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Cruise control and 6-speed transmission


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Is anyone experiencing problems with large swings in vehicle speed when using their cruise control on hilly roads?

 

I have a 2011 Z71 Suburban with the 5.3 engine, 6-speed transmission and 3:42 rear-end. I see +/- 4 mph when driving moderately hilly highways. I have also noticed it doesn't matter whether I'm running empty, loaded or even if I lock out 6th gear. Dealer and GM tech center say it's within spec and inherent to the 6-speed. "it needs longer to think". I've never had a vehicle with a cruise like thie. I have a '92 K-Blazer (5.7TB, 3:42I w/ 400,000 mile), a '99 Buick Regal (3.8 w/ 250,00), and previously owned a 2005 Suburban (5.3. 4-speed, 3:42 w. 75,000) and drive the same roads. These vehicles don't vary by more than +/- 1mph.

 

Currently I'm not real satisfied with the answers I'm getting and thouoght I'd ask forum members to weigh in.

 

Don't get me started on AFM. I'll save that for another post.

 

Thanks

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Mine overshoots sometimes though it isn't constant. I hate using cruise in the hills because it's too damn aggressive IMO and needlessly drops more gears than needed on hills. Living in Michigan and driving in the surrounding states is fine though cause there are no hills here. :lol:

 

 

If AFM is a bother around town, drive the truck in M4 or M5. All the newer vehicles love to run in high gears for fuel efficiency and it's stupid. Keeping it out of 6th prohibits AFM from turning on.

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Yes, really annoys me. Do you have a tune in that truck, or is it stock? My 6.2L has a tune, so I was thinking that might be partially to blame, just wondering if you do....

 

Cruise on my GMT-900 is brutal. Pretty sure it could force a drop to 5th if the speed starts taking off (but it doesn't). I find it's stupid late to accelerating when the speed drops as well.

 

Heck, if I go waaayyy over the set point in my 3-series BMW, it'll yank on the brakes to bring the speed back in line to the set point.

 

BTW, another user pointed out to me if you read the manual, they already have the cleavat in there that hills will cause it issues. Nice of them to cover their @$$ on this one.... :)

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No trouble on my 2012. Much better than the 07 Classic that would wait and then just give it's all up hills and be screaming in 2nd. The new one does it nice and calmly but will allow some leeway on hills which I am fine with. Oh and if I get overspeed it will downshift to bring it back down too.

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Yes, really annoys me. Do you have a tune in that truck, or is it stock? My 6.2L has a tune, so I was thinking that might be partially to blame, just wondering if you do....

 

Cruise on my GMT-900 is brutal. Pretty sure it could force a drop to 5th if the speed starts taking off (but it doesn't). I find it's stupid late to accelerating when the speed drops as well.

 

Heck, if I go waaayyy over the set point in my 3-series BMW, it'll yank on the brakes to bring the speed back in line to the set point.

 

BTW, another user pointed out to me if you read the manual, they already have the cleavat in there that hills will cause it issues. Nice of them to cover their @$$ on this one.... :)

 

If you were asking about my truck...nope, no tune. All stock, I can handle the over shoots but yes when it slows down due to hills, the truck tends to wait waaayyy too long to down shift, by then it'll jump down to 2nd and go like...the dickens, then all is well until the next hill. However, this is nothing worth trading my truck in for and even to get it "fixed".

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Thanks for the responses. No my truck isn't tuned, completely stock. When I say hilly roads, I'm talking about I-94 through MN and WI, HWY 29 across WI or some the better rural 2 lanes in MN and WI like MN-89 and 64 from Bemidji to Little Falls. Not the hills of TN or the long grades out West. Incidentally, the truck handled I-70 through the Rockies better than it handles the MN and WI hills. And I do like the 6-speed as there is not the huge downshift that leaves you screaming up a hill with the tach reading 4000RPM, which was my experience with the 4-speed.

 

Wires - I agree wholeheartedly with your statement "I find it's stupid late to accelerating when the speed drops as well".

 

The other part of the problem is that the truck is "stupid late" backing off the gas when it reaches the set point.

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My 2012 does this and my '07 Vmax did as well. My '02 HD 6.0 did not, or at least not as bad. I'm told that it is the throttle by wire causing a slight delay in response, and that it takes more time for the throttle body electric motor to respond than it did for the good old fashioned throttle cable. Sometimes on steep hills I will help with my foot to not let it get so far behind, which seems to help it to not overshoot so bad at the top of the hill.

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Thanks for the responses. No my truck isn't tuned, completely stock. When I say hilly roads, I'm talking about I-94 through MN and WI, HWY 29 across WI or some the better rural 2 lanes in MN and WI like MN-89 and 64 from Bemidji to Little Falls. Not the hills of TN or the long grades out West. Incidentally, the truck handled I-70 through the Rockies better than it handles the MN and WI hills. And I do like the 6-speed as there is not the huge downshift that leaves you screaming up a hill with the tach reading 4000RPM, which was my experience with the 4-speed.

 

Wires - I agree wholeheartedly with your statement "I find it's stupid late to accelerating when the speed drops as well".

 

The other part of the problem is that the truck is "stupid late" backing off the gas when it reaches the set point.

 

 

My 6.2L does exactly that. Quick rolling hills really messes it up. Pretty sure one could drive hammered and still maintain a more consistant speed! My GMT-800 was right on the money all the time. I'm going to try tow/haul this weekend to see if that helps the swings.

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Mine acts the same way as the OP's. Will drop 5-7 MPH going up hills and increase 10-15 MPH down hills.

 

Towing a trailer with the cruise on is a test of patience, to put it nicely.

 

I only tried towing my trailer and using cruise once....and never did again. What a pain in the....neck!

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So I played with this on the weekend. I found running the 6 speed in tow/haul mode (without a trailer) does help maintain speeds a lot better. The truck still fails to deal with overspeed going down hills (at least on my '08 -- maybe there was an update for the newer trucks?) I set the cruise for 66MPH in tow/haul mode. Going down a steep hill it hit 80MPH, still in 6th gear, with no hint of a downshift.

 

I did find that with tow/haul the underspeed/overspeed isn't as pronounced as without it on. Overall MPG ended up being about the same, since it would bump back to 6th on the flats, but dropped into 5th quicker under slight hills.

 

With tow/haul off, it was just brutal. The speed would drop 4MPH before it started to compensate. And cresting a hill it would overshoot by 3MPH.

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