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Old 10-26-2021, 05:46 PM   #1
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Mt643 mode??

Hi, i used servicemaxx software to disable the speed limiter and while i was looking at the parameters i noticed one for the transmission, pics attached.
Now, currently i hit 60mph at 2500 rpm , which is yoo high.

Does anyone know if this can be fixed by adjusting the tranny mode to either power or confort?
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Old 10-26-2021, 09:40 PM   #2
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Mt643

I donít think there is anything you can do with software. The 643 is a mechanical transmission.

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Old 10-26-2021, 09:48 PM   #3
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I have a 2000 international 3800 with a dt466e and an allison mt643

I cant figure qhy the rpm are so high at 60mph. I have 4.10 differential.

I was told it should be at 2200 rpm when going 60mph

It seems to shift all gears. I feel a tug when going from 1st to 2nd and one from 2nd to 3rd and finallt one more when 3 to 4th. The only odd thing is that i feel a bump when slowing down fast from 45 mph to a stop sign. Like a little push at the end before it stops completely. Please any help apreciated.
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Old 10-26-2021, 09:55 PM   #4
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Tires

What size tires? Do you know for certain tachometer is correct ? Speedometer correct? My speedometer is off 2mph

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Old 10-26-2021, 10:05 PM   #5
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11r225 tires. The tachometer and spedometer are on point. I double checked with a nexiq and a laptop using servicemaxx. Any ideas
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Old 10-26-2021, 10:22 PM   #6
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Checking on numbers

Give me a few minutes

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Old 10-26-2021, 10:28 PM   #7
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You should be seeing 70+. Maybe your 643's TC isn't locking up. slippage could cause the rpm's to be higher. Hows the transmission temp? Fluid color?
Or- your bus could be like mine. Mine had its gears changed out by the school that owned it before me. They decided to go from 4.10 to 3.42. Yours could have lower gears than came in it. Just a possibility.
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Old 10-26-2021, 10:33 PM   #8
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Math says

About 500 revs per mile for 11r22.5…… at 60 mph that is a mile a minute…..tire turns 500 revs. Times diff ratio gives 2,050 driveshaft revs per minute. Fourth gear converter locked. Or around 2,700rpm in third gear…….

Something doesn’t add up.

A 5.2 rear gear would give you 2600 rpm, in fourth with locked converter. I have to wonder if you are in third gear……

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Old 10-26-2021, 11:42 PM   #9
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Fluid is new, twmp stayed around 190 to 200 when at full speed
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Old 10-27-2021, 12:03 PM   #10
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Quote:
Originally Posted by magnakansas View Post
I don’t think there is anything you can do with software. The 643 is a mechanical transmission.

William
Quote:
Originally Posted by magnakansas View Post
About 500 revs per mile for 11r22.5…… at 60 mph that is a mile a minute…..tire turns 500 revs. Times diff ratio gives 2,050 driveshaft revs per minute. Fourth gear converter locked. Or around 2,700rpm in third gear…….

Something doesn’t add up.

A 5.2 rear gear would give you 2600 rpm, in fourth with locked converter. I have to wonder if you are in third gear……

William
I've quoted these both, as the op needs to read them again.

Nothing in the engine ecm is preventing you from going faster if you're already at 2600 rpm @ 60 mph. Don't mess with those parameters, it's not going to change anything.

Here is the math explained differently.

11r22.5 tires have an assumed overall diameter of 41.4 inches.

A tire diameter of 41.4 inches, coupled with a 4.10 rear axle ratio and an mt643 in 4th gear w/ converter locked will have you doing 1997 rpm at 60 mph. If you're not doing that, but doing 2600 rpm at 60 mph, one of the variables used above is wrong.

To get that 2600 rpm @ 60 mph, You'd have to have:

31.9" diameter tires (Not likely at all)

5.3x gear ratio (possible, but also not likely)

transmission that isn't in a 1:1 gear ratio(possible as well)

If you'd crunch the above numbers again, but with the transmission in 3rd gear(1.39:1), you'd end up with 2600 rpm @ 56 MPH. It's closer to being correct, but it's still not 2600 rpm @ 60 mph.

So are you sure you're at 2600 rpm and 60 mph?

