Allison Transmissions...

[Iver]

My transmission has a tag which reads  HT 750 CRD.

On the Allison Website: http://www.allisontransmission.com/myallisontransmission/mytransmissioninformation/
When I input my serial number it says I have a HT 754 CR.

Can anyone describe the differences between these transmissions?
    Thanks,  Iver.

[Nellie Wilson]

One's newer than the other?

Or maybe they're like printer designations... like an HP 500 covers everything in the '500' series.

But don't take me seriously... I haven't a clue, just guessing.

Nellie Wilson

[johns4104s]

When the tranny changes from 1st to 2nd and the down from 2nd to 1st does it change hard? Mine does.

John

[RickB]

Iver,

The 754 is a 5 speed transmission rather than the more common 740 which is a 4 speed. I have the 754 as well but mine is a 2nd gear start which I didn't know for the first 6 months I owned it. I was told by an Allison tech that the the 2nd gear start 754 was trying to remedy a major problem greyhound was having with lead footed drivers hammering the throttle before people were seated and they were falling and getting injured so they put a gear between 1rst and 2nd that made the buses ease away from stops. I have heard that some 754's are 1rst gear start transmissions. Here was the kicker with mine, If I was on soggy ground after a rain or pulling away from a stoplight on an uphill incline (like every stoplight in Atlanta!!) my transmission would shimmy and slip so before I had a major rebuild I went under the bus expecting a HT 740 and lo and behold it is a 754. But my shifter only had 1-4 gears to choose from, then I noticed an "extra" unmarked notch on the selector and Voila! I had a another gear which is quite peppy I might add. Maybe, and I stress maybe here. the difference between a 754 and a 750 is yours is a 1rst gear start. On your transmission gearshift selector how many gear are numbered on it? In all honesty unless I can find a 1rst gear start 754 I may switch to a 740 when I finish my repower this spring. There are far more of those on the road and they are cheaper to rebuild.
So, short answer the main difference between your transmission and most of the other folks here is:
You have another gear between !rst and 2nd.

Hope it helps.

Rick

[JackConrad]

    7 is the Allison transmission family (heavy duty), 5 is the number of gear rations (speeds), the last number is the specific model. I think the 750 is mechanical and the 754 is the electronic version. I do not remember what the CR vs CRD denotes, although I know there is a "close ration" and a "deep ratio". The deep ratio was frequently used in garbage trucks and first gear was a "granny gear".  
   A 644 is a medium duty 4 speed, a 740 is a heavy duty 4 speed.  Jack

[luvrbus]

The difference in the 2 is the torque converter stall speed and lockup and the deep low gear , Tomc beat to the other difference in the 2



good luck

[TomC]

HT754CR is the Allison 700 series 5 spd Close Ratio.  It comes standard with first gear start.  Optional is second gear start with manual selection of first gear.  On the close ratio 5 spd, the first two gears and 5th are the same as the 4 spd.  The difference is that Allison put two gears in where 3rd gear was on the 4spd to give it better hill climbing capability.  While this works well on big rigs, is not really necessary on a light weight bus, but it is very nice to have.
The other 5spd was the HT750DR.  That was a 4spd with a low granny 1st gear-mainly for trash trucks that needed the extra oomph to get out of mushy land fills.  1st gear was manually selected and was NOT supposed to be shifted to 2nd while moving. Allison has a 7spd version of the 4000 series called the 4700.  It is a close ratio 6spd with a 7th gear low granny added to the back of the transmission.  Unlike the HT750DR, the 4700 can be shifted while moving.  Good Luck, TomC

[RJ]

Rick -

Back in my charter bus days, I put over 100,000 miles on MC-9s with the same powertrain you've got, and absolutely loved that combination - except for the couple of fleet units that had the 8V71T / HT-754s in them - those were even better!

Anyway, based on my experience, I recommend that you NOT change out the 754 for a 740.

Why?

