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Old 02-01-2021, 09:50 PM   #1
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MT643 Downshift 3rd lockup to 2nd under load

I switched out my 5.11 rear diff for a 4.10 for lower RPMs at cruise (2K @ 60mph). I have not driven my bus much at all before doing this, so I'm not sure if it acted this way before the swap.

Anyway, everything seems to work great except in one situation, that's when going up hill. It goes from 4th lockup to 3rd (assuming it stays in lockup), but it really slows down before shifting into 2nd. By then, my RPMs in second are out of their power range and it struggles.

Now, I had not changed any of the axel ratio or axel ratio pulse parameters, so the speedometer was not reading correctly. Yet, the RPMs, throttle position (meaning pedal to the metal), etc., were all the same. Since this is a mechanical tranny, I'm not sure if the speedometer has anything to do with shift points?? (I know enough to be dangerous).

I recently got linked up and have updated the rear diff ratio and pulses, yet it's not insured at this time, so I haven't been able to test it.

Would the speedometer being off or the aforementioned parameters not being updated cause a shift issue like this? Or, is all that a red herring?

If it is a red herring, what would cause the tranny to not go out of 3rd lockup and down into 2nd at a more power band friendly point when pulling up hill?

Thanks!
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Old 02-01-2021, 10:49 PM   #2
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Speedometer has nothing to do with it. Governor pressure and modulator pressure control shift timing. Modulator is throttle position and governor pressure is output speed. What kind of modulator do you have? Is it cable or air? Is this a RE or FE? 4.1 may be a little steep for the T444E.
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Old 02-02-2021, 07:33 AM   #3
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How big is the bus, what hp rating do you have, and what size hill are you talking?

Like s2mikon stated, your speedometer won't have anything to do with it. If it's a t444e it will have an electric modulator, which is known to go bad. Cruise on flat ground around 35 mph and then floor it. If it doesn't downshift into a lower gear, your modulator isn't working and needs to be fixed.
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Old 02-02-2021, 09:25 AM   #4
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Now for a few questions. What is the rated full governed engine speed 3000? Will it do it? This is your starting point in diagnoses / adjustment. Next what is the stall speed of the torque converter? Is the shift / range cable adjusted properly? Be sure to check the cable adjustment before doing a stall test. If the range cable is out of adjustment it will toast the forward clutches. Make sure that the parking brakes and wheel chocks are in place and that your service brakes are good before doing a stall test. And don't have it parked in front of something just in case. No kids playing in front of the bus. These 3 values have to be known first. Engine and transmission at normal operating temperature. If these are to spec, we now have to know at what engine speed full throttle second to third shift and third to fourth shift occurs at. Ignore first to second at this point. If correct it should be 200 rpm below governed speed. What is the shift quality? Firm shifts, or slipping / flare up shifts? These were known as very durable transmissions. School buses and trash trucks were hard service. The things that caused most failures were range cable adjustment, modulator adjustment and overheating. All preventable. If you have any questions as to how to proceed please ask.
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Old 02-02-2021, 10:45 AM   #5
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booyah is right it will have an electric modulator.


S2mikon there is no adjustment on the electric modulator.. and the earlier the ECM the bus has the worse that modulator will work.. in fact so bad on some that WW williams in columbus Ohio created a fix where they put a Micro-Switch on the foot pedal to initiate the modulator..



the electric modulator is nothing more than a kickdown.. it is either full on or full off.. the idea was the ECM would activate it when the engine is under heavy load, whether the trans actually downshifted or not was up to the transmission valve body and governer it either was modulated or not..


Gear ratios play into part with downshifts.. to prevernt Hunting, transmissions typically wont downshift themselves right into an upshift situation.. for instance the MT643 is designed to upshift around 2500-2600 RPM on IH diesels.. (except for 4th gear which is usually 2400).. this is fully modulated..


if you have your foot to the floor and your 643 is up-shifting at much lower RPMS then it means the modulator or its electrical circuit isnt working


now back to downshifts.. on my 643 it upshift to 4th at 2400 RPM.. so lets say thats too much for it so im slowing down in 4th gear.. it will drop RPMs down so that when it downshifts back to 3 that it is going to have at least 200-300 RPM of rise before it upshfits again.. so yes because the ratios are pretty wide on most 643s (that ive encountered).. it means that to get that downshift you will be below your desired horespower band of your engine..



if you want the shift sooner, you can drop the T-handle down a gear and as lomng as the trans isnt exceeding its max governed input shaft RPM in the gear you select, it will downshift..


