Free 7 Day Trial RV GPS App RV Trip Planner Campground Reviews RV Maintenance Free 7 Day Trial ×


Reply
 
Thread Tools Display Modes
 
Old 05-05-2021, 11:18 AM   #1
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
Allison at643 lower gear troubles

So I have a 8.2 Detroit diesel and Allison at 643 transmission on my 1990 Chevy medicine bus. It doesn’t want to go into lower 3rd gear when going up hill. So it will kick into gear but only after it has shifted all the way up in regular drive on the highway. So it won’t shift into this lower gear in 1st 2nd or 3rd in drive. Only after I’ve gotten it up to 50 or 60mph. I just flushed out the system and changed pan filter and screw on filter. It shifts feel better but still having this problem. I can shift into 2nd lower when going slowly up hills but a lot of the time it seems to low for my needs. Any ideas?

MedicineBus is offline   Reply With Quote
Old 05-05-2021, 11:35 AM   #2
Bus Nut
 
Simplicity's Avatar
 
Join Date: Aug 2019
Location: Auburn, WA
Posts: 612
Year: 2000
Coachwork: IC / Amtran
Chassis: 3000 / 33' Flat Nose
Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
I've got three or four threads on my experience with my MT643 not shifting.

The first thing to check is to see what type of modulator you have. It's either vacuum, mechanical or electrical.

If it's electrical, then this is very possibly a problem. The electric modulators on the AT545 and MT643 are notorious for failing, so are the relays and the ECM not sending signals to the relay.

If you search on my profile and look up threads I've created, you'll see the information.

The modulator is located towards the narrow end of the tranny on the left side (if looking from driveshaft towards bell housing). If there are wires connecting to it, it's electrical (duh, right).

Hope this helps.
__________________
Steve
Simplicity is offline   Reply With Quote
Old 05-05-2021, 11:48 AM   #3
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
You have called it an at 643 I have never heard of one, AT 545 or MT 634 would be correct. Most likely it has an AT 545 with a cable modulator with the adjustment at the governor. Please check the tag on the right rear of the case.
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-05-2021, 12:06 PM   #4
Bus Crazy
 
Join Date: May 2014
Location: West Ohio
Posts: 2,332
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
As others have said, clarify what transmission you have. Is it an mt643, or at545? There is no at643,

Secondly, retype your issue to help me understand, because I'm not following the lower 3rd gear comment. What do you mean by lower gear? Your bus won't downshift into 3rd going up a hill? Are you talking about the converter clutch engagement? It won’t downshift when the shifter is in 1,2 or 3?
__________________
My build: The Silver Bullet https://www.skoolie.net/forums/f11/p...llet-9266.html
Booyah45828 is offline   Reply With Quote
Old 05-05-2021, 12:13 PM   #5
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
If the tag is gone: MT 643 is 30.5 inches long. AT 545 is 27.25 inches long.
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-05-2021, 01:34 PM   #6
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
It’s a mt643 not at 643
MedicineBus is offline   Reply With Quote
Old 05-05-2021, 04:36 PM   #7
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
Does this have a cable shifter or does it use a rod?
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-05-2021, 08:05 PM   #8
Bus Nut
 
Simplicity's Avatar
 
Join Date: Aug 2019
Location: Auburn, WA
Posts: 612
Year: 2000
Coachwork: IC / Amtran
Chassis: 3000 / 33' Flat Nose
Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
I just re-read your original post. It is confusing, so if you can go through what's happening step by step, that would be helpful.

When I re-read it, something clicked in my memory (rare these days) that the MT643 works like this:

1st > 2nd > 3rd > 3rd lockup > 4th in lockup.

I "think" that if you're going up a hill in 4th it should downshift to 3rd lockup, but it won't shift into 3rd. It will wait to get so slow that it shifts into 2nd.

Since adding in my pressure switch, I still have to test the downshifting when going up a grade (I've only done it twice and the first time it didn't need to shift and the second time I got caught in traffic) to see how it shifts.

