Cruise Control

marpel · January 12, 2023

'22 Sierra 2500HD, 6.6L gas.

Was driving along a rural road today, through rolling hills. Up and downhills of fairly short distances. Can't relate the exact incline but nothing too serious. Speed limit at 60 kmh (not mph), and decided to use Cruise Control. so set it at 63. Did as expected on flat sections but on the first downhill, the speed reached 82, before I pressed the brake. The truck had downshifted and RPMs up to 3500 - 4000, before I applied brake, so it appears it was responding to the speed, but I was not prepared to see how far this would go.

I have driven previous truck (Sierra 1500, 6.2L gas) many times along same stretch and also used CC at same speed, and it never went above 70-ish maximum before it slowed itself down, which was why I was quite surprised on this occasion.

CC works fine on level terrain or slight inclines. I recognize CC will not keep speed right on set speed and will vary, depending on steepness of incline, but is at least 19 over the set speed a bit excessive?

Marv

rdonarski · January 12, 2023

That surprises me. I have a 2020 2500HD with the 6.6 gas. I sometimes pull a trailer with a skid steer on it (probably around 10K lbs total). I have been amazed how well it handles going down a large hill by my place in cruise. Downshifts, I don't think it has ever sped up over 5 MPH. Never hit the brake. Actually, does better on that hill than my car does.

sheath · January 12, 2023

I've noticed cruise control works better for controlling speed in Tow Haul mode. The down side is Tow/Haul mode is more aggressive with shifts up and down. At least that is the way it seems to work on my truck.

KARNUT · January 12, 2023

Regular cruise control can only do so much going downhill over speed is possible. Adaptive cruise or hill descent control uses brakes and can maintain speed going downhill. If increased speed is a problem going downhill then you probably have regular cruise control. When I was pulling loads I usually would engage the trailer brakes to save on overheating the truck brakes.

marpel · January 13, 2023

Yes, I have regular Cruise Control, not Adaptive or HDC. However, as initially noted, I have driven the same route a number of times in previous truck (also no HDC or Adaptive) and CC has always worked as I expected. Just seemed weird this time and for a second I actually thought CC had somehow deactivated itself. But the higher rpms then seemed to indicate otherwise. One would think, especially for an HD, that if reasonably gentle foot braking can slow the truck that CC would be able to do same.

NaturallyAspirated · January 13, 2023

When I’m in CC not in Tow Haul Mod, I’ve had a nice experience of controlled speed when I was in a low gear before the descent. You got me thinkin… maybe I should try some experimentn’ with tricking her into a lower gear before the descent.

Edited January 13, 2023 by NaturallyAspirated

Jettech1 · January 18, 2023

On 1/13/2023 at 1:08 AM, marpel said:

Yes, I have regular Cruise Control, not Adaptive or HDC. However, as initially noted, I have driven the same route a number of times in previous truck (also no HDC or Adaptive) and CC has always worked as I expected. Just seemed weird this time and for a second I actually thought CC had somehow deactivated itself. But the higher rpms then seemed to indicate otherwise. One would think, especially for an HD, that if reasonably gentle foot braking can slow the truck that CC would be able to do same.

That is kind of odd. Loaded or unloaded with the cruise set I've never experienced more then 4mph difference up and down hills. The trans will downshift going down steep hills enough to keep it within range we will say. So I have no idea why yours is that excessive. That would definitely concern me too but I can't think of any reason that would be happening....very odd.

marpel · January 19, 2023

Seemed odd to me too. And I forgot to initially mention, this was with no load.

I have driven my prior truck to/from Vancouver, B.C. and Denver, Co. multiple times (also unloaded), mainly along the Interstates, with CC on for much of the drive and even on the steepest descents I don't recall it ever getting anywhere near this far over the set speed. Having said that, although it was the 6.2L with tow package, it was lighter than the new one.

Jettech1 · January 19, 2023

1 hour ago, marpel said:

Seemed odd to me too. And I forgot to initially mention, this was with no load.