Lockup and downshifts on the 643.. ive found on mine that if I drop down low enough in RPM during 3rd gear, that it will unlock the converter before downshifting to 2 (this is with my foot to the floor (modulated shift).. it will try to gain extra power from the engine by running unlocks through the first part of 3rd.. if you speed up enough it will pull the lockup back engaged..



NOW if the modulator is not on (no foot on pedal) then it will stay locked all the way through third till it gets slow enough to downshift into second then it unlocks..


so for you to test.


1. from a dead stop drive at full throttle till you reach 4th gear.. you should notice that the transmission is shift from 1-2 and 2-3 somewhere around 2500 RPM.. Lockup will pull-in midway through 3rd and will feel almost like a solid shift. the 3-4 shift should occur at somewhere around 2400 RPM..


if thats not happening.. if your shifts are much lower RPMs then check the modulator and circuit..



I burned up an AT545 because of a bad modulator I was unaware of at the time.. when i fixed the modulator it was too late, damage had been done to the trans..
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Old 02-02-2021, 11:19 AM   #6
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Mine has an air throttle and it does modulate line pressure some what when it isn't leaking air pressure. It has to be changed as a regular maintenance item. It will loose part throttle down shifts when leaking.
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Old 02-02-2021, 11:33 AM   #7
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Quote:
Originally Posted by s2mikon View Post
Mine has an air throttle and it does modulate line pressure some what when it isn't leaking air pressure. It has to be changed as a regular maintenance item. It will loose part throttle down shifts when leaking.

the IH electronic engines dont have a mechanical throttle linkage to an injection , they are drive by wire.. the OP has a T444E.. it uses the electric modulator which is a simple solenoid.. it kicks on or off it doesnt have in between settings..



busses with mechanical throttle linkages to a fuel pump.. (ie P-pumps, MW pumps, etc) can have a cable modulator goign t othe trans itself from the linkage at the pump. your air throttle most likely just actuates the link age at your pump therefore you can have a modulated trans..
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Old 02-02-2021, 11:44 AM   #8
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It has an air cylinder on the transmission that actuates the modulator pin in the valve body. It is fed by air pressure that actuates the pump lever. More air pressure = more pump travel and more line pressure.
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Old 02-02-2021, 11:50 AM   #9
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great way to do it!! I wish navistar had done something similar with the electronic engines.. but alas not.. and alas why I threw away the crappy 4 speed transmission and installed the allison 1000.. that I modulate with the computer program instead
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Old 02-02-2021, 12:05 PM   #10
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great way to do it!! I wish navistar had done something similar with the electronic engines.. but alas not.. and alas why I threw away the crappy 4 speed transmission and installed the allison 1000.. that I modulate with the computer program instead
Ya know christopher, I wonder if someone could actually modulate the electric modulator. Create a standalone box that takes the % engine load PID from the j1708 data bus and turns it into a pwm signal. Granted, you'd have to have a solenoid valve that can handle such a thing, so maybe the standard on/off electric solenoid that's currently used wouldn't work, but I don't think it would be hard to set something up like that.

It would be better then the on/off system that those transmissions currently employ.

You could even setup an operation check system to trigger a warning light in case of failure, which seems to frequently happen with the other system.
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Old 02-02-2021, 12:32 PM   #11
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a servo driven device would work fine and they exist.. however just like our discussion on whether i should build my J1708 / J1939 / Transmission translator box.. the market dwindles every day.. less and less of these trucks exist and even fewer with mechanical transmissions out there...



the Allison 545 shared resemblence with the TH-400, which the TH-400 used an internal mechanism that was electric for "kickdown".. the GM 350 series (350,700, etc) used true modulation to control shifts.. the 400 also used a true modulator in some applications but many people activated their trans in hotrods using the kickdown solenoid and it worked well for what it was..



my guess is allison felt in the later years it is enough just to get kickdown vs actually control shifts.. that electric modulator showed up not only on navistar but also on CATs and on later CUmmins as well..



the solenoid that is inside of the electric modulator isnt really designed to modulate.. it has no position control really.. its just an electromagent and a big spring.. i could drive it with a PWM but if I remember right the pin it pushes has pressure behind it that varies with main pump drive so it would be tricky to know just where the modulator pin is as it could push back against the PWM drive.. being a strong magnet I could probably drive it with a relatively low frequency from a microcontroller and it would work..



