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Old 09-22-2017, 07:58 PM   #1
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Allison 3000 shifts hard

We have a 2016 Tiffin Phaeton 40 QBH with 380 Cummins and Allison 3000 transmission. When the engine brake is on, the transmission downshifts extremely hard when the brakes are applied going down a hill. No problem with the engine brake is off. Seems like the transmission downshifts from 6th gear directly to 4th and since the rpm are low it jolts the whole coach. Anyone else experiencing this problem
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Old 09-23-2017, 08:04 AM   #2
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Sounds like you have engine brake preselect enabled. When the engine brake is turned on, the transmission uses a preselect downshift schedule (higher shift points) from sixth gear to fourth. The shift points are higher, so that keeps the engine speed up, which helps the engine brake work efficiently. The shift quality of these preselect shifts will be different that normal closed throttle downshifts. There may not necessarily be a problem. If it's not something you want your coach to do, you cold have it turned off, but the trade off is a lot less engine braking going down hills.
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Old 09-23-2017, 09:28 AM   #3
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Not sure what the engine brake preselect is or how to turn it off or on. Only switch I am aware of is exhaust brake on/off. Would prefer that the transmission shift from 6 to 5 before 4. As I mentioned the jolt is severe to the point i worry about the drive shaft or u-joints failing.
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Old 09-23-2017, 09:36 AM   #4
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Quote:
Originally Posted by curtsr View Post
Would prefer that the transmission shift from 6 to 5 before 4. As I mentioned the jolt is severe to the point i worry about the drive shaft or u-joints failing.
The engine brake can be programmed to downshift to either 4 or 2 when it engages. I too don't know anything about an "engine brake select" switch.

I have sometimes noticed a hard downshift on mine, thinking it is most likely to occur on a more gentle grade transition, particularly with cruise control involved. I am rarely able to predict it, and it doesn't happen that often on mine.
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Old 09-23-2017, 09:44 AM   #5
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Engine brake preselect is an option in the transmission programming. It's not something you can turn on or off with a switch. What the transmission does is like moving the shift selector from 6 to 4. It does shift from 6 to 5 to 4 based on vehicle speed. It only goes directly from 6 to 4 if you are going slow enough.
If the coach is new, these shifts do have to adapt.
If the coach is not new and it's something you want to disable, or you think there is a malfunction somewhere, it will have to go to a service outlet to get looked at.
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Old 09-23-2017, 09:47 AM   #6
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Originally Posted by dbarton291 View Post
Engine brake preselect is an option in the transmission programming. It's not something you can turn on or off with a switch. What the transmission does is like moving the shift selector from 6 to 4. It does shift from 6 to 5 to 4 based on vehicle speed. It only goes directly from 6 to 4 if you are going slow enough.
If the coach is new, these shifts do have to adapt.
If the coach is not new and it's something you want to disable, or you think there is a malfunction somewhere, it will have to go to a service outlet to get looked at.
Thanks for the info. I have to bring the coach in for a service recall on another item and will inquire about the transmission downshifts.
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Old 09-25-2017, 03:09 PM   #7
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I called Freightliner and discussed the transmission downshift and jolt issues. The technician explained that the transmission and engine brake are working as programmed by Freightliner. The engine brake will not activate until cruise control is deactivated and the transmission downshifts from 6th gear directly to 4th gear once the brakes are applied. He asked how often I drove in the mountains as the computer learned my driving habits and the terrain to optimize performance. Since most of my driving is on flat terrain (I live in south Mississippi) The computer must adjust to the mountain environment and the jolts would subside over time. He assured me that the computer will prevent any damage to the drive train and not downshift unless it was safe.

Thanks everyone for the advice and assistance.
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