99% of the time a second gear start is fine, especially since it starts in 2nd unlocked, converter locks up, then 3, 4 & 5.  As you've discovered, you can manually select first for those times that a little extra "umph" is needed getting out of the hole.  Few & far between, right?

The biggest advantage to that gearbox, IMHO, is the fact you've got the extra gear to play with for those minor grades where it begins to bog in 5th, but pulls nicely in 4th and you're still almost at the speed limit.

If you're shifting it manually in those situations, that is!

BTW, the extra 100 hp that an 8V71T puts out compared to the stock motor you've got. . . well it really, really makes the bus a pleasure to pilot.  Extremely dependable, too.  Of course, you could swap out the 71 and put an 8V92TA back there and have an additional 200 hp to play with - the gearbox can take it, but the cooling system might not! 

Guess what I'm saying is if you're going to rebuild anything, I'd put in a turbo motor and leave the gearbox alone.  And if the gearbox actually needs rebuilding, your choice as to whether or not to have it converted to a first gear start, but don't swap it out for a 740.  Obviously the 740 is a good transmission, but why throw out an extra gear???

FWIW & HTH. . .

[luvrbus]

Rick, I think the 740 is a better transmission they can be made to shift smooth and the 754 shifts hard no matter you do and with your 475 hp 8v92 there will no need for a lower gear plus a 754 runs a little hotter and you don't need that. 



good luck

[Brian Diehl]

Rick, I think the 740 is a better transmission they can be made to shift smooth and the 754 shifts hard no matter you do and with your 475 hp 8v92 there will no need for a lower gear plus a 754 runs a little hotter and you don't need that. 



good luck

In my 96A3 I had a 6v92 and a 754.  My 754 had 1st gear start.  I don't know about the 754s you've worked with, but my 754 shifted very smooth.  I had the trimmers adjusted to match my 6v92.  Again, my 754 downshifted and upshifted smoothly.  The only time it would get hard is when I put it into lockup in 1st.  Then you really noticed the shift from 1st to 2nd.   I also had the little plastic "balls" (about b-b size) put into the valve body to get it to shift exactly in the way I wanted it to in the power band.  I really liked that transmission!  Now I have an Eaton Autoshift behind my ISM and would not trade the gears in it for the 754 since all the gears are great for hill climbing out west!

[luvrbus]

Brain, that is great but behind a 8v92 they will  rattle your teeth and you can do all the work on the trimmers valves and springs and flow restricters you choose and they still shift hard. 
I can take the lockup jerk out of a 740 with the lockup control valves but with the same control valves in a 754 I have never been able to do that


good luck

[Brian Diehl]

Okay, I understand.  Must be the greater torque of the 8v92 compared to the 6v92.  Thanks for the clarification!  Probably a good thing it wouldn't fit behind the ISM then!

[PP]

If mine doesn't shift hard does it mean there's something wrong with it? The only time it shifts hard is if I manually hold it in first when leaving a campground and then upshift when I hit the highway. It prefers to do its own shifting I guess. When I first got this bus I was impressed with how smooth and 'car-like' the transmission was compared to several other buses that I'd driven. It feels solid and dependable, but definitely not harsh, Will

[Iver]

I think my question has been answered.  Thanks everyone for the input.

I like the transmission especially climbing and descending hills. 3rd, 4th, and 5th are very smooth up and down.
The only issue I have is 1st and 2nd.  When starting out level, it shifts too quickly out of 2nd and into 3rd.
If I apply a fair bit of throttle it goes from 1st, 2nd, 3rd and then jumps back to 2nd and then on to 3rd and it is fine after that.
I had Allison replace the modulator and cable, but they said there was nothing more they could do.  Or they didn't want to?

A mechanic friend told me that the shift points in the transmission can be adjusted. It required taking out part of the trans. and taking it to Allison to set the shift points and then putting it back in.

I can control the shifting manually,which I do most of the time anyway, so not a big problem.
  Thanks again,  Iver.

[Lin]

Interesting information on Allison's.  Just for academic classification, what is the difference between a 754CR and a 755CR?