I dont know.. I think ill spend more time on my Glass Cockpit and better-controlling my Diesel coolant heater.. as well as seeing if I can recover more heat from its exhaust.. the efficiency of those heaters seems low.. maybe an EGR cooler and a secondary cooling loop (demonstrated by a youtuber) is the way to go.. likely more merit than dealing with transmission modulation for units out of production since 2003.. that could all change if I buiy a bus that has an E engine and a mechanical trans.. oh wait.. i did.. and junked the POS mechanical trans..
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Old 02-04-2021, 09:45 PM   #12
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Quote:
Originally Posted by Booyah45828 View Post
How big is the bus, what hp rating do you have, and what size hill are you talking?

Like s2mikon stated, your speedometer won't have anything to do with it. If it's a t444e it will have an electric modulator, which is known to go bad. Cruise on flat ground around 35 mph and then floor it. If it doesn't downshift into a lower gear, your modulator isn't working and needs to be fixed.
Thanks to all that have replied. My bus is a 33' flat nose, rear engine, T444E 230hp. With the new 4.10 gears, it cruises at 60mph at 2000rpm. I have a pretty steep grade that's about a mile long by my house and it pulled it at just about 55mph with a strong 65mph run at the bottom. The problem is when I don't have that fast run at the bottom and I start loosing speed, it "seems" to not go out of 3rd gear lockup soon enough to have enough rpms in 2nd.

I'm not sure what type of modulator I have (seems most of you think it's electronic), so I'll get under the bus and get a picture.

I'm hoping to have insurance on it in the next week or so, and with the newly updated parameters, take it out and see what she does. I'll have more details then.

Until then, I'll try and take all that everyone has asked, organize it and try and come up with some answers, yet more likely I'll have more questions!

Thanks again!
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Old 02-05-2021, 08:46 AM   #13
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You may have to help it down shift with the manual lever. Remember the VW buses and beetles? They had hash marks on the speedometer noting downshift speeds 3 hash marks for 3rd 2 marks for 2nd and 1 for first. Besides if your trimmer valves are set right it won't let you downshift at to high a speed any way. I think that this was the real problem with the at545's. When I had a towing service I bought a new gmc 6500 with a 545 and 8.2 fuel pincher and it would not down shift until it was to late. I took it to the Allison dealer and had them install an extra cooler with an electric fan and a temperature gauge on the converter out line at the transmission. The Allison dealer told me to never let it get over 250 degrees converter out and downshift it manually to keep the rpms near 2-3 hundred rpm short of governed speed. I towed motorhomes, 1 ton trucks and more than a few 2 ton trucks over I-5 and Tehachapi grade. A few times I was down to first gear when I went over the top, but I never had a problem with it. When I sold it it had 185000 miles and only had a few oil and filter changes. I kept the International truck with the gas motor and At545 and still have it. Take care my friend.
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Old 02-05-2021, 08:04 PM   #14
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Remember the VW buses and beetles? They had hash marks on the speedometer noting downshift speeds 3 hash marks for 3rd 2 marks for 2nd and 1 for first.
Did they really? I used to drive a Beetle (old style) and I never noticed that. Of course the speedo didn't work so maybe that's why.
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Old 02-06-2021, 11:03 AM   #15
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MT643 Modulator Type

The modulator is electric (see pic). I've consolidated all the tests you have all suggested, although I will pass on the stall test for now, and hope to run all of these in the next week or so. Thanks again!
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Old 02-17-2021, 12:50 AM   #16
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UPDATE:

Don't ask me why the system stopped using my nickname and switched to a old pic, but I'm the OP.

Milestone day, I registered my bus as an RV today! Insured and street legal, yay Baby!

Anyway, with a new fuel filter, fuel additives and air cleaner, I hit the road today for initial shift tests.

Starting from zero, pedal to metal:
1st > 2nd 2400 rpm
2nd > 3rd 2400 rpm
3rd > lockup 2200 rpm, but then the rpms drop to 1400 and I loose a lot of torque until I start to reach about 1850, then it gets up and goes
3rd lockup > 4th lockup 2200 rpm

Downshifting @45mph in 3rd:
Going 45, applied hard full throttle and no shift. The bus started to build speed and as it built rpms into the 1800 range, it really took off.

Going 45 again, applied hard full throttle and no shift, so I manually shifted into 3rd and it downshifted to 1800 and quickly went up to 2100, then I manually shifted into 4th.

Again, T444E with MT643 and electronic modulator. I (think) understand how the modulator affects the transmission valve body governor pressure that adjusts the shift points. Since this type of modulator is either on or off, and based on previous feedback, I'm assuming my modulator is probably not working. So, I need to test it. I'm assuming this is done by measuring resistance?

