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Old 11-23-2008, 07:55 PM   #471
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Re: The Millicent Chronicles; two foot roof raise, big tailgate


Time to bump the thread and boost my ego again!

Finally installed a proper door for the bathroom:



The framing is all done with an unholy mixture of different materials and fasteners -- which will all be covered up eventually, so no problem.

Learned a bit about the transmission issue. Turned out a spring had broken. No wonder the throttle pedal felt so light! Earlier, I had removed one throttle return spring because the pedal was so hard, and it was acceptable with the one spring. (Though not safe, as we shall soon learn.) Lately it was even lighter, and that was because of the loss of this transmission modulator cable spring. This spring apparently keeps the transmission modulator cable synchronized (for lack of a better term) with the throttle cable. When the spring disappeared, the modulator cable would no longer return with the throttle cable, and the tranny apparently "believed" I was still on the power. I still don't understand how it works.

I slapped a spring on it, to make the tranny work right (I think), and the pedal became rather hard again. I have studied this mechanism quite intensely, and I see that the tranny spring actually pulls on the throttle linkage also. Looks like a bad design to me. It ought to be possible to attach the spring to the throttle linkage itself, so the force of the spring pulls the two cables together without pulling the entire works back against my foot.

I'm now ready to present a conclusion, for Your benefit: It is DARNED DIFFICULT to work on these cables and springs. Even getting my head in there to see the stuff is difficult. I took the dog house off and removed the driver seat. But the engine is crammed into such a tight space that it is almost impossible to even touch these parts with my hand. I'm calling this a MAJOR REASON not to get a front engine flatnose.

Oh... an other thing. The single remaining "real" throttle return spring has also broken recently. Twice! This happened after the tranny spring went away, and.......... Yep -- you guessed it. First time, it happened to me, and I was able to reach down behind the pedal and tug on the cable. Second time, it happened to Peter, and I opened the dog house and pulled the linkage back each time we needed to. The first spring was worn thru from 200.000 miles of driving. The other was a "cut-and-bend-to-fit" spring, and the steel is far too soft -- it wore thru in no-time, riding against the good-n-hard steel of the fuel pump lever. So there's another lesson.

I have been removing electrical wires and relays and such related to the pupil transportation equipment. And have found a couple of wires that were worn thru to metal against a sharp edge. Do watch for that sort of thing, guys!

Then there was the defroster/ventilation fan that hangs above the center of the windshield. It quit working at some point this year. Turned out the wire was pinched and shorted against the edge of its own sharp-edged hole, right there in the base of the fan. So the breaker was tripping. Sloppy work in 1992 finally showed up.

Sorry, no other progress. I'm anxious to get the enlarged front door finished, but my welder won't reach since I completed the wiring to the kitchen stove in the house and did away with the jerry-rigged 220 extension to the garage. Proper extension cord soon.
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Old 11-23-2008, 08:27 PM   #472
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

Elliot-

I've spent the better part of the afternoon reading through all 33 pages of threads on your conversion project. As I have just recently joined Skoolie.net, I am very intrigued by how others are converting their busses. You have done a knock out job with Millicent in my opinion - especially taking on a 2 foot roof raise as you did. I'm not sure that I could handle a project like that myself . Heck, I'm still trying to figure out what I want to do with my bus. And although mine is technically not a skoolie, I have gotten some great ideas from this site on how to go about converting my coach - as this will be my first conversion project . I'm looking foward to seeing how yours progresses.
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Old 11-23-2008, 08:35 PM   #473
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

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Originally Posted by Zcommanager
Elliot-

I've spent the better part of the afternoon reading through all 33 pages of your conversion project.
So sorry.

Raising a roof two feet is no different from raising it two inches -- except for the result. Well, not much different.

Welcome aboard, and don't forget to read some of the other guys' threads also!
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Old 11-23-2008, 09:48 PM   #474
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

Hey Elliot...just a bit of background on how that modulator works and why it is important.

In the transmission there is a pump that produces what is called "line pressure." This is the life blood of the transmission. It is what circulates the fluid to the torque converter, applies the clutches and bands, and also what controls shift points. The shift points themselves are controlled by two valves which act to lower line pressure appropriately. There is no greater pressure acting than line pressure which is why shift kits (and newer transmissions with electronic controls) raise line pressure. Ok...

So we need to control when the transmission shifts, correct? There are two inputs necessary to determine what gear you should be in, engine load and road speed. Each is "monitored" by a valve within the transmission. In the tailhousing will be the governor. Since it is splined to the output shaft it will spin at output shaft speed. This serves to provide input on driveshaft speed. As speed increases its pressure also increases.

