Changing shift points on a 740 Allison transmission

redbus · October 20, 2009, 06:06:46 PM

I have a recently rebuilt Allison 740 transmission that shifts too soon. I shift the transmission manually to keep the RPMs up on my 6V92TA. I would really like to change the shift point. Is this hard to do? Does anyone know who to take this to? I am presently in SE Kansas and will leave just before Christmas when I will head toward the Coastal Bend of Texas.

Dallas, do you do this kind of work and if so, where will you be in this timeframe?

TomC · October 20, 2009, 07:19:04 PM

Most likely, the throttle position modulator is not adjusted right. What you need to do is to make sure that when someone up front steps on the accelerator to the floor, that the throttle position modulator on the transmission is also open up wide. It sounds like it is not opening up enough, hence the transmission thinks it is at partial throttle, so you have premature shifting. Good Luck, TomC

redbus · October 21, 2009, 07:42:49 AM

Thanks Tom.
Is the Modulator located on the outside of the transmission or inside.Can an OLD farm boy do this or does it need to go to a shop.

TomC · October 21, 2009, 08:07:03 AM

You either have a cable or air operated throttle. With a cable, you just follow the second cable from the governor down to the transmission, and that's the modulator. With air operated, there will be a T in at the governor cylinder, once again follow that down to the transmission. It is relatively easy to adjust the cable type, I'm pleading ignorance to the air operated one. Good Luck, TomC

redbus · October 21, 2009, 08:35:08 AM

Thanks Tom
It is cable operated so when we get out of Kansas I may try to adjust it as it will be too cold to work on it here at Christmas time.

JackConrad · October 21, 2009, 08:54:22 AM

Quote from: TomC on October 21, 2009, 08:07:03 AM
I'm pleading ignorance to the air operated one. Good Luck, TomC

Only adjustment I know on an air operated modulator is per Williams air throttle installation instructions. They state that air tubing from the T fitting to the throttle actuator on the governor and the air tubing on the transmission modulator must be exactly the same length. Jack

buswarrior · October 21, 2009, 08:57:39 AM

By shifting too soon, what do you mean? What did it do before?

Most all old hands manually shifted the HT740 to get smoother operation out of it.

Start off in 2nd, the 740 does the 1-2 and 2-1 thing ok by itself.

Eliminates the part throttle upshift/downshift banging.

There are many times having the flexibility to let it upshift earlier, or hold it to higher RPM comes in handy for what driving environment you find yourself in.

happy coaching!
buswarrior

redbus · October 21, 2009, 09:09:54 AM

It used to shift into high gear between 45 & 50 mph at full throttle now it shifts at 40 mph.

buswarrior · October 21, 2009, 09:19:18 AM

Will it hold the gear and pull once it shifts?

If so, I'd like to know how yours is set up now, so I can try it with mine!

Always bugged me about the 740 not letting the engine pull on a partial throttle early shift.

Lots quieter for getting out of Dodge if you can get it into a higher gear early, like the folks with a manual transmission can CHOOSE to do.

2 stroke DD doesn't make friends at high revolutions at 0'dark thirty.

Thanks!

happy coaching!
buswarrior

redbus · October 21, 2009, 09:44:19 AM

Yeah but when you are trying to merge into traffic and it shifts at 40 it takes a long time to get up to speed especially on a hill

buswarrior · October 21, 2009, 10:14:29 AM

I am one of those aforementioned "old hands"

I shift it myself, always have, both commercially and with my conversion, so I won't mind, it only gets to upshift early if I choose to let it!

It's about having choice and having control over which gear it is in, something I'd like to have in my own coach.

On the 2-3 and 3-4, mine has a few mph between the 740's desire to shift and governed speed under full throttle. Under partial throttle, the dumb thing upshifts way earlier than it can pull, and will downshift again if you squeeze the throttle a little harder. These shifts are harsh on the passengers.

Also, full throttle acceleration is not conducive to good fuel economy, So partial throttle launching and the 740's regular set-up requires driver involvement for a smooth ride, and some compromise on high RPM running in town to avoid unwanted automatic downshifts.

