Cruise Control Throttle Blipping

[StevenRogers]

i have not seen any postings regarding cruise control problems so here goes:

we have an 06 m45 sport. we recently took a trip to vegas from san diego. i noticed on the way home, through the mountains on the 15, when in cruise control at about 65- 75 mph, the rpms would drop from 3000 to 2900 then go up to 3000 then down to 2900 and so on (depending on speed, but about a 100 rpm drop every 1- 5 seconds) until i would cancel cruise.

at first i thought the trannie was shifting gears from 4th to 5th, but put it in tiptronic in 5th and the throttle blips would continue. and so i thought it was just because we were going uphill. but i tried it again on a straightway and going downhill, same throttle blipping occurred.

another thought i had was the intelligent cruise control being affected by all the big rigs, reflectors and other cars, since no highway lighting is available in the mountains.

after the mountains, cruise returned to normal. no more throttle blips in cruise. is this something that should cause concern? i have not encountered this in other cars.

[ken in az]

sounds like the torque converter locking up, then when more throttle is needed it goes into partial lock - or unlock depending on how you view it. Most cars and trucks nowadays have this "partial lock" programmed into the transmission to make full lock up feel less agressive.

I recomend to put the cruise into normal mode and see if it does the same thing and then possibly visit the dealer but most likely it's just an anomaly. I think the manual says no to use ICC in the mountains - maybe this is the cause?

[infinitiandbeyond]

I have encountered same. The effect is basically as if it wasn't on cruise control and you were lifting and pressing back down on the accelerator pedal. Strange indeed.

[Sazar]

You

i have not seen any postings regarding cruise control problems so here goes:

we have an 06 m45 sport. we recently took a trip to vegas from san diego. i noticed on the way home, through the mountains on the 15, when in cruise control at about 65- 75 mph, the rpms would drop from 3000 to 2900 then go up to 3000 then down to 2900 and so on (depending on speed, but about a 100 rpm drop every 1- 5 seconds) until i would cancel cruise.

at first i thought the trannie was shifting gears from 4th to 5th, but put it in tiptronic in 5th and the throttle blips would continue. and so i thought it was just because we were going uphill. but i tried it again on a straightway and going downhill, same throttle blipping occurred.

another thought i had was the intelligent cruise control being affected by all the big rigs, reflectors and other cars, since no highway lighting is available in the mountains.

after the mountains, cruise returned to normal. no more throttle blips in cruise. is this something that should cause concern? i have not encountered this in other cars.

You put your car in manual mode and then were able to use the cruise control?

Isn't it supposed to cancel out when you are in manual mode?

And when you use cruise in mountains or inclines, your car may adjust speeds, in my experience as it will slow down/speed up more or less than would be expected on a flat road.

I have had this occur in pretty much every car I have driven, unless I am misunderstanding your issue.

[StevenRogers]

nope you got it.

im almost sure i threw it into tiptronic mode, so that i could keep it in 5th gear (i thought originally it was going back and forth between 4th to 5th gear).

i have never experienced this in any other car ive drove, but i cant remember if ive ever cruised in the mountains either.

what bothers me is that it did not only throttle blip when inclining, when declining and i believe it did a little bit on the short flat straight aways. but all in the same time frame of about 20- 30 min. then gone.

but like ken in az referred to in the manual, cruise probably is not to be used in the mountains. i need to look at what else is in that manual!

[ken in az]

nope you got it.

im almost sure i threw it into tiptronic mode, so that i could keep it in 5th gear (i thought originally it was going back and forth between 4th to 5th gear).

i have never experienced this in any other car ive drove, but i cant remember if ive ever cruised in the mountains either.

what bothers me is that it did not only throttle blip when inclining, when declining and i believe it did a little bit on the short flat straight aways. but all in the same time frame of about 20- 30 min. then gone.

but like ken in az referred to in the manual, cruise probably is not to be used in the mountains. i need to look at what else is in that manual!

Did you try it with the conventional cruise control engaged yet? This type of behavior could be signs that your lock up solenoid is going out in the transmission or some type of CAN communication error hapening while in cruise mode.

And yes, you can engage it into manual mode then engage cruise. That is how I normally do it but I'm not sure if it disengages cruise when you go from auto mode and switch to manual while the cruise has already been engaged. I'll have to try it.

Steven, try the above and see if it does it, I would also recomend to videotape it and take it to the dealer the next time you are in and they should be able to tell you if it is normal operation or not. I know when I first drove my car in the mountains the cruise control was acting very strangley, but that was too long ago to remember the specifics.

Good Luck

[szh]

Cruise control can easily be fooled by changing slopes, and also when going steadily up or down hill. The optimization for maintaining a speed is done for level ground, and while the feedback system can correct for a bit of changing speed conditions, there are simply some things that cannot be corrected for.

Since going uphill at a steady speed requires more HP than level ground, this can throw off the calculations.

I suppose that if an electronic inclinometer of a sort were incorporated into cars, then that could be another variable into the feedback calculations, but until that highly unlikely thing happens , cruise controls will have difficulty with slopes.

Finally, one thing I would encourage on long uphill slopes: downshift to one lower gear than overdrive. In my 2003 M, I do this simply by moving the lever into manual mode (like I do when driving around town in slow traffic - why have the transmission upshift to highest gear too often and too much).

Then, the cruise control compensation for speed is easier - since the cruise control doesn't have to react to a "I gave it gas, but the speed did not go up like I expected" kind of moment. Plus, the transmission will be happier when the rpm's are higher and more power is available to avoid "lugging".

Z