mt654cr transmission shifting charateristics and tendencies discussion

[lindsey97]

hello everyone, I would like to start a thread specifically about how the mt654cr trans acts while upshifting/downshifting.

my truck is a 1990 am general m923a1. I have owned it about a month, and have driven it approx. 600 miles. seems to be a very solid truck mechanically and cosmetically. all fluids are clean and filters are fresh. I have adjusted the shift cable 2 turns towards the front of the truck and it responded well by holding gears longer before upshifting and downshifting somewhat more firm.

what I am concerned with is the 2-3 shift and the 3-4 shift.

i'm very pleased with the 1-2 shift and the 4-5 shift while upshifting, they are quick firm and quiet.

downshifting automatically is also quick and firm, and the truck will compression brake almost like a standard. when the truck is put into gear at an idle, it immediately goes into gear and will begin to move without use of the accelerator. what I am saying is I don't suspect that I have a worn out or weak transmission. temperature of the trans fluid doesn't change my shift characteristics either. it runs 180-190f on the trans temp gauge.

my main concern and questions are how the trans shifts 2-3 and the 3-4 upshift. it makes "errrr aagh" sound and then the tachometer settles in and starts slowly climbing again. doesn't act like the trans is slipping, the truck is accelerating the entire time this strange sound is occurring.

what I suspect I am hearing is the torque converter lockup clutch. when in hilly terrain and lugging along, then trying to accelerate it will make the sound; then I intervene and manually make a downshift to 4th or 3rd, then step on the gas bringing the rpm to 1900-2000. this seems to work quite well to limiting the trans by selecting the highest gear available manually.

there is a place near my home at exit 55 I-35 that is slightly downhill at that intersection. if I take off from a dead stop and accelerate moderately thru all 5 gears letting the trans pick the shift points, it will shift very quickly and quietly, like driving a car.

what I am asking is this normal? does the truck have a torque converter lockup clutch? is it adjustable? is there a spring kit available for the valve body that changes shift characteristics? can you install a switch that manually controls torque converter lockup from inside the cab?

so far all of my experience with this truck has been unloaded hauling no weight or trailers.

please let me know how your trucks up shift and act. some real world advice and experience would be helpful here.

 

[quickfarms]

Have you read the Allison operators manual?

The Allison is not like your car automatic.

There is a lot of adjustment on these trans but they are best adjusted on the dyno.

Do you feel an extra shift where the RPM's only drop about 200? This should be in 4 the gear, if my memory is correct.

All shifts except the last should be 300 rpm below governed and the last is 500 rpm below governed.

 

[Jason O]

The converter locks up in 2nd gear and stays locked thru 5th. After the 1-2 upshift there will be what "feels" and sounds like another upshift where the rpms drop a few hundred. Thats the converter locking up. To my knowledge it remains locked until slowing to a 2-1 downshift.

I find that when adjusted properly it does perform much like a car. In the higher gears you can force a down shift by hitting full pedal, or encourage an early uphift by letting up on the pedal for a second.

 

[davo727]

Doesn't sound like any problem to me, and the noise description also not a problem. Yes these things make all sorts of noises that Cadillac or lexus don't.

 

[162tcat]

Take it to an actual allison shop, not a tranny shop that will work on them. Have the fluid and filter changed if not already done and get it checked out/adjust. You could drive 10 of these trucks and all 10 will shift differently. The allison is a quirky but solid tranny.

 

[wheelspinner]

Mine is much much smoother when loaded heavy, really seems to like to work.

 

[lindsey97]

Have you read the Allison operators manual?

The Allison is not like your car automatic.

There is a lot of adjustment on these trans but they are best adjusted on the dyno.


Do you feel an extra shift where the RPM's only drop about 200? This should be in 4 the gear, if my memory is correct.

All shifts except the last should be 300 rpm below governed and the last is 500 rpm below governed.

I have read the TM, 9-2320-272-10, pages 2-105 and 2-106.

from what the book says, it sounds like I have been making the right decisions by limiting the upshifts by manually shifting down one or two gears on hilly steep county roads.

no extra shift in 4th gear with a 200 rpm drop. I do not believe that my torque converter is staying locked up in gears 2-4. It does seem to lock up but comes unlocked when I am cruising along at say 30-40mph and encounter a small increase in grade. this is when it makes the eerie noise.

when traveling at highway speeds at 50 mph to 60mph, the torque converter stays locked when encountering an increase in grade, loads the engine, and pulls the hill like one of my dump trucks with a standard trans. very rarely does the torque converter come unlocked above 50 mph.