Internal to the valve body (generally) is the throttle valve. This is the one you're playing with. It really is a simple device. As you push the throttle harder (indicating higher load) the cable opens this valve further. At wide open throttle it will be running wide open and you should see close to line pressure, save for some loses due to friction. At low cruise you will see less pressure.

So...now the magic happens. Shifts are controlled by the differential pressure between these valves. Let's say you take off in first gear at a gentle clip. Both valves see a low pressure (meaning lower apply pressure on the bands and clutches also). As you speed up the driveshaft spins faster and faster producing more governor pressure. Meanwhile you haven't crowded the throttle much so TV pressure remains relatively low. Eventually the differential pressure is great enough between the two and the governor forces an upshift. The governor slows down, pressure decreases, and the world keeps going round until the next upshift. It will keep doing this until top gear.

Now...there's a big hill in front of you. You start crowding the throttle which increases TV pressure. Meanwhile, you slow down. This results in lower governor pressure. Eventually the TV pressure is enough greater than governor pressure that a downshift is forced. You might not back off the throttle, but the output shaft now spins much faster so governor pressure increases preventing a downshift too great (like to 2nd at high speeds). Hopefully you will climb the hill just fine, albeit at a slightly lower speed. If your speed continues to drop and you keep your foot in it you're going to continue to downshift.

Yep, that's about it. Hopefully you can see why the modulator cable is so important. If it's too loose you will not downshift aggressively enough putting a huge strain on the clutches and bands. Line pressure modulation will also not allow for higher pressures (there is a bleed off in the transmission) which only worsens problems. If you adjust it too tight your transmission will hold on to lower gears longer and shifts will be harsher though this might keep the transmission alive longer at the expense of ride quality and downstream driveline components.

Make sense?

Class dismissed.
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Old 11-23-2008, 10:13 PM   #475
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Re: The Millicent Chronicles; two foot roof raise, big tailgate


Oh Teacher! Teacher! Me! Me!

[The Teacher calls on me.]

Teach, could you please clarify how the output shaft changes speed when the transmission changes ratio? I used to think that the output shaft followed road speed, and it was the input shaft that changed speed with each shift?

And then I need to learn about the torque converter lockup. One of the boys on School Bus Fleet Magazine Forum told me that the MT653 shifts 1, 2, 3, 3 lock, then directly to 4 lock. And during acceleration, I can feel and hear this sequence. But I wonder if mine sometimes unlocks in 4th when I'm coasting. Is this possible?

Thanks Teach!


Edit: P.S. I think you ment "pump" a couple of places where you typed "output shaft" and "governor".
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Old 11-23-2008, 10:54 PM   #476
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

I think you're right. Sometimes my fingers don't keep up with my mind. At any rate, output shaft speed changes the amount of fluid that comes out of the governor. The pump pressure drops when an upshift is made so the governor spits out less pressure. Yeah, governor pressure changes with road speed, but drops with shifts because line pressure drops, not because the output shaft spins slower. I probably worded that incorrectly, but TV and governor pressures are always a percentage of line pressure since line pressure is king. Line pressure varies with engine speed so physically output shaft speed is the same even after an upshift, but line pressure drops since engine speed drops effectively simulating a decrease in output shaft speed. Basically, governor pressure is a percentage of line pressure and line pressure changes either through road speed changes or engine speed changes, but is dependent on output shaft speed in either case.

The torque converter locks up to make things more efficient. Without lockup there is no solid link between the transmission and the engine, only the fluid coupling. Non-lockup converters will get into what is called "coupling phase" where turbine speed will match impeller (engine) speed and the stator will actually start spinning as well, but it still is rarely a 1:1 ratio.

Your lock up clutch operates much like a clutch in a manual transmission in that it will physically lock the input of the transmission against the output (flexplate) of the engine. The guys on SBF are correct in that it will shift 1-2-3-3 lock-4-4 lock. It wouldn't surprise me that you can see/hear/feel it on 4th gear. I wouldn't expect you to notice it on 3rd lock up, but it is possible. Torque converter lockup only occurs during light throttle cruise type applications so the torque converter is typically in that coupling phase already, but a drop of 200 or so RPM wouldn't surprise me at all.

