Question about cruise control

[Lightning]

While I was driving my 2001 Tundra 4x4 V8 in a 55 mph zone in cruise control mode, I approached a downhill part (not too steep) of the road. To my surprise when I looked at speedometer at the bottom of the hill, I was going 63-64 mph! And once the road leveled, it came back to 55 mph and stayed there. I always thought that cruise controls maintained your speed, regardless of up or down grade (to a reasonable degree). Has anyone else noticed this? On other roads, when I'm going 70 mph in cruise control, it usually maintains 70 mph, up or down grade (although I hate the constant downshifting on uphills). I'm guessing at 70 mph, there's enough wind resistance to keep the truck at around 70 mph (unless it's a really steep downgrade), and maybe at 55 mph, gravity can still overcome the wind resistance, and let the truck speed up? But then again, I always thought cruise controls held your speed constant...

[RockyMtnRay]

Originally Posted by Lightning

While I was driving my 2001 Tundra 4x4 V8 in a 55 mph zone in cruise control mode, I approached a downhill part (not too steep) of the road. To my surprise when I looked at speedometer at the bottom of the hill, I was going 63-64 mph! And once the road leveled, it came back to 55 mph and stayed there. I always thought that cruise controls maintained your speed, regardless of up or down grade (to a reasonable degree). Has anyone else noticed this? On other roads, when I'm going 70 mph in cruise control, it usually maintains 70 mph, up or down grade (although I hate the constant downshifting on uphills). I'm guessing at 70 mph, there's enough wind resistance to keep the truck at around 70 mph (unless it's a really steep downgrade), and maybe at 55 mph, gravity can still overcome the wind resistance, and let the truck speed up? But then again, I always thought cruise controls held your speed constant...

All the cruise control can do on a steepish downhill is close the throttle. If the hill is steep enough...or if there isn't enough wind resistance...or if you're close to the speed at which the transmission's torque converter in OD will unlock and freewheel (or doesn't generate much engine braking when locked), then gravity will indeed speed you up significantly. I encounter this quite a bit here in Colorado with our mountain grades, particularly on the 2 lane roads with 50 to 60 mph speed limits.

The easiest way I've found to solve the problem is to keep an eye on your speedometer during descents and if the speed starts to increase while using the cruise control, just turn off the overdrive. The transmission will go into 3rd and the torque converter will lockup to provide quite a bit of engine braking...enough that the cruise control will be able to maintain the set speed. At the bottom of the hill, just turn OD back on and go your merry way.

[Lightning]

Thanks for the info! I feel better now, knowing there's nothing wrong with my truck's cruise control. I just wish they would eliminate the annoying frequent downshifting that occurs when using cruise control in slightly hilly areas. This is the only car I've owned that downshifts so much in those conditions. During my long drives, it will downshift dozens of times, and it gets old really quick. Our engine is supposed to have a lot of low end torque; I'm guessing they do the downshifting to prevent 'lugging' the engine and to improve emissions; however, it can't be very good for the tranny in the long run.

[RockyMtnRay]

Originally Posted by Lightning

Thanks for the info! I feel better now, knowing there's nothing wrong with my truck's cruise control. I just wish they would eliminate the annoying frequent downshifting that occurs when using cruise control in slightly hilly areas. This is the only car I've owned that downshifts so much in those conditions. During my long drives, it will downshift dozens of times, and it gets old really quick. Our engine is supposed to have a lot of low end torque; I'm guessing they do the downshifting to prevent 'lugging' the engine and to improve emissions; however, it can't be very good for the tranny in the long run.

Glad my explanation helped your understanding.

Totally agree with you on the excessive downshifting issue! And if you think it's annoying in relatively flat and low elevation Texas, I can testify that it's a lot more annoying when crossing Colorado's mountains...where the grades are steeper and the air is thinner (the engine has roughly only half its sea level torque). I've also discovered that doing a "resume" if you're more than about 1 mph below the set speed causes the same commotion. It also tends to overshoot the desired speed by as much as 2 mph before it finally settles down.

And no, that much downshifting under fairly full throttle is not helpful to long transmission life. It's bothersome to have to do it, but I've found that if I turn off the OD in hilly/mountainous areas then no problems.

The cause of the problem, I believe, is the ECU's logic for the cruise control is fairly aggressive and not very proportional to the difference between current speed and set speed (it opens the throttle fairly rapidly even if you're only one mph below set). Plus, on a relatively pricey vehicle like this one, it could use additional inputs such as the accelerometer that's in the Prodigy brake controller so the truck would know how steep a hill you're on. A controller with "fuzzy logic" might help this thing...as cruise control "brains" go this one isn't the sharpest tool in the drawer. But given that the truck really has very few other bad traits, I'm willing to put up with this one.

[AZ TUNDRA]

This down shifting is just a pain! Does anyone on the TS Forum work for Toyota or know someone at the factory that could reprogram whatever is making the transmisson downshift and fix the problem for all of us?

[SATundra]

Originally Posted by RockyMtnRay

It's bothersome to have to do it, but I've found that if I turn off the OD in hilly/mountainous areas then no problems.

Thats what I do if its starts kicking in/out of OD, when the road flattens back out just kick it off.

Seems like the truck used to do this ALOT more when it was newer, but as the miles rack up it hasnt been so much of a problem.

[Mark440]

I've made the trip from Dallas to Shreveport more than a few times. I set the cruise at 78....and it stays within 1 mph all the way...hills or not. It usually doesn't downshift at all.

I know...last thing you want to hear is somebody's stuff that is working ok....

[Verndog]

Originally Posted by RockyMtnRay

The cause of the problem, I believe, is the ECU's logic for the cruise control is fairly aggressive and not very proportional to the difference between current speed and set speed (it opens the throttle fairly rapidly even if you're only one mph below set).

I would agree with this. Cruise control is at its best on the long flat streches. And because of the issues with maintaining speed, and shifting I just kick it off if I have varying types of terrain.

Also you will lose mileage using cruise on hills, because it over compensates trying to maintain exact speed, where as I would pick up 5mph at bottom of hill and keep constant gas and maybey lose a couple by top on a steep incline, just to conserve resourses. Its easier on the running gear and mileage, and when you live in hilly areas like I do makes complete sence.

Kevin

[Verndog]

Originally Posted by Mark440

I've made the trip from Dallas to Shreveport more than a few times. I set the cruise at 78....and it stays within 1 mph all the way...hills or not. It usually doesn't downshift at all.
I know...last thing you want to hear is somebody's stuff that is working ok....

That is because of the year truck. The 03 and newer trucks have a higher emmisions level, and attempt to stay in the power band more to keep emmisions down. That is also why the ECU's changed in 03.

I believe both your trucks are working normal. Also at 78 MPH your closer to the power band and it will naturally shift less.

Kevin