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Discussion Starter · #1 ·
On the highway today I noticed that the transmission would downshify going up hill but would not bo back to 6th gear after the hill was over everytime. Several times it stayed in 5th gear until I toggled the cruise down one MPH and then it would shift and I could raise it back up 1MPH and it would stay in 6th, This only occurred when ther was not a significant downhill when reaching the top. If you crested the hill and drove on level ground it would hang in 5th. Mine was built only two weeks ago so it should have the new trans calibration in it. Anyone else notice this?
 

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The fifth and sixth gears on this car seem to be set at about 1800rpm. I'm guessing that if fifth gear is steady at 1800, its not going to upshift. Have you noticed your rpm's? Overall speed is also a factor. How fast were you going. I know at 70, I am crusing at 1800 in 6th.
 

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Discussion Starter · #4 ·
You are right. At 70 is should be approximately 1800 RPM, but what I am referring to is at 70 going uphill it downshifts to 2300 RPM and stays there. It should upshift to 6th gear and return to 1800 RPM after the hill is doen, but it stays at 2300 RPM.
 

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This probably isn't the case for you, but I remember one of the other posts talking about how the car "learns" your driving style or the type of road. This person was constantly going up and down small hills, and after a while the car tended to stay in the same gear to keep revs up. It almost sounds like you may have engaged the "tow/Haul"mode (see page 373). You also might want to check out page 133 on Automatic engine braking. Hope some of this helps.
 

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mine does this intermittently as well. this is the 3rd day in the shop to address this and various other issues. still no word when i'm getting it back. tranny definitely has some problems shifting at the proper time.

for all of you in the dallas area, here is an example. last weekend, i had to make several trips across north dallas on the bush tollway. the first trip going west to east, all on cruise control, the tranny downshifted 5 times (on small inclines), and on those 5 downshifts (2 gears, first to about 3000 rpms, then to about 2300 rpms) twice it stayed in 5th at the 2300 rpm range for a period of time before going back to 6th gear. the second trip across later in the day (going the same direction as above), i did not use cruise and just used my foot to control the speed. with my foot, i was able to keep it at 65 and go all the way in 6th gear. all the downshifts really kills your highway mpg.
 

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I do get shifting down on inclines as well, but I don't seem to have the sticking in 5th issue.

Yesterday evening, after filling up with gas in Front Royal, the DIC then registered 25.9 MPG avg for the remaining 65 highway miles back to Falls Church. Using cruise set at 68 and some requirements to pass a semi or two. Then posted speed limits of 60 and 55 back in the DC area. I was impressed. Even with tremendous head and side wind we had...no...it wasn't tail wind.
 

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The transmission will "hold" a gear at times and not upshift. This is intended to reduce shift busyness. This should be most noticeable on grades or rolling hills. Most times you will notice that the transmission will stay in the gear climbing the grade and then shift at the top of the hill. Sometimes it will shift a few seconds after cresting the hill. There is nothing wrong with the transmission as it is supposed to do this. It is trying to pick the appropriate gear for you to climb the grade. In most all cases, it does a good job of this, but there can be times when it might hold just a bit too long. Problem is trying to account for all driving conditions.

On the highway, I'm sure people have noticed that a slight incline can cause a downshift from 6th to 5th gear. It is only doing 1 shift. When you see the tach move a bit after the shift, that is the Torque Converter Clutch (TCC) coming back on in 5th gear. Depending upon the length of the incline/grade and the wind, yes the wind can make a big difference, the transmission will stay in 5th gear. Once you crest the hill, it will shift to 6th gear or if you can maintain your acceleration in the next higher gear, it will shift.

I think the point here is to help people understand that there is nothing wrong with the transmission. If it didn't do this, everybody would complain that it shifts MUCH more than it already does. Don't forget this is a 5000 lb. vehicle with a V6. There might be times where you feel it gets "stuck" in the gear and cruise control might have a better tendency to do this, but most times it's doing the right thing.

I can believe a certain hill at a certain speed might have an issue, but try it on a different day when you're going downwind instead of into the wind. It most likely won't do it then. Holding the gear is normal and nobody should be "stuck" in a gear. If you think it's "stuck" in a gear, most always it needs to be in that gear to maintain your acceleration.

I've been driving around Nevada the past week and there are times when I'm on the highway doing 75 mph and "stuck" in 5th gear for miles. The reality is that I need to be in 5th gear though because I can't drive 75 mph in 6th gear on that road with the wind that's there. The next day though I was able to do it. Same road, but different wind. Just something to keep in mind next time you're out driving and think that the transmission is "stuck" in a gear.
 

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lzwhb2,

Great explanation. Thanks.
 

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great explanation, but i'm not buying some of it. first off, for me, the part of texas i live in is flat. if i can easily make it stay in 6th, then the cruise should do it as well. second, on flat land at 70 or better, unless you are driving into a hurricane, this engine should have enough torque to easily keep it in 6th gear. after all, at 70+ i would imagine you are turning about 2000 rpm, and a little wind (because of the low coefficient of drag) should have very little effect on the engine and transmission shifts. as i said before, mine's been in the shop for 3 days now, so i'm anxious to see what changes they have made to the computer that controls the transmission.
 