One issue with mine may be that putting the shorter gears (5.11 > 4.10) in may be messing with things a bit, but I'll test and let you know what mine is doing.
__________________
Steve
Simplicity is offline   Reply With Quote
Old 05-06-2021, 09:35 AM   #9
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
Well it looks like there’s a cable and a rod. Rod that goes to the shifter arm and a cable from there that attaches to the T shifters rod. What should I be looking at?
MedicineBus is offline   Reply With Quote
Old 05-06-2021, 09:39 AM   #10
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
Yeah I mean the manual low gears it won’t go into. The automatic gears shift fine up and down. It’s just when I’m going up hill I try to put it into the 3rd lower gear and it won’t shift down. It only shifts when it has already gotten up to highway speed. So works fine above 50 but useless when needed for climbing hills slowly since it won’t go into gear at slower speeds
MedicineBus is offline   Reply With Quote
Old 05-06-2021, 09:54 AM   #11
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
Shift cable adjustment is very important on these transmissions. It has to be fully engaged at each position or it WILL burn up clutches. Put your gear selector in reverse and go under the bus and remove the cotter pin or clip and make sure the transmission is in reverse detent. The hole in the lever and the round pin part of the cable rod end should slide in and out freely with having to move the transmission lever to get it in. Then check all other positions. When the cables wear inside it will pull fine but will not push correctly. The end of the cable rod is adjustable to make it right. Rear engine buses wear the cables out faster than front engine buses. Then you will need to check if the modulator is adjusted and working correctly. If I remember correctly all 8.2 Detroit’s had cable modulators. It is connect at the governor in the front of the engine. The modulator is at the left rear of the transmission above the pan gasket. It should have a cable going to it. Check and let us know. I don’t think it will have an air modulator.
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-07-2021, 02:05 PM   #12
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
So took off the pin in the cable on the shifter. Went through all gears and cotter pin goes in and out of hole smooth on all. It is a cable modulator. How to I check to make sure it is Working and adjusted properly? Thanks a million for all the help
MedicineBus is offline   Reply With Quote
Old 05-07-2021, 02:36 PM   #13
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
Modulator cable at the throttle seems really rather loose. Can wiggle it almost and inch with my hands. It is in neutral with this check but seems rather loose but I’m not sure if this normal
MedicineBus is offline   Reply With Quote
Old 05-07-2021, 03:16 PM   #14
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
Get a pan and put under the modulator area and take the bolt out of the modulator retainer strap and pull the modulator out. There should be a small metal pin about 1/8” diameter inside the modulator. Have an assistant press the throttle down to the floor and make sure that the pin moves freely and in a smooth manner. Some modulators are push type and some are pull type. If I remember correctly the a fore mentioned pin should move out of the modulator as you press the throttle. DO NOT have the engine running when you test this. I would highly recommend that you keep the key in your pocket. No need to tempt someone.
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-07-2021, 03:18 PM   #15
Bus Nut
 