I have driven my prior truck to/from Vancouver, B.C. and Denver, Co. multiple times (also unloaded), mainly along the Interstates, with CC on for much of the drive and even on the steepest descents I don't recall it ever getting anywhere near this far over the set speed. Having said that, although it was the 6.2L with tow package, it was lighter than the new one.

With no load makes this even odder. I wish I had an answer for you, but I can't imagine it being normal. What the fix is for it, I honestly have no idea. I hope you report back if you find one in the event others might have the same problem. I think most will agree that's not normal what you are experiencing. But I'm racking my brain trying to think of why? It makes no sense and is certainly not how my truck reacts with or without loads with the CC set. Have you had the codes read? Do you have a check engine light on? All I can think of is not just one but two bad wheel speed sensors. I'm probably way off but there is something that's not reading right to the ECU.

Edited January 19, 2023 by Jettech1

Dr1ft3r · January 28, 2023

I noticed a similar issue with my truck. I was going down a 7% grade for 10 miles with CC on and I had to keep tapping the brakes to keep the truck from going 5+ mph over the set CC speed. The truck was empty and I wasn't towing anything.

I tried using the manual shift button but I think I need to shift to "L" to use it. The truck was brand new at the time and I'm still learning it. My previous vehicles would downshift automatically so this was new to me.

Next time I'll try turning on tow mode to see if that helps. 5+ mph over the CC set speed seems excessive to me.

marpel · January 30, 2023

For those who didn't happen to read another post, in which I added an update about this thread as well, I visited the dealer the other day, for another matter, and asked a senior service rep about the Cruise Control and described my previous experience. He advised CC is not really intended to slow down a vehicle, but to accelerate up to the set speed and "maintain" it in that general area. I didn't really understand that, and as he gave me some questionable info on another topic, I left it there.

Anyway, as has been my intention, I had a chance to drive the same route I initially posted about, and set the CC to the same speed (63kmh in a 60 zone). I was prepared to just let the CC handle the up/down hills and not manually press the brake, just to see what it would do.

On some in/declines, it was like the truck was a bit schizophrenic. I can only describe the management of the set speed as "clunky". The transition was not smooth and not even close to my prior truck.

On the exact decline as in the first post, I just let it do it's thing and by the time it reached the bottom (fairly short hill as well), it had reached 85kmh. It then downshifted and hit 3000 rpm and slowed, although by that time, it was at the bottom of the hill anyway. Not sure what it would have accomplished (or not) had the decline been longer in distance.

On relatively flat grade, the CC acted as I expected it should.

Not really that impressed, though, especially for an HD.

Marv

Jettech1 · January 30, 2023

12 minutes ago, marpel said:

For those who didn't happen to read another post, in which I added an update about this thread as well, I visited the dealer the other day, for another matter, and asked a senior service rep about the Cruise Control and described my previous experience. He advised CC is not really intended to slow down a vehicle, but to accelerate up to the set speed and "maintain" it in that general area. I didn't really understand that, and as he gave me some questionable info on another topic, I left it there.

Anyway, as has been my intention, I had a chance to drive the same route I initially posted about, and set the CC to the same speed (63kmh in a 60 zone). I was prepared to just let the CC handle the up/down hills and not manually press the brake, just to see what it would do.

On some in/declines, it was like the truck was a bit schizophrenic. I can only describe the management of the set speed as "clunky". The transition was not smooth and not even close to my prior truck.

On the exact decline as in the first post, I just let it do it's thing and by the time it reached the bottom (fairly short hill as well), it had reached 85kmh. It then downshifted and hit 3000 rpm and slowed, although by that time, it was at the bottom of the hill anyway. Not sure what it would have accomplished (or not) had the decline been longer in distance.

On relatively flat grade, the CC acted as I expected it should.

Not really that impressed, though, especially for an HD.

Marv

There is something wrong, what it is I wish I knew. But that is not how my truck reacts when in CC. There is definitely something wrong, I wish I could help.