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Old 11-16-2017, 04:20 PM   #1
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Swapping an MT643 for a manual

Hi all,

This is my first time posting in here, so here goes nothing.

Me and a few friends from college stumbled upon skoolies and decided to give it a shot and convert one for a national parks trip we plan to do after graduation. We looked for a bit and pulled the trigger on a '97 AmTram Genesis with a DT466 paired to an Allison MT643. Great combo right? Well it would be if the transmission were functioning properly.

It seems that the lock up clutch in the torque converter isn't locking up, so we only ever get to 3rd gear, unlocked. From what I've read, this will essentially spell death for the transmission if operated like that long term, from elevated temps. And we only ever get up to ~45mph, with much higher RPMs than I feel comfortable maintaining for too long.

So from my understanding, we replace the torque converter. Doesn't sound like too difficult a job (I have access to a diesel shop with a lift and heavy duty equipment), but I can't find a used one or the proper new one anywhere online. Quotes to get the work done in nearby shops are around $2000 for parts and labor.

We're not willing to bite the bullet on $2000 just yet, so I started looking into replacing the transmission altogether. For another MT643, I've seen a few in the $900 range. But what I'd really like to do is replace it with a manual, such as an Eaton Fuller T14607. I found a truck parts store that sells refurbished units for ~$400, just for the transmission itself (no shift linkage, no nothing). If i'm not mistaken, both the DT466 and T14607 have an SAE #2 bolt pattern so they'd match up just fine.

So what I'd like some guidance on is whether or not I should try to keep the MT643 and if there are any tips/tricks out there to fix it before I have to take it to the shop; or if I should go ahead and invest the time and money swapping the transmission altogether. I'd like the reliability and additional control with a manual (and the extra gears would be nice if we decide to swap out the rear end gearing), but at the same time if there's an easy repair out there that I'm overlooking I'd like to explore that before diving into an entire trans swap.

If we do go that route, there are a bunch of questions I still have:
How do I find a clutch/pressure plate that fits the application between the trans and DT466?
How do I deal with having shift linkage further back than ideal (since its a flat nose bus)?
What's the best way to go about mounting the clutch pedal?
What's the best option for a speedo if I swap the trans?
What do I do for transmission mounts on a new unit?

The trip isn't until the summer of 2019, so we've got a bit of time to figure it out and having a group of people to pitch in on repair money is nice too.

Any advice you can give will be greatly appreciated!

Thanks in advance!
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Old 11-16-2017, 04:44 PM   #2
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The MT643 is a pretty durable and strong transmission. I would be surprised if it really needs to be replaced. I am going to assume you have checked the fluid level etc.

When I first drove my bus I was sure the trans was messed up (and walked away from the deal), it feels like it shifts though like 6 gears or it was slipping really bad when it really is just going in and out of lockup, what kind of RPMs are getting at 45MPH? Most will not shift until 2000-2100RPM. I think I have a video of my trans let me know if that would be helpful to you. It should something like this 1-2 then 2-3 then it will drop RPM into lockup then it will unlock shift to 3-4 and re lock up.

How many miles are on this bus?

If the trans is really bad, I honestly think about scrapping the bus if you don't want another MT643 in there (I would totally keep one in there), to add a manual to something like that would takes lots of work money and trouble, cool to see in the end but not easy by any means.
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Old 11-16-2017, 06:15 PM   #3
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Yup, fluid level is good. The bus has 200k+ on it. The engine was swapped for an overhauled unit around 20k ago, not sure if the trans went with it or this is the original.

I want to say 2300-2500rpm, and it didn't seem like it was going through multiple gear ranges. It went through 2 shifts, then held the high rpm and stayed there. But I didn't know it would unlock again, then go to 4th locked.
How long does it take it to lock up? We've been driving it in 20min intervals and I get up to 45mph, then coast and the rpms drop back down to idle. If the lock up were working, it should stay at the rpm slowly lower with vehicle speed, right?

Is it possible that because the bus only saw occasional use from the previous owner that the valve body is gunked up and needs to be driven hard to dissolve any deposits?
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Old 11-16-2017, 06:57 PM   #4
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A new filter and fluid may help, I'm no mechanic but just changing fluid has fixed issues for me in the past. In the end even if the 643 needs replaced it will probably cost less then a swap even if you can get a great deal on the manual transmission. If you really want a manual finding a donor vehicle with all the patts needed is one way to go about it if one can be had cheap enough
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Old 11-16-2017, 07:07 PM   #5
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It should lock up right away, after the shift point below is a video of mine. The DT466 won't redline until 2800RPM but if your hitting 2500 and it's not shifting there is something up. I don't think it's gummed up unless the fluid is dirty or burnt, check the fluid and see if it nice and clean bight red and clear and not smelling burnt?

Check out this site also TransDiesel of Cental Florida - Servicing Allison transmissions and parts

My MT643 shifting up to 3rd at 25 MPH: https://youtu.be/9bGbGlLV3PI

If it was mine I would take it back out on the highway before anything more and drive up to 2700RPM hold here for a few mins and see if it wants to shift. I would try some lubegurad (red bottle) also if my highway drive was not eventful. But get some more advice on the lubeguard don't know if it's good or bad for big truck stuff.

