Strange Cruise Control behavior

[Mudeprived]

I been driving jeeps since forever and they don't have CC so I'm not familiar with how CC on this rig is supposed to function. Please tell me if this is normal or not:

I set CC at 60 mph and it holds that speed. When going up a hill it downshifts as expected to keep that speed and as I crest the top of the hill it starts going to 64-ish. As soon as the road becomes flat or starts downhill it feels like it downshifts or brakes are applied (rpms drop) because I'm thrown forward a little bit as it suddenly starts slowing down to the set 60 mph then rpms go back up and it holds at 60 mph. It's like that sudden forward thrust when downshifting from 5th to 3rd in a manual.

I tried maintaining 60 without CC and I let off the gas as I go downhill and there's no downshift or braking feeling like when using CC. It begins coasting and maintaining that speed.

So what's up with the CC?

 

[gamefreakgc]

I can't remember if a 99 is cable driven or not. If it is, make sure the cable does not have any slack. You can adjust it by loosening the lock nuts on the bracket and then threading the nuts up or down the threads to loosen or tighten the cable.

 

[averyislost]

Mine does this as well, even on minimal hills (33's without regearing definitely doesn't help).

Never realized it could be due to the slack in the cable! I'll have to get that adjusted.

 

[phattyduck]

When the cruise control is active, it will downshift earlier and hold lower gears longer to avoid excessive shifting. The cruise control actuator also adjusts the throttle position slowly as much as it can. In some driving situations, this can cause odd behaviors.

What you are feeling:
Up hill, downshift required.
Top of hill, lower gear held longer (more power) but slowly letting off throttle.
Too much power applied until the computer gets out of its 'smooth' range of adjustment to its 'OMG, gotta get the speed right' range - completely releases throttle, but the trans is still in 3rd.
Completely releasing the throttle in 3rd gives you the 'brakes' feeling, then it upshifts to 4th/OD, then gets back to the set speed by coasting in OD.
When it gets back on the throttle (slowly, remember?) it takes the trans just a bit to lock up the torque converter and get back to cruising normally.

This weird cycle only happens when you get to fairly sharp crests in the road... but does get worse with larger tires, etc. (more downshifting)

So, normal behavior even though its a bit uncomfortable.

-Charlie

 

[Mudeprived]

Awesome info, thanks.

 

[Mudeprived]

I wanted to bump this because I've had everyone ask what's wrong with the truck. It has also become quite annoying and I cannot convince myself that this is normal.

I don't think the truck should overshoot it's set speed, release, and then repeat every 10 seconds. I haven't taken it on a long highway commute yet but I daily drive it to work on 10 miles of highway and it is getting old.

Is there any way to remedy this bobblehead action? There is no slack in the cable FYI.

 

[Drew76]

I just did 3000 miles in the last week using cruise as much as I could, the behaviour you describe sounds exactly like what mine does.

It does downshift early and overshoot going uphill, then you get that lurching feeling when it's trying to slow down to the set speed going downhill, it is always reacting to grade changes varying the speed to maintain set point.

It's not nearly as annoying as the people who don't use cruise and constantly vary their speed by as much as 20mph on the interstate....

 

[gamefreakgc]

I wanted to bump this because I've had everyone ask what's wrong with the truck. It has also become quite annoying and I cannot convince myself that this is normal.

I don't think the truck should overshoot it's set speed, release, and then repeat every 10 seconds. I haven't taken it on a long highway commute yet but I daily drive it to work on 10 miles of highway and it is getting old.

Is there any way to remedy this bobblehead action? There is no slack in the cable FYI.

I would see if there's any issue with wiring or look to replace the cruise control module entirely with someone else's or a cheap used unit. I am not aware of any way to diagnose a faulty cruise control, if there is one.

 

[kolelt]

It’s normal. No need to replace or fix anything. It’s how the computer works in CC mode. Truck will go few mph over when going up hill. Other wise it will be maintain the set speed.

 

[jgue467]

...

Is there any way to remedy this bobblehead action? ...

Yeah, on these under-torqued 3.4 engines, I simply don't use CC when I can tell the system is gonna have to do its shifting thing on moderate hill.

It's no different on a 4 banger camry or corolla, exact same thing due to underpowered engines.

For a really smooth operation, use your own foot. LOL :cowboy:

 

[SpidermanGeek]

I don't think the truck should overshoot it's set speed, release, and then repeat every 10 seconds.

Both my Matrix and my 4Runner do this.

Matrix because it's a manual (so it can't downshift with the CC on, so often times it'll overshoot the set speed while it's "under load" or going up an incline).

4Runner does the same as well. Goes into "oh crap, I gotta make it up this hill" mode by stomping down on the throttle and holding that until the "under load" is done and then releasing when over the crest. This happens more abruptly than in my Matrix because of many factors. Heavier vehicle, larger M/T tires, 4:30 gearing, throttle and CC via cable (Matrix is DBW), bad aerodynamics, etc.

 

[CircuitCircus]

This is why I love my 5-speed

 

[phattyduck]

It's no different on a 4 banger camry or corolla, exact same thing due to underpowered engines.

That was definitely the case pre motor swap on my Camry. 115 hp, auto and AWD - basically the slowest Camry ever made. Now with a 5-speed and more than double the horsepower, it is perfectly fine up all but the steepest stuff.

-Charlie

 

[gamefreakgc]

I've never experience this behavior, but maybe because I'm supercharged? My engine does not have to work very hard on hills so there's no jerk at all, it's very smooth.

 

[AntleredRuin]

Pretty sure all it is is just the CC kicking it back to 4th gear.
It's the same feeling when you turn off and back on OverDrive. Turn it off and it drops to 3rd and the RPMs raise. Turn it back on and it lurches as it raises to 4th and the RPMs go back down. The RPMs bounce for a second as it finds the gear.

 

[Mudeprived]

Yeah, on these under-torqued 3.4 engines, I simply don't use CC when I can tell the system is gonna have to do its shifting thing on moderate hill.

It's no different on a 4 banger camry or corolla, exact same thing due to underpowered engines.

For a really smooth operation, use your own foot. LOL :cowboy:

I do for now but if I wanted a vehicle that required doing this for regular highway cruising then I would have kept my TJ that I sold for this. I just find it really odd that it performs this way. I never experienced this in any vehicle I have ever ridden in.