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Old 03-20-2016, 05:35 PM   #1
Mini-Skoolie
 
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Transmission Question

Every time I post here I have to admit I cringe because I think people are reading my questions and thinking "What a moron" but if I don't know I have to ask, so here goes.

Reading posts I see most buses from my area would have transmissions not designed for highway speeds.

Generally speaking, what are the costs comparisons to having a transmission re-geared over just buying a transmission with highway gears?
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Old 03-20-2016, 05:39 PM   #2
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Rear end needs to be re-geared as well
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Old 03-20-2016, 05:41 PM   #3
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And there's my glaring ignorance of bus mechanics showing again...

So cost comparison for both please.
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Old 03-20-2016, 06:52 PM   #4
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Almost all school buses built are built with automatic transmissions.

Almost 100% of those transmissions are made by Allison.

Starting in the mid-90's the older AT/MT/HT series mechanically controlled transmissions were phased out in favor of the 1000/2000/3000 series electronically controlled transmissions.

The vast majority of mechanically controlled transmissions were 4-speeds with the top gear a direct drive.

The vast majority of the electronically controlled transmissions have been 6-speeds with the 5th and 6th gears overdrive gears. In almost all school buses the 6th gear is locked out. I have heard some people have been able to get the bus manufacturer to give clearance to allow the 6th gear to be enabled. I have also heard of a few people who were able to get into the electronics to get the 6th gear enabled.

It is probably best to consider school bus transmissions as having only 5-speeds with the 4th speed direct and 5th speed overdrive.

What determines the top speed of the bus is the maximum RPM's the engine can turn and how many times the drive shaft can turn per tire revolution. The fewer times the driveshaft turns the faster the bus can go.

Max engine speed is fixed unless you want to spend a lot of $$$ to fix a broken engine.

Transmission gearing is basically fixed. It either has direct or OD as the top gear. Swapping a direct drive transmission to an OD is going to cost more than what it would cost to purchase a similar bus with OD.

Final gearing in the rear end is the only variable that can change the number of times the driveshaft turns in relation to tire revolutions.

The easiest way to change the variable is to purchase taller tires. Going from a 10x22.5 to an 11x22.5 will raise the top speed about 5 MPH. The same will happen going from an 11x22.5 to a 12x22.5. You can do this only if you have room in your fenders for the taller and wider tire. If you have room and you have to purchase new tires anyway this is a great option.

The difficult way to change the variable is to swap the rear gears from slow gears to highway gearing. This will cost in the neighborhood of about $2,000.00 installed if you can find a good used gear set that will fit your rear axle.

If you can't find a good used gear set it can cost considerably more. It may even be easier to find a complete axle to swap in which could give you bigger rear brakes and/or air suspension.

Be aware of the fact that changing the rear gearing in any fashion can mess up the speedometer. On an electronically controlled engine and transmission this can cause other problems.

The moral of the story is it is best to find a bus with a power package and gearing that will fit your needs and expected use at the start than to try and swap things around later.
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Old 03-20-2016, 07:13 PM   #5
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Quote:
Originally Posted by BigPaul View Post
Every time I post here I have to admit I cringe because I think people are reading my questions and thinking "What a moron" but if I don't know I have to ask, so here goes.

Reading posts I see most buses from my area would have transmissions not designed for highway speeds.

Generally speaking, what are the costs comparisons to having a transmission re-geared over just buying a transmission with highway gears?
Never, ever be embarrassed to ask a question. If no one asks questions, no one will need to find answers.

Now and then a transmission can be swapped with little more than time and a few tools. More often, a drive shaft lengthening or shortening will have to be done. But for the most part, a transmission swap is not a big deal and can be done for very little money.
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Old 03-20-2016, 07:34 PM   #6
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Hopefully, the days of newbs being afraid to ask questions has come to an end.
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Old 03-20-2016, 08:11 PM   #7
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Please ask all the questions that you have. That is how you learn. That is one of the things we do here. We actually enjoy helping people. Ask away!
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Old 03-20-2016, 09:14 PM   #8
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Cowlitz - Great info!! It's nice to have access to your unique inside information.
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Old 03-21-2016, 10:42 AM   #9
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Changing transmissions is pretty straight forward and is no different than changing a transmission in a pickup except for the fact a bus transmission is much larger and heavier.

Swapping one for another is a different kettle of fish altogether unless you have a donor vehicle with all of the required parts, pieces, linkages, pig tails, brain boxes, etc.

For all practical purposes, swapping a mechanical to an electronic isn't going to work. With the way in which the electronics work you may or may not be able to get the transmission to work, particularly if your engine is not electronically controlled. There were some electronic transmissions installed behind mechanical engines. Those few can be made to work in another vehicle but you need to get everything from the gear selector down to make it work in your vehicle.

Even swapping an AT500 to an MT600 series transmission is not easy. In order for the swap to work you need to find a donor transmission that has been set up for your engine. An MT643 behind a Cat 3208 will not work properly behind a Cummins 5.9L. In many cases it will require a different bell housing. It may or may not require changing the driveline to accept different u-joints or changing the length to account for the difference in size of the donor transmission. It may also require the need to fabricate a different transmission mount.

In other words, yes you can swap transmissions to a heavier duty or an OD transmission but it isn't going to be easy or inexpensive.

If you haven't purchased your bus already it is much better to shop around and get the power package and gearing that you want rather than purchasing a bus that happens to be close by or inexpensive and swapping stuff into it later.

If you have already purchased a bus, converted it, and you really like your bus but are not happy with the power package it might be easier to purchase another bus with the power package and running gear you want and swap it all into your bus than to try and swap piecemeal and try to make it work.

I have a friend who works for a company that has a 2003 IHC chassis Krystal Coach medium duty bus. The cam sensor went bad and basically trashed the engine. They picked up a good running DT466E in a junk yard and were not able to get it to work with the transmission. They could get it started but no matter what the IC dealer and the Navistar Truck dealer (tow different IHC dealers) did they couldn't get the donor engine electronics to work to work with their transmission. They ended up purchasing a crate motor that had all of the electronics on the engine flashed with the specific codes dictated by the VIN and the transmission serial number. In hindsight they wished they had purchased the new engine instead of messing around with a junk yard donor.

I am sure there are some people on this forum who are much more electronically savvy than me and can understand the lines of code in the brain boxes. I am sure given enough time those of you that understand it all could have made my friend's bus work with the donor engine. But when time = $$$ you can't stand around with a bus that isn't able to make revenue while someone spends time "figuring it out".

With enough time and $$$ you can do just about anything. But the reality is, if your bus does not have the transmission you would prefer the options for you to change it out are not cheap and they are not easy.
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Old 03-21-2016, 10:54 AM   #10
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Ya...the whole electronics issue has made both engine & tranny swapping a nightmare. Not like the "old days" when all it took was some basic mechanical savvy and an adapter. And to make matters worse, even if you are good with complex electronics, many manufacturers use proprietary programming languages that they will NOT share with anyone. There are folks over on the 4BT forum that have been trying to get various tranny and engine combos to communicate for years now with no luck. The new (5th generation) Allisons are, to my knowledge, the only electronic/computer transmissions that have the ability to work with just about any engine...even all mechanicals like my older 4BT Cummins. All this particular trans needs is input from a mechanical TPS (throttle position sensor). The trans then figures the rest out via a single built in computer.
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