they computerized things.

Of all the folks experiancing hard first shift how many have d-deck? Any?