Bosch Motronic - Component Datasheet

Knock Sensors I & II (G61 & G66)

Located on the left side of the engine block, both of these sensors play an important job in keeping the 20V turbo engine in good health by ensuring that detonation or 'knocking' is always kept to a minimum.

They operate not unlike little microphones attached to the engine - with the Motronic software programmed to look for the waveforms associated with the sounds on knocking/pinking during combustion. This is no easy task !

Excessive detonation in any engine (particularly a turbocharged one) can cause serious engine damage due to the increased amount of heat endured by the pistons, cylinder head and valves.

Anybody familiar with the prices of internal engine parts on their S2 will know how important these innocuous looking chaps are !

The knock sensors are sensitive enough to let the ECU run the engine strongly into the performance envelope of the ignition map to provide more advance when it is needed. Should the ECU detect "excessive detonation", via the knock sensors, on account of poor fuel quality or high air temperature then the ignition is retarded and boost pressure is reduced. Better to cool the engines than melt the credit card I say :)

Knock Sensor I (G61) is located next to cylinder #2 and serves the needs of cylinders #1, #2 & #3. Knock Sensor II (G66) is located near cylinder #4 and it provides knock detection duties on cylinders #4 & #5. The reference signal from G4 is used to help determine which cylinder is knocking and the ignition timing for that cylinder is retarded accordingly.

In the event of failure (or disconnection) of G61, the ignition timing for cylinders 1, 2 & 3 is retarded by 6 degrees. Similarly for cylinders 4 and 5 in the event of failure of G66.

Audi used different knock sensors on the 3B and ABY engines throughout the years. I assume this was to optimise sensitivity and/or reduce costs along the way. These various sensors have different length cables and different coloured connectors to make it easier to spot them in the engine bay. Many of them are common to other Audi engines such as NG, AAH and ABC.

Application 3B
G61, BROWN 930mm 077-905-377 A
G66, GREEN 740mm 054-905-377 A

On the ABY engine, the parts system appears to conflict with the workshop manual on the details here, so please send me an Email if you find any concrete info on this one. I've cross-checked with RS2 information to get it as accurate as possible.

Application ABY
G61, BLUE 535mm 054-905-377 G
G61, BLUE 520mm, RS2 054-905-377
G66, GREEN 740mm 054-905-377 A

Knock Sensors can cause running problems when they go faulty. Unfortunately, like many sensors around the engine, the ECU can only detect faults when the sensor goes open or short circuit. It could be providing a rubbish signal to the ECU but not cause a fault code.

NOTE that knock sensors must NEVER be overtightened due to their sensitivity - the correct torque setting is 20Nm on these.

SJM's site has some more info about knock sensors here - and potential ill effects when faulty. They are not easy to replace on the S2 due to the number of other items in the way !

Last Updated 9th November 2007