Cruise Control Anomaly - A6 2014.

[F4TCT]

Hi Folks,

I've recently purchased a 2014 reg A6 3.0 TDI Black Edition. All is well, besides an annoying issue with the cruise control. Basically the selected cruise control speed is always 3mph more than what the actual speed is...

For example.

I'm travelling along at 70mph, both the dial and digital readout sync at 70mph, I then select the hold speed button on the end of the cruise stork and the cruise speed registers at 73mph... The same happens if you pre-select and at any speed aswell... (if i select 60mph, the car will do 57). Passenger took a photo of the same thing at 40mph.

Cruise issue.jpg

The second issue - Are there supposed to be red LED's which run around the outside bezel of the speedo to indicate the selected speed etc? I also run a brand new S6 (literally delivered a few days ago) and obviously it works fine. Its the same C7 shape and instrument cluster and would have thought they would be the same LED light wise?

Dan

[Guest 2]

Are the tyres fitted to the car the correct size for the chassis? For the 4G BE the tyres should be 255/35/20

The cluster LEDs you're referring to come on Audi vehicles with Adaptive Cruise Control, you have normal cruise control hence why you don't see them.

[F4TCT]

Yes mate, tyres are correct. been double checked. Makes zero sense.

Dan

[Alan02]

Okay, are your tyre pressures correct due to TPM light on dash? maybe faulty? Or do you need to set them in MMI? I`ve never had a mismatch on cruise speed to actual.

[F4TCT]

Yes Alan,

Doesn't matter what the pressure is, the readings remain the same. Iv'e tried varying pressures to fault find. Tried different combinations of setting the cruise.

What remains constant is the mis match.

Dan

[Alan02]

Have you checked the value of cruise set speed to the digital display? ( not needle) as my C6 model had a slight difference at times. I never bothered looking in to it. If you had VCDS or similar you could look into it in more detail.

[ChrisCi]

In my experience, all 6 or so cars which I had in the past with cruise control always had slower real speed than indicated. Usually it is progressive, e.g. at 30mph indicated the real speed (from external GPS) is 28-29, but at 80mph indicated, the real speed is 75-77mph. It differs with tyres, even to the point that fresh ones give more accurate reading. I suppose all manufacturers set the real speed slightly below indicated, so nobody can sue them in case of speed camera ticket or anything similar. I don't think I had one car which would have the real speed the same as indicated, there is always slight mismatch to the lower side.

[F4TCT]

Have you checked the value of cruise set speed to the digital display? ( not needle) as my C6 model had a slight difference at times. I never bothered looking in to it. If you had VCDS or similar you could look into it in more detail.

Yes as previously mentioned.

The digital readout matches the dial perfectly.

Dan

[F4TCT]

In my experience, all 6 or so cars which I had in the past with cruise control always had slower real speed than indicated. Usually it is progressive, e.g. at 30mph indicated the real speed (from external GPS) is 28-29, but at 80mph indicated, the real speed is 75-77mph. It differs with tyres, even to the point that fresh ones give more accurate reading. I suppose all manufacturers set the real speed slightly below indicated, so nobody can sue them in case of speed camera ticket or anything similar. I don't think I had one car which would have the real speed the same as indicated, there is always slight mismatch to the lower side.

Yes I'm aware the speedo will under read however it doesn't answer the issue.

There must be one single pickup where the car gets the speed data from. all the systems in theory work off that. So there's no reason why the cruise should be 3mph over.

Dan

[Uberman]

Yes I'm aware the speedo will under read however it doesn't answer the issue.

There must be one single pickup where the car gets the speed data from. all the systems in theory work off that. So there's no reason why the cruise should be 3mph over.

Dan

Maybe better to get to the bottom of tyre deflation indication first. Not sure if your car has the sensors fitted to the tyre valves for puncture indication or if it infers a puncture from a speed difference between the wheels (uses abs sensors fitted to each hub). It would seem probable that if you can't clear the deflation indication that there may be an issue with one of the sensors which in turn could be a factor in the cruise always "adding" 3mph to the speed setting (why Audi would program it to react in this way who knows!)?

Thinking more about it- it could be that when cruise is activated (and only when activated) it uses a different speed input for the digital readout? The needle speedo continues using the original speed signal. So when you engage cruise it is actually "copying" the speed from a faster wheel/defective hub sensor (or under inflated tyre) to the digital read out hence the apparent difference in speed? Builds in a kind of redundancy which would make more sense for Audi to have done?! Unlikely to be a faulty sensor.

Are you running a spacer saver spare at the moment on one of the hubs?

What speed does the large numerical speed display show on the MMI when you engage cruise?