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Old 05-01-2021, 04:10 PM   #1
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Year: 2000
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Chassis: 3000 / 33' Flat Nose
Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
Solution: Allison Electric Modulator Activation

I have about three threads on various aspects of my journey to resolving how to get my Allison MT 643 electric modulator to work.

This is the solution I came up with.

After much troubleshooting I determined that the ECM was not sending the signal to the modulator relay. Why, I don't know. I decided it was such a common issue that a workaround was the best solution.

Workarounds:
Electric switch at accelerator pedal. This problem was so common, W.W. Williams actually made a workaround using what appears to be a wobble type limiting switch. You can place the switch under the accelerator pedal so that when the pedal goes down X amount, the switch sends the 12v to the modulator.

Pressure switch. It was suggested I instead use a pressure switch off my MAP port. This was appealing to me because I could not get an actual W.W. Willaims switch, and I was not too excited about fabricating an adjustable mounting system and finding an appropriate switch. In addition, because I have a rear engine, running the wiring and such wasn't as simple or clean as I would like. Upon investigating this, I ended up with the following:

1 - Tee. My T444E MAP sensor uses a M10x1 male thread, but the pressure switches have a NPT 1/8" male thread. I was able to find a Male M10x1/Female M10x1/NPT 1/8" tee. They ranged from $8 - $20.

2 - 8-13psi SPST Normally Open floating point pressure switch. Mine was factory set at 10psi and seems to work well there. The pressure range was based on determining where my MAP / Boost pressure ranges were running for my bus. You'll need to determine this using a boost pressure gauge and going out and driving your bus in different situations (city, freeway, climbing grades, etc.).

How I installed/connected the pressure switch:
1) I removed my MAP sensor, installed the Tee, replaced the MAP sensor and installed the Pressure Switch. Do NOT use any thread tape as it may interfere with grounding of the pressure switch.

2) I ran a wire from one of the terminals of the pressure switch to the positive wire of the modulator connector. I cut the modulator connector positive wire about 6" from the connector and connected the wire from the switch to the connector wire.

3) I ran a wire from the other pressure switch terminal to the positive from the existing positive wire out of the engine compartment flat blade fuse panel.

Your situation may be different from mine, but no matter where you get your 12v+ source, make sure you have an inline fuse.

My modulator measured ~2.5amps when 12v was applied.
My pressure switch is rated at 8amps @ 12v.
My OEM flat fuse was a 15a, so I kept it there, but may lower it to a 10a...not sure yet.

How do I know it all works?
1) I will be adding a light from the positive modulator connector wire to the drivers area. This will tell me when the modulator is receiving 12v. It will also give me a heads up if the modulator, pressure switch, fuse, etc. is having an issue.

2) I know what my bus shifted like before applying this solution, what it was supposed to shift like (thanks to a few key posters on here) and what it shifts like now.

It now shifts like a warm knife cutting through room temp butter...smoooth!
It shifts between 2600 rpm (low gears) to 2400 rpm (high gears) and sustains the high rpms the entire time. So sweeeet!

I'm not sure about this, but I think now that it's shifting properly, the engine ECM isn't trying to compensate and the entire bus is running so sweet.

Note:
Some situations may arise where your boost range when shifting into higher gears may fall below the pressure switches psi setting. A solution for this is what the call a cut-on / cut-off pressure switch. You need one that allows you to set the cut-on at a higher psi and the cut-off at a lower psi to work with your boost psi. I know these type of switches are available in the refrigeration industry because my friend works in that industry and he's the one that told me about them.

Since the AT545 transmission also uses this same modulator, this solution may work for these trannys too.

Thank You!!!
This has been a few months in getting to this point. I truly believe I would not have resolved this issue in such an elegant and functional way without great advice and generosity from this site. So very appreciated.

Hope this helps!

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Old 05-01-2021, 07:50 PM   #2
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When you run the wire to the light you can use the same wire to do a bypass if you want. I would use a momentary switch for that.
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Old 05-01-2021, 08:09 PM   #3
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Quote:
Originally Posted by s2mikon View Post
When you run the wire to the light you can use the same wire to do a bypass if you want. I would use a momentary switch for that.
I'm not sure what you mean by a bypass.