s2mikon's Avatar
 
Join Date: Nov 2014
Location: NM USA KD6WJG
Posts: 777
Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
Here I just lifted this from Allison:
• Connect the engine (throttle) end of the modulator cable housing to its
mounting.
• Open the engine throttle fully and check whether the throttle linkage will push
or pull the cable core when the throttle linkage is moving toward full throttle
position. If it will push the cable core, then push the cable core until it reaches
the end of its travel. If movement of the throttle linkage toward full throttle
position will pull the cable, then pull the cable to the end of its travel.
• Adjust the clevis or rod end on the cable core until it registers with the hole in
the throttle linkage lever and the connecting pin can be freely inserted. With the
pin removed, rotate the clevis or rod end one additional turn counter-clockwise
(viewing cable core from its end) for pull-type arrangement, or
one additional turn clockwise for push-type arrangement. Install the clevis
pin or rod end to connect the throttle linkage and cable. Tighten the lock nut
against the clevis or rod end. This adjustment assures the ability to achieve full
throttle on the engine without interference from the modulator control.
• Check the travel of the cable core when the throttle is moved from fully open to
fully closed position. The system is designed to provide a travel of
1.187–1.560 inches (30.15–39.62 mm).
• The most common type of mechanical control is the cable-operated actuator
with a lever to vary the force on the modulator valve. This type of control is
convertible; either push or pull force can be used on the cable when the throttle
is opened. Ensure the modulator control, when connected to the throttle linkage,
provides an increasing force against the modulator actuator rod in the
transmission when the engine throttle is moved toward the open throttle
position.
• Later model actuators include a lever marked PUSH on one side and PULL on
the opposite side. When the modulator control cover is removed, the word
PUSH or PULL can be seen and indicates how the device is assembled.
• Conversion to the opposite mode of operation is only a matter of reassembling
the internal parts. Reverse the positions of the lever, spring, and thimble to
convert it.
• Be sure the modulator control action is as required.
• Install the O-ring seal onto the modulator control. Coat the O-ring with oil-
soluble grease.
__________________
“Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?
” George Carlin
s2mikon is offline   Reply With Quote
Old 05-07-2021, 06:55 PM   #16
Bus Nut
 
Simplicity's Avatar
 
Join Date: Aug 2019
Location: Auburn, WA
Posts: 612
Year: 2000
Coachwork: IC / Amtran
Chassis: 3000 / 33' Flat Nose
Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
The MT643/AT545 modulator attacks again!

I have a true love hate relationship with my modulator. I love it when it works and hate it when it doesn't.

FYI, on my electric modulator, the piston pushes out when activated.
__________________
Steve
Simplicity is offline   Reply With Quote
Old 05-10-2021, 08:14 AM   #17
Bus Crazy
 
Join Date: May 2014
Location: West Ohio
Posts: 2,332
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
Quote:
Originally Posted by MedicineBus View Post
So took off the pin in the cable on the shifter. Went through all gears and cotter pin goes in and out of hole smooth on all. It is a cable modulator. How to I check to make sure it is Working and adjusted properly? Thanks a million for all the help
Is it on the pedal or governor arm on the engine?

Regardless, with the throttle held wide open(full throttle) adjust the modulator cable adjustment until it only has like an 1/8" of play in and out. Test drive the bus and see how the full throttle shifts are. If the shifts seem delayed(holds the gear too long before shifting), loosen it an 1/8" more and retest.
__________________
My build: The Silver Bullet https://www.skoolie.net/forums/f11/p...llet-9266.html
Booyah45828 is offline   Reply With Quote
Old 05-13-2021, 01:10 PM   #18
Mini-Skoolie
 
Join Date: Sep 2020
Posts: 19
The gears shift fine. Just when I try to manually put it in 3rd climbing hills if the bus isn’t up to 40-50 it won’t shift into it. I’ll give this a go. Thanks so much for all your help.
MedicineBus is offline   Reply With Quote
Old 05-14-2021, 01:13 PM   #19
Bus Nut
 
Simplicity's Avatar
 
Join Date: Aug 2019
Location: Auburn, WA
Posts: 612
Year: 2000
Coachwork: IC / Amtran
Chassis: 3000 / 33' Flat Nose
Engine: IC T444E / Allison MT643
Rated Cap: 72 Kids / 48 Adults
The MT643 has a valve body that is designed to semi-think for itself. It "may" be saying to itself that despite being manually told to do something, the valve body may not agree.

There are others that know this stuff much better, but I know even if my modulator is activated, the tranny will still decide when it will shift.

So, whether the modulator is activated systematically, or you are manually shifting, maybe it's just the way it works.
__________________
Steve
Simplicity is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


» Featured Campgrounds

Reviews provided by

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 03:23 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2021, vBulletin Solutions, Inc.