I use the cruise control every day. Over the past two days, the engine does not stay consistent. It accelerates and then coasts about once every second. The car drives fine without cruise control.

I know that the cruise is probably a part of the engine computer and that the computer is controlling the throttle through the drive-by-wire engine controls. Is a speed sensor going bad?

Anyone had this problem?

are you sure you have drive by wire? my 99 doesnt have dbw. check out the TPS sensor. you could have bad abs pulse generator. get a shop manual to get specs for it.

I am pretty sure 99 was the first year of dbw for the 540i. I believe there are two TPS associated with the mechanized throttle plate and no idle air control valve.

When I hold the accelerator steady, the car does not behave that way, that's why I am leaning away from something like a TPS.

The speed sensor seems more likely, however the speedometer does not ever appear to move around. I wonder if the cruise uses a different speed sensor.

my civic does the exact same thing.

however, i cannot help you though.

In addition to numberous speed sensors there should also be a throttle position sensor. If it is acting up it will lead to cruise hunt, I would think that you would have erratic idle also.

Idle is OK... I'll check the Bentley manual later tonight to see if there is any description of the cruise control system.

After doing more research, it appears that the right rear speed sensor controls the cruise control. I have ordered a replacement speed sensor and I will swap it out.

There is a reason they write books. Good research. Post up after the install please.

There is a reason they write books. Good research. Post up after the install please.

Yeah that'd be great to know too when you install. I have ghosts in my cruise control AND I have a bouncy idle sometimes. Could be your same fix or maybe MAF sensor too?

There is a reason they write books. Good research. Post up after the install please.

I can't take all the credit for that - I had some help from another forum too. However the advice that I got matches the Bentley manual.

RR speed sensor replaced today... No change. Cruise control still erratic. I don't know what it could be. Car idles fine and drives fine. My next best guess would be the ABS/DSC module, but that's not a cheap fix.

I've noticed that the cruise is much more erratic the hotter it gets outside. I've also noticed that when it is hot, I can sometimes feel the antilocks kicking on for split seconds when I am slowing down... More things that point to the DSC module.

I have solved this problem. After more research I found that the two front wheels play a part in determining the correct speed for the cruise control. And then I had two cheap tires on the front, one of which was worn pretty badly due to a bad lower control arm. Replacing one of the front tires and the control arm did not work, but once I got both tires replaced with new ones, the cruise is now perfect again. I think the computer was confused because it had two differently sized wheels (and two different speed signals).

I have the same problem time to time on my auto 525.. but mine will jump rpm for a second.. then work fine for a while.. then do it again a minute or two later. Its weird. I have just decided to live with it until it gets bad.

I have solved this problem. After more research I found that the two front wheels play a part in determining the correct speed for the cruise control. And then I had two cheap tires on the front, one of which was worn pretty badly due to a bad lower control arm. Replacing one of the front tires and the control arm did not work, but once I got both tires replaced with new ones, the cruise is now perfect again. I think the computer was confused because it had two differently sized wheels (and two different speed signals).

Thanks, FD. That is extremely useful information that I'm sure will save me some grief some day.

I have a94 539 manual. the cruise will work fine for several minutes than increase 4-5 mph then coast back down. It will cycle like this a few times then slow down 10mph and back up. The control lever has not effect during this. If I reset the cruise, it will work fine then start oscillating. The car has new tires on it 5000 miles and did not do this when they were installed. suggestions please.

Check the air pressure in the tires and then check to see if any of them are worn out.