07M45 cruise control issue

[Trendwho205]

I just bought a 07 M45 and I drove it from Miami to Birmingham Alabama I noticed when I was in cruise control that my RPM Gauge drops RPMs when going down hill in Cruise control I thought I was crazy at first but every car I have had in the past when cruise control was on the rpm gauge stayed the same meaning if it was cruising at 2200 rpm on flat land it was good and when going down hill it remained at 2200 rpms... The only time I have ever seen the rpm gauge move while in cruise control in any car is when you are going up hill the any car will probably downshift and apply gas for more acceleration thus raising rpms does any one else M45 do this or am I just crazy . I have also noticed that this car downshifts a lot is it normal to feel the car down shifting when coming to a stop its almost as if the car slows itself down when its downshifting . car only has 44,000 miles on it the only upgrades done to it is a Megan exhaust system

[The00Dustin]

That is not an issue, what other cars have you owned? The only reason the RPM wouldn't drop when going down hill with cruise control would be if the car was actually resisting gravity to such an extent that the engine needed to continue to push to maintain speed. IOW, I have to guess you were driving maybe a Jeep or something else with a beastly transmission before, or perhaps had four bad wheel bearings on each of your previous cars? Realistically, while some manufacturers would clearly beg to differ, one shouldn't expect a car to slow down just because his foot came off the gas IMO (that's what brakes are for). Presumably your previous cars slowed down much quicker than the M45 when you took your foot off the gas. My previous cars all coasted much better than the M45 and gained free speed going down hill. What's annoying with the RPM and cruise control on my 06M45 is that it has to keep jumping back up after dropping while going down hill because it is geared so low there is always a bit of engine braking. None of the three cars with cruise control that I owned before the M ever did that, because they had decent gearing and could actually coast.

Regarding going uphill, depending on the hill, this is normal if annoying (especially considering how low it's geared). However, you might search for instructions to reset your ECU and manually drive up some hills while trying to maintain speed without shifting. It can be done, and as the ECU learns your style, the cruise will start to fit your style a little better. Unfortunately, for bigger hills, this won't work. My previous cars would do this on some hills, but since the pedal moved with the cruise control, I could actually apply a little more gas in anticipation of the hill in order to prevent it. It is possible to do the same thing in our M45s, but it is trickier since it is drive-by-wire and the pedal isn't already in the current throttle position. Because of that, if you don't push it far enough, it will slow you down (at least with intelligent cruise active), and you won't feel when you get to the sweet spot, so you can easily push it too far and cause the same downshift yourself anyway.

Regarding the downshifts when stopping, this is normal, but if it seems a bit much, maybe you need to change your transmission fluid and/or differential oil. At 44,000 miles, you might even be OK flushing the transmission, but I don't know with the age on the fluid. Depends if you believe that transmission flushes can cause problems, I guess.

[Trendwho205]

I actual got the car out of the dealership todayand themaintains a constant rpm the whole time which is what I'm use to. The rpms no longer drop while going downhill in cruise control It also doesn't down shift no where near as hard as it use to they kept my car for two days and it was all electrical I had the gas pedal recall done and after that nothing seemed to work right but they had to reset the ecu and everything seems fine now even my mpgs. My car had a rough idle and the last day before I took it to the dealer the rpms idled at 1k rpms which was a absolute nightmare but all is well now I'm happy with my M

[The00Dustin]

Nice, your original description still sounds normal, but apparently it was more drastic than it should have been. Perhaps they performed a transmission TSB as well as the pedal recall. The ECU reset certainly helped either way and now you don't need to figure out how to do it for yourself.

[Larz]

I live in Florida so the terrain is basically flat. However, crossing the 17th causeway bridge to get to my house, there is a long, steep incline and decline. I reckon its about 1/2 mile up to the peak and 1/2 mile back down. I tested that drive with the cruise control ON and the revs did not change at all on the decline.

[The00Dustin]

Speed and conditions would also matter, especially in our M's. If you are driving with cruise set at 70+ MPH (ie Interstate driving) and going down a hill, RPMs likely won't drop because the wind and engine resistant at that speed would require you to keep pumping gasoline into the engine to maintain the speed in spite of gravity. Same would apply driving into the wind at lower speeds. OTOH, if you're driving <60MPH on back-highways that have real hills, gravity is going to do its thing with less wind and engine resistance getting in the way. Cruise control maintains speed by adjusting the throttle, and if the car is gaining speed, it will let off the throttle to counter that, causing an RPM decrease. It is counter-intuitive to expect the RPM to stay the same when the acceleration isn't needed, unless you're used to driving a car where it's never needed, i.e. a 3rd generation prius or another "EV" with gasoline engine charging where the battery's power is all that fluctuates. Go drive through some mountains with cruise on and come back and tell me you wished the throttle stayed at the same RPM when you were already gaining more speed than you needed and coming into a sharp turn just over the top of a peak.