I have had some issues with the cruise control of my 07 4-matic recently and am looking for any answers/solutions to the problem. Several times when I attempted to set the cruise by pushing down on the lever a message appeared, "speedtronic inop." It also happened when the cruise was engaged and I attempted to lower the set speed by pushing down on the lever. It always resets by turning the car off and restarting. It has never tripped the cruise and displayed the message when pushing up on the lever, only when pushing down. Today it worked just fine no matter which way I pushed the lever but when it does shut off the cruise you either keep driving without cruise control or pull over and momentarily shut off the car and restart. Anyone with similar experience? Any suggestions Glyn?

 

Oh boy. This can be numerous things. The most common cause used to be the brake switch on the pedal but it was deleted by your model & the feed comes from the BAS system. Any corrupted output from the BAS unit will cancel the cruise control.

It can be a transmission tailshaft speed sensor issue or the amplifier etc circuitry from there on.

Check fuses 56, 57 and 62 in the front SAM. I doubt it's any of those because you'd have other issues or ESP/BAS errors, but check them just in case. Also visually verify that your brake lights are working. Again, I doubt that's it because you usually get a ESP/BAS error indicator.

The problem can be the column switch itself that requires replacement. Not too difficult.

Good luck. This can be a pain to diagnose.

 

Thanks Glyn, I will check the items you mentioned. My guess from the start is a faulty column switch since it works just fine with up movement but was causing the cruise to trip off when moving the lever down only. Anyway, yesterday it worked fine no matter which way the lever was moved. If it reoccurs and everything you suggested is OK I will attempt to replace the switch. Thanks again, Les