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Old 05-21-2009, 10:29 PM   #1
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Forcing tranny into OD

On my Allison 5 speed tranny I have to reach at least 56 mph with no loading on the engine in order to shift into 5th gear OD. Can't get it to shift at 55, 53, etc., just above 55mph.

However, there are times when traveling in extended 45 mph zones, straight and level, with the tranny in 4th at 2,300 rpm, it would be great to shift into OD and save a little gas.

In order to get it to shift into OD I have to accel to greater than 55 mph, let it shift into OD (down to 2,000 rpm), then slow my speed back down to 45 mph (where the engines loves to be) and motor merrily along till the next stop light, and do it all over again. Does this make sense?

Seems like in this day of technology, something as simple as the ability to shift into OD without having to break the speed limit by 10 mph would be standard.

Is there a secret to forcing the tranny to shift into OD at, say, 45 - 50 mph? Cannot find anything about it in the Allison or WH manuals.

Thanks

(Note: this post was orginally posted in the Allison forum, but it does not appear that forum is visited too often, so I am reposting here for greater exposure.)
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Old 05-21-2009, 10:58 PM   #2
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Our's operates the same. I would not want to put that much load on the engine at such a low rpm.
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Old 05-21-2009, 11:07 PM   #3
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Mine goes into 5th gear at about 58 MPH, and I find that it would be nice if this would happen earlier. For that reason I went and spoke to Allison about this.

Allison tell me that you can not change manually into 5th gear so you are not able to go to 5th sooner than the transmission will automatically allow.

I argued that in many places there is a speed limit of 55 MPH and for that reason it would be nice if the change happened at that speed. Allison then told me that the transmission can be re-programmed to do this. This would take about 10 minutes and would probably cost you about $100 (all depending on if the Alison garage you asked to do this would have the software needed already or if the had to purchase it just to do your job).
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Old 05-22-2009, 04:10 AM   #4
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I guess I have a good combination of transmission and rear axle ratios - mine shifts to O/D at 50 mph unless I really have my foot heavy on the accelerator. On level terrain it can stay there in O/D as low as 45 mph.
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Old 05-22-2009, 05:12 AM   #5
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I sure wish mine would shift to OD at 50 mph.

The engine seems to run very well at speeds below 55 mph to 45 mph once it's in OD.

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Old 05-22-2009, 06:21 AM   #6
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I wanted to edit my post but even though the edit was still showing my edit was denied.

So here goes

When I had my transmission fluid changed at a Allison service center I ask them if the transmission could be reprogrammed to shift below 55 mph and they said no.

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Old 05-22-2009, 09:23 AM   #7
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Reprogramming the tranny

"I argued that in many places there is a speed limit of 55 MPH and for that reason it would be nice if the change happened at that speed. Allison then told me that the transmission can be re-programmed to do this. This would take about 10 minutes and would probably cost you about $100 (all depending on if the Alison garage you asked to do this would have the software needed already or if the had to purchase it just to do your job)."

Has anyone had this reprogramming done to their Allison tranny? If so, any problems or concerns you have?

I just know on mine, in those instances where I am already in OD but can decelerate down for the 45 mph zones and remain in OD (at ~1850 rpm), the engine/tranny combo just seem to love it. No lugging, no hesitation, etc. and I can at least dream that I am getting an actual 10+ mpg for that 10 or 15 minute time frame.
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Old 05-22-2009, 09:47 AM   #8
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There might be. GM releases updates to address customer complaints but does it very quietly. A bulletin search can be done, but it's sometimes hard to find. The best way is to get a tech to hook the scanner up, go into the programming mode and see what they have available for your chassis. There's a menu that has the various fixes to choose from. Find a good shop, ask to speak to a tech because they can explain to their service writer that even though you can't make it do anything, sometimes there is a program to fix a complaint.
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Old 05-22-2009, 08:00 PM   #9
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After driving this thing for almost 7 years ya sorta get used to it.
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Old 06-02-2009, 04:44 PM   #10
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I just called Florida Detroit Diesel-Allison today and ask them if they could reprogram my 5 speed Allison to shift sooner and they said no. To change it and make it shift sooner would cost upwards of $500.00 which would included rewiring and reprograming, and a lot of research.

The 6 speed push button can be reprogramed to shift earlier in about 1.5 hours.

So I'm stuck at 55 mph

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Old 06-02-2009, 05:31 PM   #11
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Quote:
Originally Posted by Ron & Dee View Post
So I'm stuck at 55 mph
Ron, Are you the first owner? The 55 MPH shift has not been a major problem for us.
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Old 06-02-2009, 10:01 PM   #12
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I was just wondering what the general concensus on this is.
Do you think it's more economical and better for the engine to lug or chug the engine at say 1900 RPM or let it purr at 2100 RPM?
I personaly feel it's easier for the engine and transmission to let it run to what seems to be 'the sweet spot' at about 2200 RPM. 60 to 65 MPH @ 22/2300 RPM gives me my best gas mileage too.
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Old 06-02-2009, 10:56 PM   #13
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Shifting

"Do you think it's more economical and better for the engine to lug or chug the engine at say 1900 RPM or let it purr at 2100 RPM?
I personaly feel it's easier for the engine and transmission to let it run to what seems to be 'the sweet spot' at about 2200 RPM. 60 to 65 MPH @ 22/2300 RPM gives me my best gas mileage too."

Don't know about others but my 8.1 doesn't "lug or chug" in the 1900 rpm and 45 to 50 mph range. In fact, when going up some long but smaller-incline hills, when my speed slows down to the 1900 rpm range, the engine seems to consider that rpm its sweet spot and just continues to run at that speed just fine until the incline increases enough to force the tranny to downshift.

Apparently the Allison computer thinks that to "lug and chug" in this rpm range is just fine at that speed since the tranny doesn't downshift on its own unless the incline increase.

As for mpg, I think I have tried all speeds for best mileage and have settled in at the 2050 rpm range that gives about 58 mph in no wind, flat road interstate conditions.

Any time I travel >60 mph the mileage drops noticeably, probably due to the greater parasitic drag as well as the barn door effect of the front end. My mileage is markedly better at 2050 rpm range since my speed is lower, as is the associated aerodynamic drag.

Beyond this point, I think you all provided an answer to my original post about forcing the tranny into 5th prior to 56 mph. Apparently Allison did not think it was either possible or practical to incorporate what I want to do. Guess there must be a good reason beyond my needs.

Unless there are other comments shortly, the moderator might want to close this thread.
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Old 06-03-2009, 02:54 AM   #14
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There are many times I like to drive at 50 mph, especially if the speed limit is 55 mph, and the same at 35 mph when the speed is posted at 40.

To me the engine sounds much happier running at the RPM associated with 50 mph rather that the RPM associated with the 55 mph, same with the 35 and 40 mph.

I hate to be driving through town where the speed limt is marked a 35 mph and my engine is revving at the higher RPM that is associated with 40 mph.

The lower RPM just makes sense to me and my engine is happy

Ron
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