There's a possibility that you're in 4th gear, with an unlocked torque converter that's slipping. But if the converter is unlocked and slipping I think it'd be obvious. Does the converter feel like it's slipping?
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Old 10-27-2021, 12:18 PM   #11
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How can i tell if the converter is slipping?
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Old 10-27-2021, 12:48 PM   #12
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To check for converter slip, be at highway speed and go from no throttle to full throttle. If your rpm swings a large amount with very little change in mph, then the converter or trans is what absorbed the speed difference, and is slipping.

However, if I had to guess, I'd say that you aren't shifting to 4th gear, and the mph/rpm your reading isn't correct. What you're likely feeling and assuming as a 3-4 shift, is more likely the converter clutch locking up. On an mt643 the converter clutch lock up occurs in 3rd, and stays locked through the 3-4 shift.
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Old 10-27-2021, 12:57 PM   #13
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I'd be 100% certain though that you have a 4.10 rear end, and not a 5.17 or something before I'd tear into the transmission.

A lot of new owners have made the mistake of assuming they have highway gearing, only to inspect it and come to the realization that they don't.
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Old 10-27-2021, 06:43 PM   #14
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Having messed around with a few medium/heavy duty rearends lately I noticed that if you look real close you can see the number of teeth on the pinion and ring gear stamped on the front of the pinion. You will have to drop your driveshaft and wire brush well to see it. Who can second that?
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Old 10-27-2021, 06:49 PM   #15
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Having messed around with a few medium/heavy duty rearends lately I noticed that if you look real close you can see the number of teeth on the pinion and ring gear stamped on the front of the pinion. You will have to drop your driveshaft and wire brush well to see it. Who can second that?
most the buses I've crawled under have a tag or marking indicating the ratio.
In my case my ratio didn't end up matching the number on the tag.
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Old 10-27-2021, 10:49 PM   #16
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most the buses I've crawled under have a tag or marking indicating the ratio.
In my case my ratio didn't end up matching the number on the tag.
Yeah that's a lazy mechanic that didn't change out or at least remvoe the tag when he did a ratio change.
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Old 10-28-2021, 09:46 AM   #17
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Mine has the IH line set ticket taped to the radiator support under the hood. If you can read it, it has every IH option installed on the bus. It's been invaluable, as I've used it to order king pins and confirm suspicions on parts.
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Old 10-28-2021, 12:57 PM   #18
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Yeah that's a lazy mechanic that didn't change out or at least remvoe the tag when he did a ratio change.
I've never really heard of a bus yard doing this, so its probably pretty rare. It was easy to figure out the difference out on the highway.
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Old 10-29-2021, 06:07 PM   #19
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You can always jack up a drive wheel, put the trans in neutral, mark the drive shaft and the wheel then turn the drive wheel. How many times does the drive shaft rotate compared to the wheel?


NOTE: Do this on FLAT solid surface and CHOCK the front tires
With air brakes you'll have to release the brakes by charging the system or by mechanically releasing them.





Determining a Rear Axle Ratio of a Non-Posi Rear

  1. Jack one wheel off the ground, put the transmission in neutral and take the parking break off. ***Be sure to use jack stands under the axle for safety precaution***
  2. Mark the wheel and the drive shaft with chalk or masking tape.
  3. Turn the wheel TWO revolutions and count the number of driveshaft turns.
  4. The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio.
  5. For better accuracy, turn the wheel 20 times and divide by 10 for your rear axle ratio.
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Old 10-29-2021, 06:22 PM   #20
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my 643 i feel a 1-2 shift, a 2-3 shift.. a good solid RPM drop which is the Lockup engagement. and then I feel a good solid 3-4 shift.. ive never driven an MT643 that didnt "feel" like a 5 speed.. seems like you are missing a gear.. an MT643 stuck in 3rd gear with lockup engaged would give vehicle with 4.10 rears and 41" tires a 56 MPH top speed at 2600 RPM.. if its really 2700 RPM you are at 58.



have you checked vehicle speed with a GPS or just the dash gauge?



if you are maxxed out then let off of the pedal does your RPM drop suddenly or does it drop slowly with the speed?


theres no settings for the tran in servicemaxx.. it should be set to Allison AT/MT
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