I'm looking for some clarification on acceptable RPMs for my ISM 450. Here's the info from the data plate on the engine.
Model: ISM 450
Mfg: 03/99
Idle: 600-800
Adv. Horsepower: 450 at 1800 rpm
Peak Torque: 1450 at 1200 rpm
Governed Speed: 1800
I've been fooling around with gears and retarder joystick position on different grades trying to find the sweet spot for ascending and descending.
The OM Manual notes the same 1800 governed speed that the data plate shows but also a 2100 vocational speed. In addition, it states that the engine should never be operated above 2300. Finally, it warns against ever operating above governed speed when using an engine brake.
So.... I'm going down a pretty steep grade and I pull the retarder back a couple of clicks (might have been three). At the same time I punch the down arrow on the trans control unit to 4 (might have been 3). I'm watching the RPMs on the VMS display and they start climbing... and climbing. It got over 2,000. I hit the brake and let off on the retarder.
So here are my questions:
1. Should I consider 1800 or 2100 as a not-to-exceed speed?
2. If I had not braked would the Allison have stayed in gear and allowed the engine to over rev?
3. Does the application of the retarder influence the Allison's shift points?
Thanks.
(Have to drive a few miles in order to get a cell connection so I might not be able to get back quickly)
jor
I do not think the allison will stay in the gear selected if it needs to shift it will just as it will not down shift too fast.
I do not exceed 2100 rpm.
Thanks, guys. I've got about 6,000 miles on it so far. Lots of different grades so I've been trying different revs. I've found that 1450 to 1550 seems to pull real well up. I had just never had it up to 2000 revs going down and it took me by surprise. Still confused about the terms I mentioned though. Also, the Allison book is not clear on the auto up shift going down. It states "... Transmission has a feature to prevent auto up shifting above the lower range selected. However, during downhill operation, if engine governed speed is exceeded in the lower range, the transmission may up shift to the next higher range."
There's that "governed speed" phrase again. My data plate indicates it is 1800. Also, the phrase Allison chose was "may up shift. I'll keep playing with it. Everyone have a great weekend!
jor
jor,
One big hint on grades is to descend a grade no faster than you would be able to climb the grade. On long, steep grades the safe speed may be 25-45 mph. If you are in an appropriate gear at an appropriate speed, the coach will not speed up unless you press the accelerator. If you have to use the brakes on a long grade, you are likely going too fast. Slow down a bit and the engine/retarder braking will keep you at a constant speed. You will be able to keep the engine in a proper range, also.
The "descent = climb" speed rule doesn't work for speed freaks like Metzger and Sichenze who have coaches that will run 70 mph uphill. :P
Yes, this is what it is supposed to do. Both models (3060 and 4060) work the same way. They are wonderful transmissions and have a lot of built in safety features to prevent the operator from doing something that should not happen. I am fairly sure that when the transmission is mated with the engine at the factory it is programmed for the max engine rpm for that engine. Even though you may have selected a lower gear like 4 or 5 normally it will not up shift past that gear. It engine rpm does get too high though it will up shift in an attempt to lower engine rpm. Like you said it will come as a big surprise as the higher gear is a loss of braking effect. The only thing the driver can do is hit the brakes and slow down until the transmission selects a lower gear. Another safety feature is that even though the driver selects a lower gear the transmission will not shift into that gear if doing so would over rev the engine. Again brakes must be used to reduce speed before the downshift will happen.
I know the theory about using the same gear going down and coming up the grade, but usually I have not come up that way, so what would that gear be? I have found that selecting anything above 3 on the retarder on a descent will overtemp the transmission after a few minutes. So I select 2 or 3 and a gear that will keep the speed and rpm under control. I don't select a gear that is so low that I never have to hit the brakes. A few intermittent brake applications to reduce speed should not be a problem. I just don't ride the brakes, and that seems to work well for me. Foretravel has installed disk brakes which have less brake fade than drum brakes, don't be afraid to use them. At the same time we are much heavier than a car, so we can't drive like we are in a car. I think you will find the happen medium very soon.
This is were I have noticed my using Amsoil in the Allison DOES have a better result than when it had the other make in (T-----yd). I have noticed that I do not heat up anywere near as quick when using the retarder and on our trips to mexico we have had some long fast-slow decents that test the system pretty good. The Grapevine in Calif is another one, but a lot shorter.
John H