 

[lindsey97]

Jason 0, I agree with what you said about encouraging an upshift by lifting on the throttle ever so slightly. my truck does just that from a stop on a slight downhill grade.

162tcat, I think I have read that before on here about going to Allison only for repairs and tune up. wish I could a few of you guys to come drive it and give an opinion. I would gladly pay for it.


wheelspinner, a farmer friend and I plan on moving approx. 300 round bales this week with the truck offroad, so that should be a good test.

I forgot to mention in my initial post that I am running 14.00 r20 super singles. my transmission oil is very clean and clear and does not smell burned. also, the trans is filled to the correct level with oil, not too high or low as per the TM.

 

[quickfarms]

I have read the TM, 9-2320-272-10, pages 2-105 and 2-106.

from what the book says, it sounds like I have been making the right decisions by limiting the upshifts by manually shifting down one or two gears on hilly steep county roads.

no extra shift in 4t gear with a 200 rpm drop. I do not believe that my torque converter is staying locked up in gears 2-4. It does seem to lock up but comes unlocked when I am cruising along at say 30-40mph and encounter a small increase in grade. this is when it makes the eerie noise.

when traveling at highway speeds at 50 mph to 60mph, the torque converter stays locked when encountering an increase in grade, loads the engine, and pulls the hill like one of my dump trucks with a standard trans. very rarely does the torque converter come unlocked above 50 mph.

I have been told that the MT654CR is considered an obsolete transmission by Allison so some of the information about this transmission is not available on the internet like the MT643 and MT653.

Once an Allison is locked it should stay locked in those gears. It should not be unlocking by it own.

The operation of the lockup removes the torque converters multiplication factor and directly drives the transmission. This function is very adjustable and can be affected by changing the modulator cable as you stated that you have already done. Did the transmission unlock in the upper gears before you adjusted it?

I would take it to an Allison rebuilder so they can test drive it. You could have a cracked lock up piston.

 

[lindsey97]

I have been told that the MT654CR is considered an obsolete transmission by Allison so some of the information about this transmission is not available on the internet like the MT643 and MT653.

Once an Allison is locked it should stay locked in those gears. It should not be unlocking by it own.

The operation of the lockup removes the torque converters multiplication factor and directly drives the transmission. This function is very adjustable and can be affected by changing the modulator cable as you stated that you have already done. Did the transmission unlock in the upper gears before you adjusted it?

I would take it to an Allison rebuilder so they can test drive it. You could have a cracked lock up piston.

I understand what you are saying about the torque converter and the lockup. when engaged, the torque converter becomes a clutch so to speak, with a 1:1 drive ratio with no slippage.

I think for the most part the lockup is working correctly in 5th gear. when offroading at slow speeds the trans temp is 200f. I can then leave my farm, pull onto a highway running 55 mph in 5th gear T/C locked up and the trans temp drops 10-20f. slippage=heat and I don't think I have a heat problem.

I read other threads on this site about the modulator cable adjustment, and the reason I adjusted it was the transmission was "short-shifting and or the T/C was coming unlocked above 1st gear or when I encountered an increase in grade.

So the modulator cable directly controls the T/C lockup? I may experiment more with the adjustment.

If I have a cracked lockup piston, can it be replaced by removing the valve body, or do you have to remove the entire transmission from the truck?

also please let me know if I have this right: from a dead stop accelerating moderately when my truck makes the 1-2 shift, then the T/C should lockup and stay locked all thru to 5th gear, and then upon stopping/deaccelerating to the point that the trans decides to make the 2-1 downshift at the correct speed, the T/C becomes unlocked again?

 

[Jason O]

Lindsey97 - Your last paragraph describe how the transmissions in my 2 trucks operate. I'm sure the T/C stays locked, as I'm able to lug the motor down to around 1000rpm before downshifting if I'm very easy on the throttle. ( doesn't mean its correct, but thats how they both run)

They do behave better with an additional load.

