Mt653 issue

[Cmatthew]

Been scouring internet for resolutions. I have a mt653 being powered by a 3208 from 1984. 6.14 ratio in back. The problem I’m having is the trans will only upshift once i reach close to max rpms. If you drive at part throttle it won’t upshift until you raise rpms until just under max revs. Pulled governor and it works properly. Modulator is adjusted according to the book. If I unhook the modulator it will shift alot better. Around 15-2000 revs give or take. The lock up seems to be working. Pulled governor filter and cleaned also. Shifts hard with modulator hooked up and not as violent with it disconnected. I know it’s a tall gear but only run about 52mph at 2800rpm.

[rossvtaylor]

Hi CMatthew, is this a new vehicle to you? Or is this a vehicle which you've had for a while and noticed a change in shifting? I've got a 3208T with a MT653 also and what you describe is pretty normal. I also had a MT643 with a 3208...and same thing. In fact, with all diesel buses, I teach to basically mash the pedal or you can cruise along in first gear all day long.

And...totally unrelated to your question...but since I guess this is a new drivetrain for you...this video might come in handy one day. https://youtu.be/VU2F-DeHqXg

EDIT to correct my rudeness: Welcome! And don't forget to add some details to your profile, like vehicle info and location...both of which can come in handy as people try to help. And we like pictures!

[Cmatthew]

Thanx for the reply. And yes it is new to me. ��

[Booyah45828]

Sounds right to me too.

IIRC governed speed of a cat 3208 is 2800 rpm.

So it shifting at 1500-2000 or so with modulator unhooked would be normal. Full throttle up shift should occur anywhere between 26 and 2800 rpm. There are adjustments on the valve body to change shift rpms, but honestly yours sound like they're correct.

You should have a change in part throttle rpm and shifting, so maybe check your modulator settings again and that you have it installed correctly. But if you see nothing out of sorts, I'd say it's just the nature of the beast.

[cadillackid]

my 643 shifts in the upper part of its RPM band.. I like it that way as i can effectively hold it in a gear esp 3rd (would ne 4th in your 653).. when climbing up.. then let off the pedal to shift it to top gear and it typically wont downshift bacj unless I crowd it pretty good...



you dont want to lug a diesel when the lockuop is engage so its better to shift it later than earlier..

you casn pull the pan and adjust the shift wheels if you dare or set the modulator a little looser.. or if you like the full throittle shifts where they are you could alter the geometry of the modulator cable to achieve earlier shifts.. but the MT series in general like to shift a little later to prevent lugging

[Cmatthew]

I will check over everything again. I was looking at shift points for mt653. They do show partial throttle upshifts at much lower rpm. I have no partial throttle upshifts. Only wot upshifts. Thanx for the replies

[Cmatthew]

Here’s what I’ve found for shift points

[Booyah45828]

And that's how it should shift.

The fact that you only get that when the modulator is disconnected tells me it's not adjusted correctly.

What adjustment procedure are you following?

[Cmatthew]

Hold throttle open all the way then adjust modulator cable so there is no more then 1/8” play at it full travel. Should be slack at fully closed throttle as well.

[Cmatthew]

I was reading 653 manual and it was saying that at wot there is no modulator signal being applied. Modulator applies pressure from closed to just below wot. So my assumption is my modulator might be faulty because it seems im only getting signal from modulator at fully retracted and thats it. As soon as you touch the throttle it must lose it signal/pressure to the governor. ?????

[Booyah45828]

That adjustment is how I do it. Does your cable retract back into the sleeve at no throttle? It should be spring loaded to automatically retract. If that spring breaks, I'm wondering if the modulator might stick full throttle.

Pull the cover off the side of the modulator and have someone in the cab work the throttle while you watch. Something seems amiss here.

Cable modulators vary the amount they apply from no pressure at full throttle to full pressure at no throttle. Default on the system is full pressure, as the cam/spring not being there won't push back on the line pressure. I'm thinking you're missing a spring, or a spring is broken somewhere, or something is installed incorrectly. I know they can be push or pull modulators, maybe that's what's flipped?

Truthfully you can continue to chase this, but I think I'd leave it be. Most of the time I'm near full throttle when my bus is shifting anyways. So long as the firm shifts are not so firm that they're shocking the driveline, I doubt you'll have issues. I'd much rather have the modulator engaged all the time, over it being not engaged and soft shifts.

[Cmatthew]

Modulator does pull back at rest with a little slack left. The linkage at throttle will only pull the cable. I see of no way for it to push the cable. The shifts are pretty hard. Especially 2-3. I will check the innards of modulator when I can. Thanx

[Cmatthew]

Pulled modulator apart. Cap came off with ball bearing. Here is pic of what I saw inside. Ball bearing rides in detent until throttle is applied, cam turn’s slightly until one of the teeth catches outer housing tooth. Then ball rides up and out of detent which pushes cam in applying pressure until full throttle is achieved and ball bearing falls into next detent relieving pressure. Springs looked and felt like they were In working order. Am I missing something?

[cadillackid]

is there more than one attachment point on your pump / pedal for the modulator cable? geometry is important.. the chart makes it Linear internal to the transmission, so any curve needs to be handle by the geometry of the application.. the pivot point of the cable.. if the cable stays 100% in-line and one end or the other(pump / pedal or anhcor bracket) doesnt deviate or pivot at all then there is zero curve to apply to the chart..



point being certain geometriesd could have the cable pull out more in the beginning of throttle travel, (increasing shift points more in the first part of the throttle).. vs cable pulling out a smaller amount in the beginning and then heavier at the end.. this how an "arc" works... again I dont know your vehicle..


it sounds like you want earlier shifts in the earlier part of your throttle but still maintain current shift points at wide throttle.. so you want a geometry that pulls out less in early throttle and heavier later.. that becomes a matter of your cable anchor point in relation to the end of the cable itself..


your cable should retract nearly fully but not slack when you are at zero throttle and should adjust to be with the 1/8" play you describe when your pedal is buried... that is your Full-range.. and an important piece and where to start.. if you cant achieve full range.. cable is either too loose at WOT or too tight at no throttle and you can get the range.. then thats the first piece of the geometry that needs fixed...



next you can work on how much pulls out when...



this is hard to explain but iom not in front of my MT643 bus to show you what I mean

[Cmatthew]

There is an anchor point just before connecting to throttle linkage. Only one connection to throttle linkage also. I have full throttle swing with no binding or stop issues with mod cable.

[Cmatthew]

Adjustment seems to be spot on.