Someone with more experience will chime in, but ya start with a fluid change before going to a shop.
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Old 11-16-2017, 07:27 PM   #6
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My DT466 redlines at 2600.
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Old 11-16-2017, 07:51 PM   #7
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With your time line and without a donor vehicle to get everything you need your only option is a filter, fluid change and if that don't work then find another one and send me your old one to replace my 545.
Trust me?
Working on replacing my old ford auto tranny with a manual tranny and it wasn't built for it? Then you have to build it for it????
Time? Money? Put it in,mark it,take it out, make what you need! Put in again and see if it works.
Auto to manual is not hard if you have a donor?
But if you have to guess at everything that could cost you in excess of 2000$ and a lot of time?
Rebuild what you have because you have because it is an exact fit and you know the exact condition of what you have transmission wise anyway?
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Old 11-16-2017, 08:17 PM   #8
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I am interested in this swap also, perhaps largely because I drove sticks for a living and simply do not like automatics at all.

You may want to check your modulator/kick-down cable. This is only a theory, but your MT643 might be in kick-down all the time.
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Old 11-16-2017, 09:14 PM   #9
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First off, just changing the converter most likely won't solve your problem. The locating pins in the drive cover break off, the clutch plate blows apart and you have metal everywhere. Oil goes from the converter to the rest of the hydraulics and spreads the metals. Get the entire trans gone thru or get a good used one.
If you decide to go standard, I may be able to help. I have a spicer 5 speed in mine and going to an mt643.
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Old 11-16-2017, 09:18 PM   #10
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Quote:
Originally Posted by bluebird90 View Post
First off, just changing the converter most likely won't solve your problem. The locating pins in the drive cover break off, the clutch plate blows apart and you have metal everywhere. Oil goes from the converter to the rest of the hydraulics and spreads the metals. Get the entire trans gone thru or get a good used one.
If you decide to go standard, I may be able to help. I have a spicer 5 speed in mine and going to an mt643.
I'm not a fan of the Spicer 5 speed. You're making the right move, imo.
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Old 11-16-2017, 10:28 PM   #11
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Whats wrong with the spicer 5 speed? I know people usually prefer the 6 and up speeds but other that that.
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Old 11-16-2017, 10:42 PM   #12
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Okay, filter and fluid flush, I will give that a try and get back to everyone.

Bigsky, will do on the highway run, but I'm unfamiliar with lubeguard?

Jolly Roger, good point and haha we'll see.

Elliot, the more I read about it, the more I'm hoping that's the case.

Bluebird, how do you like the spicer? I've heard mixed reviews on them, as Eastcoast seems to share the opinion of
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Old 11-16-2017, 11:14 PM   #13
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I actually like the spicer, was even going to put a easy pedal twin disc clutch in it but my wife needs to be able to drive this thing and she is only 4.6 ft and has a hard time with the clutch.
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Old 11-17-2017, 06:22 AM   #14
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Whats wrong with the spicer 5 speed? I know people usually prefer the 6 and up speeds but other that that.
No overdrive, and I don't like the ratios.
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Old 11-17-2017, 09:00 AM   #15
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im a fan of the 643, I also second the post about doing more than just replacing the converter.. your fluid and filter flush should be telling.. when you pull your pan down look for metal.. if you in fact have broken up hard parts you will have pieces of usually snap rings in the pan, and of course if the clutches are burned up you'll also have a lot of sludge in the bottom of the pan.. some is normal on a high mileage transmission, but a big ole mess of sludge is telling that the trans slipped its way through clutch material..

and yes check the modulator cable.. if the modulator cable has broken or has stuck completely pulled out it will mess up your shift points.. and if its the electric modulator they are notorious for going bad..
-Christopher
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Old 11-17-2017, 09:41 AM   #16
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Couple things.

A Modulator cable out of adjustment would still give you 4th gear. So if you're sure that you don't have it then it's likely something internal. Lock up wouldn't allow the bus to free wheel like that with it engaged, so that's strike 2 for your trans. You can try a fluid and filter change. But I'm with bluebird90 in that I don't think it will be that simple.

As far as fixing it goes. I'm leery of suggesting used units to people because of the unknown factor that is present with the trans. Most used stuff comes with no warranty so what you're buying may not be any better then what you've got. You just missed the truckparts of ohio auction back in october. That's where I bought my mt643, brownie box, and 2 apu's. There should be another one coming up in the summer. If you can't find one locally, I'd wait and buy one at the auction.

As far as swapping goes. Go for a 10 speed or greater with OD. You're doing the swap already, you might as well make it worth it. You'll need access to illustrated parts manuals in order to determine what clutch will work with your trans/flywheel, either that or a very skilled parts counterman. The shifter can be done by using a forward shifter housing used on the old cab over semi trucks. I'd also suggest using the clutch pedal and movement assembly out of a cab over as well because that will be the closest thing that is similar to what you have. Matter of fact, you might be better off just buying a scrap cab over and taking all the pieces you want off of it. That way would be alot easier then fabbing up stuff from scratch.