When climbing the big hills, the 740 downshifts way below governed speed for the next gear, stealing some power out of the top of the next gear, and downshifts quite harshly.

Try: Lift throttle, pull the shifter back 2-3 mph above governed speed, squeeze throttle open again so that the shift happens in the lift, and the power is coming back on as the gear engages. Takes a little practice to perfect the timing for it to be smooth as silk, unnoticeable except for the change in engine note to the passengers.

In the old school, the only thing automatic about a 740 was the absence of a clutch pedal, they never were smart enough to work smoothly for paying and TIPPING customers...

And the B500 is dumb in the other direction, it upshifts and then holds the high gears, so it won't accelerate in town if left in the "drive" position.

Oh well...

happy coaching!
buswarrior

John316 · October 21, 2009, 11:48:15 AM

Quote from: buswarrior on October 21, 2009, 10:14:29 AM
And the B500 is dumb in the other direction, it upshifts and then holds the high gears, so it won't accelerate in town if left in the "drive" position.

Oh well...

happy coaching!
buswarrior

BW, could you explain a little more? Our B500 has trouble dropping into the next lower gear, all of the time, if we are going at that "sweet" speed. Is that what you are talking about? And what did you mean about not accelerating in town?

Thanks.

God bless,

John

buswarrior · October 21, 2009, 12:36:24 PM

The B500 will upshift and hold the higher gear when cruising around town.

The HT740 will upshift, but then crash back down if you try to squeeze the throttle.

Neither is desirable for either a smooth ride or driving performance.

Trouble with the B500, the engine revs are way down, and it can't pull very well from down there.

Same as the old days, shift it yourself!

To get good driveability out of a B500, I start it in 3rd, let it take care of 1-2-3 and 3-2-1 and then I choose when to allow it to go into 4,5 and 6 using the keypad.

If joining the freeway, I'll just tap "drive" and let it do the job, under other more interesting conditions, I'll choose the shift points myself. Same with the 740, just advance the shifter to 4th if the business at hand is within its capacity.

The top of 3rd gear in a B500 produces a similar road speed to the top of 2nd in a 740.

You also get much better jake/retarder performance if you do some downshift in a complimentary fashion to your deceleration desires.

Some B500 are programmed to downshift under those conditions, again less than convenient.

Choosing 5th just at the bottom of a big climb will get the engine into power territory without the degrade in speed that leaving it to shift itself produces. Running with another coach that is left in drive is instructive. Even with the big engines, a proper mountain pass on the interstate will seperate the two coaches, if one uses the transmission for full effect.

For a loaded coach, a 6% grade often can be climbed in 5th or 4th at the same road speed, somewhere around 50 mph, with 4th helping the cooling. The transmission will try to climb in 5th, a problem if the cooling system is starting to show its age.

The B500 doesn't need as close attention, and does more of its shifting in a pleasing way to provide a smooth ride for the passengers, and the desired performance for the driver, but the world is still too variable for the computer to know better than the driver what gear we need right now.

happy coaching!
buswarrior

mikelutestanski · October 21, 2009, 04:13:29 PM

Hello: I find that with the L10 B400R with retarder I have to use the keypad occasionally to get the engine where I want it and to keep the Rpms up where they need to be. The L10 will run at 1500 but it likes it better at 1700 and the mileage is not much different. The reason is this is a mechanical engine. I suppose that a computer engine would work at a lower rpm but I dont have one.
Since I upped the HP to 300 on the engine I need to downshift more because the engine will not shift down where it used to. now it will try to lug more so I have to manually donwshift to get more power that a lower shift will bring especially on a hill.
Use the keypad , the up and down arrows will select you up or down.
I dont have alot of experience with this engine only 10 or 12 K miles so every time is a new adventure.. and we are learning..
Regards and happy bussin. mike

redbus · October 21, 2009, 04:27:52 PM

All good advice. Maybe I will just keep shifting manually. I just want to do what is best for the transmission and engine.