Downshift Prevention Switch ?

[Ross]

Hello, all you wizards --
I have a 2005 Thomas mid size dog nose with MBE 906, Allison 2000 (5 speed), and air brakes. GVW is 27,500 lbs, actual bus weight is approx 19,000 lbs.

My concern is transmission downshifting. In every respect, the trans works fine, but I notice it downshifts too easily, or too soon. At least it downshifts earlier than I would like. I have changed the fluid and external filter to Valvoline Synthetic. (previously had Transynd, I believe) The trans operation was previously fine, and still worked fine after fluid change.

On our first long trip this past winter, pulling a 2800 lb toad, I would start up a grade at 55 mph (1900 rpm) and the trans would downshift fairly quickly as our speed slowed down. I could not press the throttle pedal lightly enough to prevent downshifting, even being willing to climb the hill slowly. I feel that the engine would pull just fine at rpms down around 1400-1600, even as low as 1200, but the trans insists on down shifting, keeping the rpms up at 2200 or more.

I'm fine with climbing hills at whatever speed is necessary, but I'd like for the rpm to be able to drop further before downshifting. I feel like if I had a manual gearbox, I could let the engine rpm drop below 1500 or so and it would just hang on and pull.

Is this some adjustment that can be performed by an Allison shop?
Or am I crazy for wishing for a Downshift Prevention Switch?

Thanks in advance.

[magnakansas]

The size of the engine, the load the engine has to pull. I think it is possible the exhaust will be too hot. More boost from the turbo will help to keep temps lower. I think you feel like the engine can pull the load, but it might not be good for the engine, rpm too low.

The Benz power train people have plenty of experience with diesel turbo engines, if they have this kind of shift strategy, until you get EGT gage on this. I can’t recommend change.

This is just my gut level reaction to the numbers you posted and based on experience with other engines.

William

[TJones]

The smaller displacement engines are designed to run at a higher RPM. If the engine is under full load at 1900 rpm It would likely down shift to a lower gear to reduce the load on the engine and increase to an RPM where the engine makes peak HP which probably somewhere around 2300 RPM.

Like William said lugging an engine will increase the exhaust gas temperature and could damage the engine.

Ted

[cadillackid]

you WANT your allison to drop to a lower gear.. otherwise you are exceeding the definition of lugging the engine..



most Diesel engine throttle strategies are essentially where the pedal position is an RPM setpoint.. unlike a gaoline car where if you are in neutral and press the throttle halfway down the engine will take off and max the rev limiter..



diesel engine in neutral you can press the pedal to 50% and the engine will hit a certain RPM setpoint and hold there.. if the engine cant make that setpoint then it fuels heavier to attempt and get there.. so if you are at 1300 RPM and you give it a little pedal where it wants to be at 1800 and the trans stays in current gear , the computer will run the engine up heavy on the fuel.. and now you are lugging it.. losing RPM amnd yes even thouigh your pedal may only be at 50% the computer can be running 100% fuel for that RPM... so the design is to downshift the trans to a gear that most matches the RPM setpoint of the current throttle position.. you WANT the program to work like this..


I have the ability to program every gear setpoint on my Allison.. and I learned a quick lesson with some of my first programs that attempted to run it like a car (highest gear possible).. I watched my EGT gauge shoot up like a rocket... I adopted a very easy downshift pattern now.. and in fact my ECM helps me out as when it detects the engine loads to be really high it sends a "kickdown" command to my allison. my allison will downshift if it is within its parameters to do so.. (wont over-rev)....



you can probably buy a programmable TCM and EFI Live then take a stab at programming your own shift curves .. but theres more to it than meets the eye..





the engine Loading factor within the ECM (if you run a Bluefire or such you can probably see it).. is calculated by how much the desired RPM and actual RPM are apart and also factors in fueling.. more modern diesels actually will monitor their own EGTs and de-fuel if you lug it.. I dont know about Merc, havent owned one..

[cadillackid]

in fact I should add that in my trans swap thread I talked about how LACK of downshifting was causing me issues at first

[Ross]

Thank you all for your responses. I may have worded my inquiry in an unclear manner. I'm not trying to aim for 1200 rpm, or even 1400-1500.
What I'm experiencing is that as soon as I start to climb, and before the rpm really drops at all, the trans down shifts and suddenly the rpms are up at 2200-2300. Maybe the load vs power available is so extremely close at cruise
(55 mph/1900rpm) that the trans is just barely hanging onto 5th gear as it is. I don't know. Obviously, I'm not REALLY looking for a "downshift prevention switch". I was just wondering if it was possible that "downshifting too easily" was an actual thing. It just feels like the trans shouldn't need to downshift quite so readily.
But you all know a lot more than I do about such things, I trust your opinions.
Thanks.

[Oldyeller]

Maybe you should increase the size of your data sample before drawing any conclusions. You suggested, in your first post, that you took the bus on its "first long trip this past winter" pulling a 2800 pound car. How many other trips have you taken; with/without the car in tow? Maybe as you get more miles in the bus towing and NOT towing you'll get a better feel for how it behaves. I'd be especially interested in how it behaves over the same route with AND without something in tow.

I have an '04 Expedition (with towing package) that I've used on trips a number of times, sometimes towing a 12' motorcycle trailer full, sometimes empty and sometimes not towing at all. The shift pattern is radically different between towing heavy and not towing at all. I imagine it would be much more pronounced in a bus with a much lower power to weight ratio.

BTW, if I make it out in my bus I'll be pulling the same trailer behind it loaded just like it would be behind the Expedition. That will make for an interesting comparison - hmmmm hadn't even thought about that till now.

[dozr62]

It’s supposed to shift like this keeps the rpm’s up and fluid flow is high keeping the transmission cool if you have a manual shifter with the 234D as soon as it down shifts manually drop it in 4 and see if it waits a little longer than manual down shift again but it’s programmed that way to prevent over heating the transmission beginning with torque converter failure ending with transmission burn up. I bought a programmer that you leave plugged into the diagnostics port it provides all the temperatures and rpm speed etc. etc. while you are moving and you can make adjustments for more turbo boost fuel economy etc. I mounted it on my top of the dash and I don’t have to look down inside the steering wheel trying to read the original cluster . It’s $150 on Amazon or eBay scan gauge.