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Old 01-31-2013, 07:03 PM   #21
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Re: How to tell AT545 from MT643, w/pix & specs

the only reson u have that type of trans is because u got a cat,, and there motor to trans bolt hole are for a cat trans,,make sure u have an allison trans,it maybe a cat trans.
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Old 02-21-2014, 11:12 AM   #22
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Re: How to tell AT545 from MT643, w/pix & specs

Ok, I get it, Allison MT643 is the transmission to have, but what does it really mean that it locks up 3rd and 4rd gears, can someone explain in a driving senerio the advantage of the transmission locking up on gears? I have a cat 3208 with MT643, just started my conversion.

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Old 02-21-2014, 11:50 AM   #23
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Re: How to tell AT545 from MT643, w/pix & specs

Quote:
Originally Posted by juliol
Ok, I get it, Allison MT643 is the transmission to have, but what does it really mean that it locks up 3rd and 4rd gears, can someone explain in a driving senerio the advantage of the transmission locking up on gears? I have a cat 3208 with MT643, just started my conversion.

J
A basic automatic transmission does not have any mechanical connection between the engine and the tires. There is no clutch, as in a stick shift.
Instead, the power is transmitted by a fluid. There is a "propeller" on the rear of the engine, and an other "propeller" on the front of the transmission. These two "propellers" are very close together, inside the unit called the torque converter. This unit takes the place of the clutch.
So the engine "prop" blows the transmission oil against the transmission "prop", which is how the power is transmitted. But oil is not solid, so there is slippage -- maybe 5 or 10 percent. This slippage becomes heat (thru friction, essentially).
So we lose some of the energy that would otherwise serve to drive the bus forward; and we get heat, which is damaging to the tranny. The slippage is typically about 200 RPMs worth.

Then somebody invented the torque converter lock-up clutch. This clutch is located inside the torque converter, and is controlled by the transmission's regular valve body (I think). It bypasses the "fluid coupling" with a solid connection, so no power is wasted as heat.
The result is better efficiency, which means more driving force to the tires, and less fuel consumed. In addition there is less wear on the transmission from the lower heat. Having torque converter lockup is most important in mountains, when a plain torque converter generates a great deal of heat, which can ruin a transmission.

You can feel and hear when the lock-up clutch engages. It is almost as if the transmission shifts gears, so it can seem as if the tranny has five or six gears.

Does this help?
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Old 02-21-2014, 12:11 PM   #24
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Re: How to tell AT545 from MT643, w/pix & specs

Wow! Elliot, that is why this site is so helpfull there is people like you that takes the time to explain it as it is, thank you this really helps.
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Old 02-21-2014, 12:22 PM   #25
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Re: How to tell AT545 from MT643, w/pix & specs

Yeah, I'm a "Wow!" kind'a guy, ain't I.
I mean... You are most welcome.

There is more going on inside a torque converter than I mentioned -- the thing is called a "torque convertor" for a reason. Otherwise it would be simply a "fluid coupling" -- which is what the first ones were, back in the late 1940s. But we don't need to know about the "converting" -- we just benefit from it.
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Old 08-20-2014, 11:55 AM   #26
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Re: How to tell AT545 from MT643, w/pix & specs

I have a 1983 ford b600 with a 370 and an Allison 545 I am at a crossroad and thinking about a engine swap as I understand this model year also offered the bus with a Detroit Diesel 8.2 and the DD 8.2 turbo. can I up engine without changing the transmission or its location. My 370 is a little long in the tooth and may be at a place of needing head work rear main leaks all the normal ailments of an older gasser etc... and the bus is of sentimental value to the family so chucking it for a newer model isn't an option. Plus we might want to head south thru the mountains and follow nascar a bit, and I am a little sketchy if it can climb up mountains well. I am not married to the DD 8.2 willing to listen to all advice on this subject I think if I do this I want to go diesel, and not a bunch of lg modifications. i would consider a more modern engine but not sure about the electronics. like the idea of keeping it simple I am a reasonable skilled mechanic but large trucks are anew area for me so I take it slow and figure it out as I go. new to schoolies but love every minute of it. And any easy way to figure out rear end gearing without taking it out?
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Old 03-24-2015, 12:55 AM   #27
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The 5.9 Cummins is a very popular engine and can produce 190 HP very reliably and it usually comes with a 545 trans. I have seen some with MT643 and some with the MD3060 which would be the ideal trans for your application since it has double overdrive and lockup converter. Biggest problem converting from gas to diesel is that
the gas engine runs at a higher rpm to produce its power and a diesel like the 5.9 or the 8.3 Cummins have their max speed at 2400 to 2800 RPM. So to avoid changing
rear differential ratios you can add an overdrive transmission. As a side note the 8.3
Cummins also came in Fire trucks with a MD3060 transmission.
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Old 03-24-2015, 09:23 AM   #28
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I would watch for a 5.9 Cummins, DT 466, Cummins 8.3 with a MT643, or a MD3060 behind it. NO AT 545 transmissions.

I would not touch any other engine. Not worth the time if your already doing a swap.

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Old 11-03-2015, 03:23 PM   #29
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The 3000 series Alison transmissions are the best of the available transmissions in school buses. Most common variant is the MD 3060 double over drive, 6 speed automatic transmission.

If it has a electronic shift panel, it's got a 3000 series or better transmission.

Not all 3000 series transmissions have the electronic shift panel, but most will.

This is what you want to see.







Edit

Is a OP able to move this post to just under the OP's original post? It won't do much good 3 pages into the thread. Thx


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Old 11-04-2015, 11:47 AM   #30
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Quote:
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Is a OP able to move this post to just under the OP's original post? It won't do much good 3 pages into the thread. Thx


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I don't think a mod can do that. Maybe someone in admin can.
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