You have the time to do it, but the fabrication/mechanic skills required will be a 9/10 level and I don't know you enough to tell if you're capable.
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Old 11-20-2017, 08:57 AM   #17
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Update from this weekend:

I took a closer look at the modulator assy. The linkage seems fine and the travel is within spec. The return spring is still attached and properly pushing the cable back after I let off the pedal (which means I have a pull type, right?). So I crawled under the bus and took a look at the modulator itself. I took off the bolt holding on the retaining plate and wiggled the modulator free and was happily greeeted by some transmission fluid. DOH! Thankfully we had a bucket nearby, so not too much was spilled.

I had my buddy push the pedal up and down while I looked at the modulator and I noticed that the little pin inside the shaft that goes into the trans wasn't moving at all. I could hear/feel the cable moving inside, but no movement whatsoever of the pin. So I don't think it's an issue with adjustment, I'm guessing whatever connects the cable to the pin (a little cam perhaps?) has snapped or loosened over time to the point where it's not controlling the pin at all. The pin that's in the transmission on the internal side of the shaft moved freely and returned to its original position via a spring after I let off of it; everything seems to be in check there.

Did some research, and found a few custom cables websites that will make Allison parts. They don't list a price on their website, but I imagine it would be around $150 for a new modulator with cable.

Hopefully this has been the issue all along. Not sure how we didn't catch it earlier, but hey, better late than never, especially before we made the effort to a drop a manual in it. We've yet to drop the pan and add new fluid+filter, but from the little bit of fluid we drained already it doesn't look terrible. Dirty, but nothing unusual.

What do you guys think? Was this most likely the issue all aong? If it is, I wonder what kind of damage has been done by driving around w/o a functional modulator for this long. Maybe the previous owner never had lockup at all.
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Old 11-20-2017, 09:16 AM   #18
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my **GUESS** is that the bad modulator wiped one of the clutch packs.. or is causing it to slip radically.. this can manifest itself as feeling like you have no lockup as the transmission is slipping so the RPM's go up like a missing lockup..

the transmission is designed to operate with the modulator pin fully retracted (no or light throttle applications).. so you would still get all 4 gears and lockup.. however the line pressure is low when the modulator is disengaged.. shift quality is much softer so at fiull throttle its more likely to "slide" into gear which can glaze the clutches and cause slippage / excessive wear..

its still a good shot to drop the pan and see hpw much sludge and if any metal parts are down in there..

as for a modulator cable.. I would look for a used one.. from what i understand the early style modulators are the same on the AT545 and the MT643, so an older 80s or early 90s mechanical-engine or gasser bus with an AT545 might very well yield you a modulator that works.

the later style is an electric one with a solenoid that pushes the pin out..(the electric was designed for fly-by-wire engines where the pedal didnt have a physical linkage to the engine) it was designed to operate when the pedal was over 50-70% depending on the application..(the ECM or a switch on the pedal controlled it) I dont like the electric one near as much as the mechanical one.. the mechanical truly modulates, whereas the electric is simply a kickdown switch..

-Christopher
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Old 11-20-2017, 09:53 AM   #19
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It seems weird to me that soft use == shot clutch pack. I don't think that's the case since there was never a observable shift point after the first two shifts. Could it be possible that the lock up piston wouldn't be getting enough pressure to actuate if the pin never moved? Or maybe it's seized entirely. I guess a drop of the pan will point me in the right direction.

From what I was reading, most gasser busses used the vacuum version of this modulator, but that's good news that my selection just grew. I think there's a small skoolie graveyard about an hour from my house. I'll have to make a run out there to see what they've got.

I did see they sell the electronic ones on ebay, but didn't know what it would take to make the switch to electronic. Would all I need for that be a simple push-button type switch somewhere midway on the application of the pedal? Maybe I could wire a variable resistor to the pedal position to make it truly modulate.
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Old 11-20-2017, 10:29 AM   #20
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my red bus had a bad electric one on it's 545.. I changed it, but was already too late.. I had wierd things start occuring on my transmission even though I now had harder shifts and downkicks at full throttle.. it had a reed switch attached to the pedal which kicked the solenoid, it didnt drive it like a stepper so im thinking that partial voltage will just make it not push out at all..

I still have that electric modulator in my garage if you want it.. I took the new (good) one off my 545 before i junked the transmission.. you can have it if you want it (my new trans doesnt use it). at the very least you could put it on, and temporarily wire a 12 volt switch to it so see if you switched on or off your transmission acted any differently before you go searching for a mod cable..

theres supposedly a skoolie graveyard near lebanon. I havent been to it yet but ive been wanting to. im l;ooking for an older bluebird (pre-1995) to take the complete front heating / defrosting / switch panel..

-Christopher
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