This was typical in older light truck transmissions as well such as the 700R4. Newer transmission will not typically exhibit that noticeable drop or torque converter shutter (thank god...that's why we have so many different flavors of fluid) because they use electronic solenoid valves to apply bands and clutches, the torque converter clutch included. Electronic valves allow the TCM or PCM to pulse width modulate the voltage to them and so they just kind of slowly slip into lock up, kind of like riding the clutch with a manual tranmission. Does it increase wear? Maybe slightly, but when was the last time you heard of someone having to replace a torque converter from lock up clutch failure?

You said you think it unlocks during deceleration. I do not entirely know the mechanics of the MT643, but I would not be surprised if it did. Typically lock up only occurs when governor and TV pressure do not display differential pressure. When you get off it the TV pressure goes very low while governor pressure stays high so the TCC will likely unlock. Why do they do this? Simple! Mileage. Modern electronic transmissions are able to coast a vehicle a mile or more on flat land from 60 mph and your bus is attempting to do the same thing.

That sounds good, right? Well it certainly is...most of the time. It becomes disadvantageous when you want to utilize some sort of engine braking such as from an exhaust brake. The exhaust brake might hold the engine back, but the torque converter will free wheel making the engine brake about as effective as an act of flatus in a hurricane. Ah...but there is an elegant solution you will often see the Dodge pick-em-up boys doing on the older mechanical Cummins trucks with detent/hydraulic shift transmissions. When they exhaust brake is activated current is sent to the brake actuator. It also goes to a solenoid physically mounted on the TV lever on the outside of the transmission case. The solenoid slams the valve wide open and TV pressure goes sky high, thus keeping the TC locked up AND providing aggressive line pressure to help the clutches and bands resist slipping under the braking load. The electronic transmission guys can do the same thing with a simple programmer.

Is an engine brake of some type what you're looking at doing? If so I'm sure a similar device could be rigged up for your transmission.
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Old 11-23-2008, 11:12 PM   #477
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Re: The Millicent Chronicles; two foot roof raise, big tailgate


Quote:
...SBF are correct in that it will shift 1-2-3-3 lock-4-4 lock.
Actually, he said 1-2-3-3lock-4lock. And that's what I feel/hear. Noticably bigger step from 3lock directly to 4lock than it was from 3 to 3lock.

I'm not planning an engine brake, but I would like to have the normal engine braking -- what little there is.

And it used to stay locked, I'm pretty sure of that. So I probably ought to remove that modulator cable and see if it is working properly. It does seem to require an awful lot of force to move it.

I may also "break down" and telephone that shop near Seattle -- the guy who is reported to be an Allison guru and willing to give advice over the telephone.

But right now I'm going to turn in.
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Old 11-23-2008, 11:25 PM   #478
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

Well...I've never stumbled across a transmission that makes a shift directly from a locked third to a locked 4th, but that doesn't mean it's impossible. I just find it improbably as that would put a TREMENDOUS load on everything within the transmission and the engine. You're only shifting from 1.39 to 1.00, but that 39% is pretty darn substantial...to the point of being 500-600 RPM's instantly. It's not impossible, only improbable in my mind.

As for things unlocking now when they used to be locked up...you did say you have played with the cable. Perhaps before it was out of adjustment? Unfortunately I am not as intimately aware of the inner workings of an Allison as other transmission so I can't tell you for sure what is supposed to happen. Perhaps that shop is the place to talk to.
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Old 11-25-2008, 07:10 PM   #479
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Re: The Millicent Chronicles; two foot roof raise, big tailgate

I have been reading aobut your back door/ramp and am wondering do you know how wide your bus is inside?
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Old 11-25-2008, 11:35 PM   #480
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Re: The Millicent Chronicles; two foot roof raise, big tailgate


Well, looking at your profile information I would say that my bus is the same width as your bus. So now the question is, which one of us is going out in the dark to measure?
...
...
...

All right, I measure 7 feet, five and a half inches. Approximately.

The door opening is 6 feet 4 inches wide. I could have made it wider, and I wish I had, but it was easiest this way.

I may be able to give you a more helpful answer if I know more about what you are trying to accomplish with the requested information.

Now your turn to go outside. Bring your digital camera. Open the dog house. Take some pictures of the throttle linkage on the left side of the engine, towards the front. Post here or send me as e-mail attachments. In fact, all of youse guys who have this engine do that.
Hey, it's meant to be a polite request. I'm just in a goofy mood. I trying to solve the mystery of the transmission modulator cable spring. The way it was on my bus makes absolutely no sense, so I'm hoping to discover that it was assembled wrong by some mechanic years ago.

Oh... I see you are kinda new around here so... Welcome aboard, bbbrt76!
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