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Topic: jake brake downshift speeds (Read 729 times) previous topic - next topic

jake brake downshift speeds

1995 U-300 3176B CAT Allison HD4060 with Jake brake.
Not running the coach right now, so does anyone know what speed (mph) that the HD4060 transmission allows a downshift to 5th gear?  As it is now the tranny will downshift to 4th when  the jake is activated only if the speed (mph) is 60 or below. Sometimes this is a nuisance and I am trying to decide whether I should reprogram the tranny to downshift to 5th instead of 4th. I may have to take it out for a run.
Thanks,
Rick
Rick

Re: jake brake downshift speeds

Reply #1 –
Rick,

Actually, the gear displayed in the left Allison shift pad window is the "pre-select gear".  That is the gear that the transmission shifts TOWARD.  So, if your pre-select gear is 4th and you are driving 65 MPH, it will likely immediately downshift to 5th and as speed drops to 4th.  Using the down arrow allows you to select a lower gear than 4th. 

The right window tells you what gear you are actually in.

Brett
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: jake brake downshift speeds

Reply #2 –
Quote from: wolfe10 –
Rick,

Actually, the gear displayed in the left Allison shift pad window is the "pre-select gear".  That is the gear that the transmission shifts TOWARD.  So, if your pre-select gear is 4th and you are driving 65 MPH, it will likely immediately downshift to 5th and as speed drops to 4th.  Using the down arrow allows you to select a lower gear than 4th. 

The right window tells you what gear you are actually in.

Brett

 To get the info I am looking for I would pre-select (left window) 6th gear and be around 68 mph and then pre-select 5th gear (LEFT window) and get off the throttle and see when the transmission (mph) will allow the dowshifting into 5th. That's the mph I am looking for. Now I'm thinking maybe I should just go to "no automatic downshifting" upon full jake activation and select gear as appropriate. Thought's?
Rick

Re: jake brake downshift speeds

Reply #3 –
Just for giggles, the local fire dept has a jake equiped truck with thev DDC 50 series and a 5 speed Allison 3000 series, 6 th is blocked.  When slowing own, the trans down shifts to 4th and over revs the engine. The local  DDC shop says it is correct :o  only way to keep from over rev is get on the brakes HARD. This over speed has caused damage to the valve train a few times requiring tow truck to the DDC shop.
I do worry about the water in this area.

Re: jake brake downshift speeds

Reply #4 –
Quote from: Rick –
To get the info I am looking for I would pre-select (left window) 6th gear and be around 68 mph and then select 5th gear (right window) and get off the throttle and see when the transmission (mph) will allow the dowshifting into 5th. That's the mph I am looking for. Now I'm thinking maybe I should just go to "no automatic downshifting" upon full jake activation and select gear as appropriate. Thought's?

Let me restate that, since all you can do (by using the arrows and/or engaging Jake brake) is "ask" Mr. Allison to do something.  That is the LEFT window.  IF Mr. Allison determines that it won't over-rev the engine by doing what you ask, it will do it and the RIGHT window will indicate the actual gear you are in.

To determine the maximum speed at which the transmission will downshift (and also note the engine RPM at same) all you do is drive 65+, take your foot off the throttle and use down arrow to select 5th or 4th gear.  That request is IMMEDIATELY indicated in the LEFT window.  When it won't over-rev the engine (as per what Mr. Allison has been programmed) you will: feel the downshift, see the tach climb (make note of max and compare to no-load max for your engine), and see the RIGHT shift pad window go from 6 to 5.

Brett
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: jake brake downshift speeds

Reply #5 –
My jack preselects to 4th. I normally drive 63-65 mph, so letting off the gas the jake is enabled and the coach begins to brake. Understand - the jake is actuated. The downshift is to bring up the RPMs for increased braking power. I 'could' always manually downshift to 5th. or 3rd.

You 'could' also hit the brakes. You 'need' to exercise them.
The selected media item is not currently available. Dave Head & Megan Westbrook
Titusville, FL - The Great Outdoors
'98 270 buying this month
Toad is a 2018 F150 XLT
 

Re: jake brake downshift speeds

Reply #6 –
Quote from: wolfe10 –
Let me restate that, since all you can do (by using the arrows and/or engaging Jake brake) is "ask" Mr. Allison to do something.  That is the LEFT window.  IF Mr. Allison determines that it won't over-rev the engine by doing what you ask, it will do it and the RIGHT window will indicate the actual gear you are in.

To determine the maximum speed at which the transmission will downshift (and also note the engine RPM at same) all you do is drive 65+, take your foot off the throttle and use down arrow to select 5th or 4th gear.  That request is IMMEDIATELY indicated in the LEFT window.  When it won't over-rev the engine (as per what Mr. Allison has been programmed) you will: feel the downshift, see the tach climb (make note of max and compare to no-load max for your engine), and see the RIGHT shift pad window go from 6 to 5.

Brett
Brett,
You are correct, in my post I indicated shifting to 5th in the right window but I know it should have said LEFT window. I am fixing that in my post so as not to confuse others.
Rick
Rick