Knock Sensor (KS) System Description


Varying octane levels in today's gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion, or burn, in the combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front that is produced by the spark plug. The rattling sound normally associated with detonation is the result of 2 or more opposing pressures, or flame fronts, colliding within the combustion chamber. Light detonation is sometimes considered normal, but heavy detonation could result in engine damage. In order to control the spark knock, a Knock Sensor (KS) system is used. This system retards the spark timing when the engine detects a spark knock. The KS system allows the engine to use the maximum spark advance for optimal driveability and for fuel economy.


The knock sensors detect abnormal vibration in the engine. The knock sensors are mounted in the engine block near the cylinders and produce an AC signal during all engine operating conditions. The PCM contains integrated knock sensor (KS) diagnostic circuitry which uses the input signals from the knock sensors in order to detect engine detonation. This allows the PCM to retard Ignition Control (IC) spark timing based on the amplitude and the frequency of the KS signal that it receives.


FastFieros personal / practical application of usage and troubleshooting/testing in house on MANY types of L67 3800 Supercharged engine in Fiero's and Grand Prix's.

First off, knock is going to be in STOCK engine running a bone stock PCM from General Motors. The code is setup to work in a 50 state environment, and GM cannot set the PCM code for each state, and climate. HEAT is the worse enemy for a supercharged engine.  FOR any engine for that matter. Heat going into the throttle body and then to the supercharger, just becomes that much more super heated from the compression of the air by the supercharger. Next problem, people want to get more 'boost'. They put nice smaller pulleys on that supercharger and spin it faster to compress more air. More compression means more heat.

Lets touch on GM's PCM code for the Grand Prix. I have scanned and tuned about 50 plus STOCK Grand Prixs. The average in Texas for KNOCK ( KR ) as we call it, is about 5 to 8 on nice hot Texas summer days. This is stock engines, with stock PCM's. So, GM puts out something that 'works' in 50 states, but is not exactly perfect for all 50 states. The same car in Wisconsin in a May day might scan only a 3 or 4 KR event since the air is dryer and cooler.

Modified engines with STOCK PCM's. I have scanned so many of these, I lost count. Again, the KR events can range from 5-8 all the time during normal driving, and WOT runs.

WHEN is the KR too much?

I have a really long write up I am not finished with on this subject, but no time this month to finish it...... more later..