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texas123 said:
great explanation, but i'm not buying some of it. first off, for me, the part of texas i live in is flat. if i can easily make it stay in 6th, then the cruise should do it as well. second, on flat land at 70 or better, unless you are driving into a hurricane, this engine should have enough torque to easily keep it in 6th gear. after all, at 70+ i would imagine you are turning about 2000 rpm, and a little wind (because of the low coefficient of drag) should have very little effect on the engine and transmission shifts. as i said before, mine's been in the shop for 3 days now, so i'm anxious to see what changes they have made to the computer that controls the transmission.
The problem is that the Top Gear Gradeability for this vehicle is VERY low. That's why it can shift from 6th to 5th very often on the highway. Cruise control might tend to make you "stuck" a little more often because it can control the throttle more precisely than you can with your foot.

I'm not sure what you're expecting the shop to do with the transmission computer.
 

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I agree with lzwhb2.

I wrote about this somewhere else here on the forum. I had a recent trip from southern illinois to northern indiana...about 360 miles. A strong headwind the entire time. 30 mpg. I was going 75-80. So in reality the Acadia was pushing thru a 100+ MPH wind.

The Acadia spent much of it's time in 5th gear. Much of it's time with the TCC locked.

I was really pushing this vehicle. It still was able to deliver 18.5 MPG on this trip! VERY IMPRESSIVE.

5th gear was still very quiet. The engine did not feel stressed and it was a smooth ride.

Of course I wished it was in 6th...but considering the conditions and the super low RPM's 6th gear delivers...you have to accept this 5th gear issue in under strong winds or rolling hills.

My M45...V8 with a 5 speed tranny does not shift into a lower gear under these same conditions....BUT at 60 MPH instead of the engine turning 1600 RPM's (as in the Acadia) the M45 is turning 2000 RPM's (WITH A V8)!!! Of course this engine will not have the same shift characteristics.

1600 RPM's at 60 MPG in a 5000 LB vehicle is going to deliver great MPG's. But when you introduce some more challenging conditions it's going to shift to 5th plain and simple.

FYI...on my return trip home I got 21.5 MPG (Fully Loaded Acadia!!!) with no wind at all and the same 75-80 MPH!!! Amazing!!!
 

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Discussion Starter · #13 ·
I agree that the transmission explanation is normally correct, but in this case I am not talking about staying in 5th for a few seconds after cresting the hill. I am talking about staying in 5th gear for several minutes after cresting the hill. In a past life I was an automotive engineer and I have never seen a trasnsmission act like this. I am wondering if the new transmission calibration might have gone too far to the other extreme to avoid the sluggishness. My Acadia was built in early April so it should have the new calibration in it.
 

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shogun95 said:
I agree that the transmission explanation is normally correct, but in this case I am not talking about staying in 5th for a few seconds after cresting the hill. I am talking about staying in 5th gear for several minutes after cresting the hill. In a past life I was an automotive engineer and I have never seen a trasnsmission act like this. I am wondering if the new transmission calibration might have gone too far to the other extreme to avoid the sluggishness. My Acadia was built in early April so it should have the new calibration in it.
The latest calibration changed this ever so slightly to avoid shift busyness not sluggishness. The previous calibration sometimes had complaints that it shifted too much under similar situations. The tough part is coming up with that balance of shifting too much vs. not enough.
 

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If you notice a lot of rollin hills coming up, just drop it into L and push the buttons. At least I think you can kick it back into 6th that way?
 

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I live in kansas not many hills but a lot of wind, on a trip with a 40 M.PJ.H. headwind it stayed in fth gear the hole trip. But on the way back 6th was not a problem. It is a heavy car and low on torque so 6th gear sometimes is hard to pull.
 

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I've noticed that also....that the tranny will hold 5th gear for a longer period of time...not just a few seconds but up and down hills as I travel down the highway.

I think the tranny takes the first couple hills...downshifts to 5th then back to 6th pretty quickly...if the tranny senses the vehicle is in a hilly area and that it is going to need to keep shifting back and forth to 6th it simply holds 5th gear until it senses the road has flattened out and will be able to hold 6th for a longer period of time.

I would love to know what the fuel mileage difference is between 5th at 60 MPH and 6th at 60 MPH. I would guess it is not that big of a difference. Maybe 1 MPG. So about 5%. If the constant shifting is bothering you...then an idea would be to leave it in 5th via the "L" mode and move to "D" or "6" when the road flattens out.

I strong headwind in a v6, 5,000 pound vehicle that is turing under 2,000 RPM's in 6th gear is going to make the tranny search for higher rev's to get more torque to maintain speed.
 

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Today on the freeway I was experimenting (again) with the button shift and found I can shift anytime up or down. I was at 60 in 5th at 1800 rpms and easily moved to 6th. Of course the really neat thing is if you want to pass, you just kick it down a gear or two and tap the gas, and very smoothly move out.
 

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RedRocker said:
I've noticed that also....that the tranny will hold 5th gear for a longer period of time...not just a few seconds but up and down hills as I travel down the highway.

I think the tranny takes the first couple hills...downshifts to 5th then back to 6th pretty quickly...if the tranny senses the vehicle is in a hilly area and that it is going to need to keep shifting back and forth to 6th it simply holds 5th gear until it senses the road has flattened out and will be able to hold 6th for a longer period of time.

I would love to know what the fuel mileage difference is between 5th at 60 MPH and 6th at 60 MPH. I would guess it is not that big of a difference. Maybe 1 MPG. So about 5%. If the constant shifting is bothering you...then an idea would be to leave it in 5th via the "L" mode and move to "D" or "6" when the road flattens out.

I strong headwind in a v6, 5,000 pound vehicle that is turing under 2,000 RPM's in 6th gear is going to make the tranny search for higher rev's to get more torque to maintain speed.
This is exactly what's going on driving this vehicle on any type of hills or grades. Doesn't only apply to 5th gear though. The transmission will do this in every gear at every speed if it is needed.
 
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