I've tried finding from where the modulator receives its signal. I've spent a good 90 minutes researching how my bus throttle system works, but I'm not positive this is absolutely correct. I "think" there is an accelerator position sensor and/or a throttle position sensor. I believe this information is provided to the ECM for injector pressure regulation. I'm "guessing" the tranny modulator gets this same information from the ECM and uses it to turn on/off the modulator???

Help! Thanks!
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Old 02-17-2021, 08:10 AM   #17
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Again, T444E with MT643 and electronic modulator. I (think) understand how the modulator affects the transmission valve body governor pressure that adjusts the shift points. Since this type of modulator is either on or off, and based on previous feedback, I'm assuming my modulator is probably not working. So, I need to test it. I'm assuming this is done by measuring resistance?

I've tried finding from where the modulator receives its signal. I've spent a good 90 minutes researching how my bus throttle system works, but I'm not positive this is absolutely correct. I "think" there is an accelerator position sensor and/or a throttle position sensor. I believe this information is provided to the ECM for injector pressure regulation. I'm "guessing" the tranny modulator gets this same information from the ECM and uses it to turn on/off the modulator???

Help! Thanks!
Your speeds might have been too fast. Try your tests going around 35 mph.

t444e will have an electric modulator. It's controlled by the Engine ecm based off of throttle input. When you're above 3/4 throttle, the ecm will send voltage to a relay, switching the relay on, giving 12v power to the modulator solenoid.

You can test it by removing the modulator from the trans, and with the key on-engine off, floor the throttle, and the plunger should push out. If it doesn't check to see where you've lost power. The fuse for the circuit could be blown, the relay failed, or the modulator solenoid itself failed.
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Old 02-17-2021, 08:14 AM   #18
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I think your modulator is ok. What are the shift speeds at part throttle? If lower rpms than wide open then the modulator is working. In the valve body there are trim adjustment rings that you can use to set the full throttle shift points. Allison states 200 rpm below governed speed. Mine is 2600 and shifts at 2400. First to second is generally set 400 below governed speed. This adjustment also changes the downshift speed at light throttle which can be annoying. You can just use the manual lever to raise your shift points to 200 below governed speed if you don't want to adjust it at the valve body. Allison dealers at one time had a jig they could put the valve body on and make all the adjustments. I'm sure a new service manager came along and had the janitor pitch it in to the scrap bin by now.
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Old 02-17-2021, 08:30 PM   #19
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UPDATE:

thank you for all the information and education.

Here are my shift test results.

Gradual speeding up:
Shifted up through all four gears.

0 - 60 pedal to metal:
1 > 2 shifted at 14mph @ 2350
2 > 3c shifted at 26mph @ 2450
3c > 3l shifted at 45mph and dropped to 1450
3l > 4l shifted at 2200

15 mph start up a two mile 6% grade:
Gained enough at the 1/2 mile run up to get close to 55, then the RPMs started to slowly drop. At 1600 I manually downshifted to 3rd and it popped up to 2000, then started climbing. I had to pull back on the accelerator keep it at 2200 and pulled the rest of the hill at 45mph.

I'll probably try to cycle the accelerator on future tests to see if it automatically downshifts, but I'm okay with an occasional manual downshift.

My speedometer, after updating parameters, was spot on!
My cruise control worked great too!

Adding onto changing my air and fuel filters yesterday, I changed my oil, including hpop reservoir, today. And now that I'm confident in the tranny, that will likely be tomorrow and finally coolant and hoses.

While not complete, she's functional enough for starting short camping trips.

Thanks again to all the great input and feedback you've provided over the last couple of years. I've learned a lot and have a lot more to learn. Hopefully, I'll be able to be more of a contributor than and a questioner down the road.

Woo hoo!
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Old 02-18-2021, 08:39 AM   #20
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That's fine and dandy that you can shift it manually and get it to function how you want, but it still isn't right.

It should downshift to 3rd automatically once you get below a certain rpm. If it doesn't, your modulator likely isn't functioning. I would say 1600 rpm is well below that speed.

It should also drop down into 3rd gear when you floor it going 35 mph. Once again, if it doesn't your modulator likely isn't functioning.

Also, seeing how your shifts happen below governed speed, and your lockup occurs at 1450 is kind of a telltale of a failed modulator as well.

You can drive it as is, ruin your trans, I really don't care. But the most common reason for failure on the old mechanical allisons is a broken, misadjusted, or non-operative modulator. Yes, the MT series transmissions are more robust then the AT series, but that doesn't make them bulletproof and impervious to this issue.
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