 

[lindsey97]

Jason O, I'm sorry but which paragraph?

BTW, my wife and I really like your bikini top in the mod section. when we were shopping for m923's we drove an older tan one that was missing the whole top. she really liked it. ever since we got ours home she's been wanting to take the soft top off. she drove it to work the other day, too.

 

[Jason O]

I was referencing your last paragraph beginning with "also please let me know". I paid closer attention to the converter when I drove the truck this AM. The converter will unlock in 2nd gear if rpms drop to around 1200. It seemed locked up in all other situations in gears 2 thru 5.

1-2 upshift, and 2nd gear lockup seem to be at a fixed rpm, not depending on pedal / modulator position. All other upshifts are dependent on pedal position, 2000rpm with foot to floor, less under light load.

Cutting out the back of the soft top has worked well so far. Almost as fun as no top, but the seats stay dry. It's taken a year for my wife to warm up to the truck, and she wouldnt be caught dead driving it. I'm looking foward to when there is an issue w/ her FJ and the 5t is her only option to drive to work.

 

[brock]

This is a great thread!

My 5 ton shifts as described above, locking in 2nd and staying locked through 5th.

I do, however, have a really harsh 1-2 shift when the trans temp is above 150 deg. or so. Any ideas why the truck would shift nice and smooth when cold then harsh when warm? I have already tried adjusting the modulator cable with no difference. The local Allison tech told me a rebuild may be in my future. My truck has low miles and looks to have a new trans in it.

 

[Jason O]

Brock - I have the same harsh 1-2 shift. It's worse under light acceleration. I'm gusseing its magnified by the backlash / takeup in the transfer case and drive axles. The 1-2 shift is much smoother under full power, as a result I floor it every time a pull out from a stop. I doubt its anything to worry about or rebuild a trans over. If my Allison fails I'll swap in a manual Fuller transmission, but thats another thread.

 

[lindsey97]

Update: went and helped 2 farmers move approx. 90 round bales on 8/5. before hauling hay, I adjusted the modulator up 2 turns shorter. I think I am now out of adjustment on the injection pump end of the cable. so that is 4 turns total that I have shortened the cable. my 1-2 upshift is now around 1800-1900 rpm, and it occurs quickly and with authority. 2-3 upshift is somewhat better and a lot smoother/faster. 3-4 upshift is ok but still makes the "errr aghh" sound, but is better than when I started.

The truck did great hauling round bales. Hauled 6 per trip, all off road in a sandy hayfield. it was 97 deg. F. Loaded, from a stop on flat ground I could put the truck in gear and it would begin to roll without any throttle input at all. all temps were the same as stated earlier. the truck required very little throttle input to move or accelerate loaded offroad. so I think my transmission is good and sound overall.

Brock-- my truck will do exactly as you described on your 1-2 upshift if I accelerate up to 1800-1900 rpm, and then quickly lift my foot off the throttle. I think sometimes I'm not accelerating enough to get smooth shifts. if you suddenly back off the throttle and the trans has already decided to make an upshift, sometimes it is a rough one. As stated earlier in this thread by another member, accelerating briskly then just barely easing off the pedal ever so slightly can encourage an upshift.

thanks for all of the advice so far. this is a great site. I would still like to know if the torque converter lockup piston can be changed without pulling the entire trans or not.

 

[quickfarms]

To change the piston you have to remove the trans and disassemble it.

If you remove the trans, do not separate it from the torque converter.

What is your no load governed rpm?

You need to know this in order to correctly drive an allison equipped truck.

There are two problems with a lot of the Allison's in these trucks. The first problem is the transmissions shift points are incorrectly set up in relationship to the engine. The second problem is the driver does not know how to correctly drive an Allison.

 

[shadowdd]

Quckfarms, what is the correct way to drive an Allison? I've read the Allison manual other than selecting the proper gear for the conditions they don't say much more about it. Am I missing something and can you give me some insight to the Allison transmission in a M923A2.
thanks, Bryan

 

[lindsey97]

quickfarms, thanks for informing us about the piston location. my no load rpm is 2150.

 

[quickfarms]

quickfarms, thanks for informing us about the piston location. my no load rpm is 2150.

Based on this the transmission should be shifting at 1850 for all but the last gear and the last shift should be at 1650