My understanding of a momentary switch is it's something I would need to press??

My plan is to use a wire tap connector to connect to the positive wire at the modulator, run a wire from there up to the cab to an existing indicator light (use to be used for the alternating lights up top).

Then, when the modulator is receiving 12v, the light would also light up indicating all is working. No light and my new understanding of how the system works and what it's supposed to feel like, will tell me somethings wrong. It's a bit of an idiot light, but since we're talking about my transmission, I'm okay with it.

How is what you're suggesting different/better/easier?
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Old 05-01-2021, 08:19 PM   #4
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[QUOTE=Simplicity;436690]I have about three threads on various aspects of my journey to resolving how to get my Allison MT 643 electric modulator to work.

Note:
Some situations may arise where your boost range when shifting into higher gears may fall below the pressure switches psi setting. A solution for this is what the call a cut-on / cut-off pressure switch. You need one that allows you to set the cut-on at a higher psi and the cut-off at a lower psi to work with your boost psi. I know these type of switches are available in the refrigeration industry because my friend works in that industry and he's the one that told me about them.

When I read this I was just thinking of a way to bypass the pressure switch with a switch at the front end. You would have to use a heavy enough wire to your light and back feed it for bypass.
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Old 05-01-2021, 08:29 PM   #5
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Oh, kind of like a manual version of activating the modulator. Interesting. I'll keep it in mind, but your suggestion for using the pressure switch is so simple, clean and functional, I'm going to leave things as is until it needs attention.
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Old 05-01-2021, 09:25 PM   #6
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Cool. I'm glad it worked for you.
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Old 05-03-2021, 07:52 AM   #7
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Sounds like a great work around. And as you said, you can fine tune it with the adjustable switch.

I assume you tested this and have logged some miles? Are you pleased? Did it downshift properly going up a grade.

Kudos for all of this, and for sticking it out for a solution!
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Old 05-03-2021, 09:13 AM   #8
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Join Date: Aug 2019
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Year: 2000
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Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
Quote:
Originally Posted by Booyah45828 View Post
Sounds like a great work around. And as you said, you can fine tune it with the adjustable switch.

I assume you tested this and have logged some miles? Are you pleased? Did it downshift properly going up a grade.

Kudos for all of this, and for sticking it out for a solution!
I've not put miles on it. That's the next step.

We did test it.

Tests & Results:
Standing start - Throttle up normally to get the turbo spinning, then pretty quick but gradual advancing of the throttle until to the floor.
1st - 2600rpm shift into 2nd at 2450
2nd - 2450 - 2550 shift into 3rd to about 2200
3rd - 2200 - 2500 shift into 4th to about 1800

Stayed in it's power range really well off the line.

Starting at 40mph - Pedal to the metal, no response because boost is too low and took longer to wind up the turbo. Better solution was to gradually advance the throttle to start to spin the turbo, which only took a few seconds, then it took off in similar fashion to the standing start.

60mph cruise on flat - No downshifting required or expected.

Climbing a steep grade - My old ugly bus was stuck behind a bunch of new "SLOW" cars that think they are the only ones on the road (I'm a school bus driver for work, so my fuse for idiot drivers is short), but I was going 45mph on the ramp from one freeway to another and it revved right up to 55mph (again, slow cars in front). At that speed it can't continue to pull, the boost pressure drops and I manually downshifted into 3rd at about 1600rpm (normally I would do this at 1800 to not strain the tranny, but I wanted to see what it would do).

I had tested this one time before with open road, same grade, attacked it at I think 60mph and it pulled it at 50mph at somewhere around 2250 rpm.

Hybrid Mode:
On grades (several around me in the PNW) my plan is to drive by my RPMs and EGT, oil and tranny temps, which will include manually downshifting to maintain engine/tranny safe numbers.

Since the bus was not designed for those types of situations, and I put shorter (5.11 to 4.10) rear end gears, so I'm aware of needing to perform a hybrid manual/automatic shifting in my bus.

Now that it is able to perform properly, I'm going to be pushing it farther and a bit harder locally to increase my comfort in taking it from Seattle to the foothills of the Sierras this Summer.

I am planning on adding auxiliary oil and tranny coolers. My bus, my dog and I all work best in mild temperatures.
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