Automatic engine retardation?

[sportq]

When going downhill with the cruise control off the car will, in certain circumstances, cut power to the engine. I notice it especially on my commute home from work on a speed bump that's on the downward side of a reasonably steep hill. It's repeatable about 80% of the time. I've also felt it happen at the bottom of a steep hill when the road levels out. According to my father who owner it for the previous 150K, it's always done this.

Kind of related, I also noticed that unlike the previous B7 I had, when the cruise control is on the car will actively slow the car down if it's going faster than the set speed, I don't know if it applies the brakes or retards the engine, I suspect the latter. The B7 would just naturally increase speed if the force of gravity was greater than the wind and rolling resistance.

Both these effects feel the same which makes me wonder if the former is some sort of safety system.

Peter

[magt1000]

When going downhill with the cruise control off the car will, in certain circumstances, cut power to the engine. I notice it especially on my commute home from work on a speed bump that's on the downward side of a reasonably steep hill. It's repeatable about 80% of the time. I've also felt it happen at the bottom of a steep hill when the road levels out. According to my father who owner it for the previous 150K, it's always done this.

Kind of related, I also noticed that unlike the previous B7 I had, when the cruise control is on the car will actively slow the car down if it's going faster than the set speed, I don't know if it applies the brakes or retards the engine, I suspect the latter. The B7 would just naturally increase speed if the force of gravity was greater than the wind and rolling resistance.

Both these effects feel the same which makes me wonder if the former is some sort of safety system.

Peter

I've felt this too even when the cruise control is not in use, often when coming over a rise into a downhill section. It feels as though the car is dabbing on the breaks momentarily, quite disconcerting!

[Guest 2]

The cruise control thing is normal, both my A3 and A6 do it.

[gregpot2000]

Just to add I experience this on the same downhill stretch near my house. Slows down noticeably

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[a8 tech]

i will post a repair to resolve this

[gregpot2000]

Repair? Didn't think this was a fault? Thought it was just a characteristic of the engine?

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[a8 tech]

Sorry I meant answer

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Technical product information
Transaction No.: 2023850/1
CRD TDI Holding back after brow of hill
Release date: 29-Jun-2010
Customer statement / workshop findings
Customer complains of vehicle “holding back” or a hiccupping phenomenon when driving on roads with aggressive changes to the gradient (e.g. humpback bridges or on undulating roads where the terrain goes up and down) and where the vehicle is driven with light throttle application.
Technical background
Audi vehicles are fitted with an ECU with a monitoring concept, which constantly compares the drivers throttle input against the vehicles speed (measured by the tyre speed sensors). If necessary the engine torque will be controlled to ensure the driver’s throttle request is maintained.
This can lead to a reduction of torque on downhill roads compared to the torque given on flat roads with the same throttle position. The reduction of torque on downhill roads is normal behaviour for all current Diesel vehicles.
This does not affect the maximum engine torque which can always be achieved by pressing the throttle pedal, regardless of the gradient of the hill.
Production change
No change
Measure
No service solution.

[Boy]

Thanks A8. I guess this just mans that the ecu cannot assure progress that is as smooth as manual operation of the throttle,but better assures better speed control which is why we rely upon the cruise in speed limit areas!