re. knock sensor[detailed]

Ben Swann benswann at
Wed Feb 18 22:53:45 EST 2004

OK this is mostly off the top of my head, so feel free to point out any inaccuracies.  Most of this applies to the CIS(KE-Jetronic/CIS-E3) and KE-Motronic, as well as the CIS/Motronic like hybrid system used on the 5000/200 turbo quattros, but there are variations on the theme.

In general no code is thrown just because knock is detected.  The code will be thrown if the sensor continually gets knock and the timing is retarded to the point of max retard and then still knocks.  If I recall correctly, a code made be thrown if the knock sensor appears to the control unit to be operating incorrectly.

Depending upon CIS/Motronic version, knock(and other) codes may be volatile and erased when engine turned off, or may be retained until least recent code is cycled out or codes are cleared by a technician.  In KE3-Jetronic and CIS/Motronic(MAC-10-14) except California, the fault codes are erased when ignition is turned off.  In KE-Motronic, faults are generally stored.

Some if not most of these Bosch knock sensing ignitions will swap out fuel maps if knock detection continues - esentially this assumes that lower octane fuel is being used, and the maps are programmed to accomodate for the lower octane.  The maps are generally programmed for ignition timing based on load(Air-flow sensor), Engine Speed(RPM sensor), Engine Timing(TDC sensor), and sometimes the pressure transducer inside of the ECU(load again).  Literally thousands of data points are used by the control unit to calculate the optimum ignition timing for the various inputs, thousands of times per second.  Knock Detection will cause a step-by-step retard from the mapped ignition value for each time the knock is detected, until it stops.  Then ignition is advanced step by step unless/until knock is detected again.

In most systems, knock suppression is enabled above a certain RPM, such as 2500 RPM.

Depending upon ambient conditions and/or/combined with type of driving, fuel mileage may or may not be affected by lower octane.  If I recall, the horsepower output does not change with the fuel, but the available torque changes, so engine will not accelerate as good.

Knock doesn't just occur because of low octane, but also for lean conditions and excessive temperatures.

Ok I tried to answer your questions in order and fill in the details.  I take no offense for anyone to add or correct the above as needed.  Generally this is how the knock sensing works on many of the Bosch systems.


p.s. in CIS-E3, knock detection occurs on a cylinder by cylinder basis and each cylinde can be independently advanced or retarded as needed.

[I am trying to understand the workings of knock sensors that are on Audi 100 and 200's.
If knock occurs will it make a fault code? 
Has anyone made a way to get instant readings if knock occurs?
Is changed timing the only action when knock occurs? (just what the ecu can do, not melt down)
Does the knock sensor operate only in a certain rpm band?

In the case of the 100, if there is no noticeable difference in performance or mileage between using regular (87 octane) and super (92 octane) can it be assumed that no knock and therefore no timing adjustment occurred?
No I don't risk running the turbo on anything but super! but it would be good to know if there was ever knock present.
TIA for any insight.

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