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Old 03-17-2019, 01:07 AM   #1
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MT643 4th Gear Lockup

I am converting a 2000 International 3000 flat Nose 33' school bus. It has a T444E engine (essentially a Ford 7.0) and MT643. It shifts up and down well, except it doesn't seem to lock the torque converter in 4th Gear. Here's what I get gear wise as confirmed by RPM change: 1>2, 2>3, 3>lockup, lockup>4. I'm running about 2400 RPM at 60mph. I believe I should be around 1900 - 2000 RPM.

I can't find what the 643 shift pattern should be to confirm it should go out of lockup from 3rd to 4th and then into lockup again. Anyone know for sure how lockup is supposed to act from 3rd to 4th and after shifting into 4th.

I did read about the modulator being a common issue, but why would it lock in 3rd but not fourth?

Thanks for any assistance.

Steve
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Old 03-18-2019, 09:33 AM   #2
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The MT643 in any configuration ever built did not drop lockup from 3-4. Its shift pattern is either 1C-2C-2L-3L-4L, or 1C-2C-3C-3L-4L.
C stands for converter, L for lockup.
When you show 3>lockup, lockup>4, are you saying you see an RPM increase when lockup is dropping after the 3L shift?

What kind of modulator do you have? Air, cable, electric?
Are you checking your shift pattern at full throttle?
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Old 03-18-2019, 09:41 PM   #3
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Thanks for your reply and clarification questions.

First, let me say I'm learning about my bus as I go. I have not been driving it as I am focused on the conversion, and since my RPMs at freeway speeds are higher than I'm comfortable with, i havent had it on the freeway in a few months. So, what i write is from memory.

Testing is NOT at full throttle. What is the difference between say 3/4 throttle and full?

I don't know what type of modulator, as I just read about it a few days ago. I have to research and find out how to locate it and determine its type, Besides the obvious way the modulator invokes it's modulation, is there any major issues/differences in how the various modulators effect shifting?

Per shifts, I guess the bottom line is there are 4. 1>2, 2>3, 3c >3l (assuming it's going into lockup, I just know the Rpms drop), 3l > 4l?

My calculations using 42" wheel diameter, 2000RPM and either a 3.9 or 4.10 reared (i havent been able to determine my rearend ratio) tells me I should be between 60 - 65mph. I'm pushing 2400 RPM at 60mph.
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Old 03-18-2019, 10:51 PM   #4
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Bet I know what you're going to say...

After my last reply I guess I was thinking about it subconsciously and realized that it's probably not going into lockup at all. If that's accurate, hopefully swapping out the modulator will solve the issue. Do you agree?

But, that leads me to the question of what happens when climbing hills (especially mountain passes). I would assume it drops to 3L, then 3C?

Also, if I manually shift to 3rd, does it prevent it from going into 3L?

I'm just trying to figure out how to best drive it in harsher situations.

Thank you!
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Old 03-19-2019, 05:57 AM   #5
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Sometimes owners feel like the transmission is missing lockup due to the feel of very low shift points. That's why I asked about the modulator type. Cable modulators that can be disassembled have a bell crank in them that depending on how it's assembled, makes the modulator either a push or pull modulator (push or pull to full throttle). It's easy to assemble those backwards. Cable modulators that cannot be disassembled are supposed to be stamped on the outside push or pull. Get the wrong one and you could be running around with closed throttle shift points at full engine throttle.


Air and electric modulators behave normally either on or off. Drive at too low a throttle position and you again, are getting closed throttle shift points while the engine is at full throttle.


If you are getting four RPM drops as you drive, you are making four shifts. Sounds to me like the transmission is probably shifting normally and is getting lockup normally. 1-2, 2-3, lockup, and 3-4. Or 1-2, lockup, 2-3, 3-4.


My opinion is you need to verify the axle ratio. School buses were not geared for highway use normally.


Spicer has an online calculator that's easy to use to enter your vitals and figure predicted top speed.


https://spicerparts.com/calculators/...rpm-calculator


Manually downshifting to third will not prevent the lockup clutch from coming on. If you have a 1C-2C-2L-3L-4L calibration, the downshifts will be simply in the reverse order. Same with a 1C-2C-3C-3L-4L


There should be a lockup tap on the transmission you can put a gauge in and tell when lockup is coming on. You can also use a photo tach type setup to measure propshaft speed while in high gear. If propshaft speed equals engine speed in top gear while climbing a hill, lockup is coming on just fine.


A 5.17 axle ratio looks like it's 60MPH at 2400 RPM with a 42" tall tire. That may be what you have. I wouldn't be surprised. It just depends on what the school corporation had in their specs for how fast they wanted their buses to run.
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Old 03-19-2019, 10:44 PM   #6
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I'll work on determining the rear end ratio, as this will help confirm the question. The Spicer calculator gave me a current 5.0 ratio at 60mph, 2400 RPM and 42in tire diameter. From other threads, it seems going from 5.0 to 4.11 should not negatively affect the hill climbing and headwind capabilities too drastically. Hey, that's what hazard lights and turnouts are for. I use to own a 68' VW bus, so I know all about slow poking!

I will also take it out on the highway to get some full throttle flat and up hill shift points.

I'm comfortable at 2000 RPM, so I've calculated a 4.11 will give me 60mph.

It seems easiest to change the rear end gearing versus anything else. I think it's governed at 2600, and the tach goes to 3500, so I have so playing room for short higher speed spurts (65 @ 2150 and 70 @ 2300).

I'll report back my findings.
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Old 03-20-2019, 05:56 AM   #7
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I agree. Changing the gearing would be the path of least pain to get what you want. I realize 5.0 gives 60 MPH on the calculator. I used 5.1 which calculates to near 60 because I've never seen a 5.0 axle. I've seen 5.1 and 5.17, 5.29 and 4.89 so I figued it must be a 5.1 or something very close to that. The speedo may be a bit optimistic. I've seen that a lot.
You probably already know this, but keep in mind the tach going to 3500 has nothing to do with the engine's capability to turn that fast. The tach is a stock gauge and not tailored to the engine. That engine running above governed speed at some point may end up with pistons ingesting valve faces.
If you really want to figure your gradeability and startability and compare the effects of the different rear end ratios, most Allison distributors have someone on staff that can run an iSCAAN and display all the relevant performance calculations for different configurations. I'm sure they will charge for it, but it would be nice to know how your gradeability would be effected as well as the ability to maintain speed on different grades.
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Old 03-23-2019, 11:17 AM   #8
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Mixed News

Upon further inspection, I found a small flap that led to a fully road gunked covered tab. I peeled it back to discover a bright and clean rear end tag.

You estimated a 5.17, I thought 5.00, and it turns out it's a 5.11. That tells me the transmission is working properly!

It also means I need to figure out how to change my sideline to give me freeway speeds at acceptable Rpms and hill climbing power.

I can think of three options, i welcome others and advice about the pros / cons of any of the options.

Option 1
New transmission
Con - expensive
Pros - increased number of gears and tighter gear ranges

Option 2
Change rear end ratio by either swapping entire axel or changing rack and pinion to a 4.10 range.
Cons - Will effect hill climbing/passing, but I might be able to do some minor engine upgrades to increase power
Pros - more affordable?

Option 3
Possible find a split range transfer case (I think this is what they are called)
Cons - more to go wrong, possibly more complicated to drive (I don't know how automated that system is)
Pros - gives me 8 gears

Let the opinions fly!
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Old 03-23-2019, 11:31 AM   #9
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Ah. Outstanding! Different trans will be very expensive. Auxiliary box- If this is a rear engine vehicle you don’t have enough driveline space available to fit one in and do it with proper drivelines. Changing axle ratio is the most appealing option in my free opinion.
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Old 03-23-2019, 05:49 PM   #10
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Changing Axel Ratio

I agree, after reading all that can go into a tranny swap, that changing the differential ratio is the way to go.

I'll have to price the cost of an entire axel swap with just changing the differential gears. Since it seems all is good with the current setup, keeping as much original would introduce less potential new problems. Agreed?

I will see about getting an Allison dealer to help with ratios for overall performance, yet based on online calculations, the 4.11 gives me 60mph at 2000rpm. Whatever i get for off the line/climb with that ill just have to work with. I'll also include adding more umph to the engine.

I read somewhere by increasing the governed engine speed also gives a boost to power??
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Old 03-23-2019, 06:11 PM   #11
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Agreed more original equals less potential for problems.
Usually getting more power involved more fuel not just increased RPM. More RPM can be detrimental to engine life in a diesel. They can only take so much.
More power usually requires more fuel. Keep in mind if you add more fuel you also have to have a corresponding increase in air or it will black smoke. If you add more fuel than it can burn you are just wasting fuel with no added power.
Also if you turn up the engine RPM you have to crank up the transmission shift points or you’ll never see that added RPM in operation.
I say change the axle ratio and research what can be done to the engine to get a higher horsepower rating without going too exotic.
Can’t wait to hear the results of the axle ratio change.
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Old 03-24-2019, 06:31 AM   #12
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Horse Power

Your poinst about "power" are good. I completely agree that increased horse power is best done in moderation. All I really want is to make up (as much as reasonable) for what I'll loose in off the line and climbing ability by going to a higher (5.11 to 4.10) ratio.

Good News is, within the t444e hp offers (190 - 250) mine is 230!

The rear end switch will take a while due to research a good shop that can not only do the physical work (rear end & engine mods), but truly understands tuning for those changes for gas mileage, hp and engine protection. Not to mention paying for it!

Thank you for all your time, quick replies and input.

Steve
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Old 03-24-2019, 06:42 AM   #13
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Your MT643 can handle some more HP, I have a 275HP and the MT643, still not the fastest up the hill but gets me there
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Old 03-25-2019, 08:03 PM   #14
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Good News.

I found a good ol` Allison dealer and they calculated the numbers to see if a 4.10 rear end will work. They seem to feel it will fo fine holding 60 in 4th on the flat (3 percent grade). Hills will downshift, but I'm okay rolling with the 18 wheelers. Guess I should add a cb radio. They are pricing the swap. Wish me luck.
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