Turbo MR2
Knock System

By Bill Wotschak

More than you ever wanted to know about the knock sensor/knock response system!

One of the most misunderstood systems on the MR2 3sgte is the knock sensor/ignition retard ("knock response") loop. This system actually operates fairly often, but since it is so sensitive and effective, you often are unaware that it is doing its job. And when it actually makes itself known, most owners look for any possible explanation, except the right one.

This is NOT a primer on knock and detonation. A Google search for "detonation" will keep you reading for quite a a while. Here are a couple of the best links I have found, but there is a lot more information out there.



But a basic understanding of knock/detonation is needed to understand what this system does. But this IS a primer on the knock control mechanism, what it does, and how it does it.

If you do further research, you will learn the difference between "knock" and "detonation". For this discussion I use the terms interchangeably, but detonation is really what we are concerned with.

1) Detonation is the "spontaneous" ignition of the air/fuel charge from a source other than the ignition spark, or more commonly, is multiple ignitions of the same charge.

2) Anything that increases the volatility of the air/fuel charge will increase the propensity to detonate. This includes increased temperature, increase pressure, (high compression, boost) lean A/F mixture, low octane, etc. (Note that the same things that increase power also tend to cause detonation!)

3) All gasoline engines are prone to knock, but it is most damaging under high cylinder pressure (forced induction) and at higher RPM.

Toyota recognized that many of the conditions that help maximize power also tend to increase the risk, and potential damage, from detonation. So they added a knock sensor to the turbo engine (and to the N/A in '93).

The knock sensor is NOT an "over-limit" device, like the boost pressure sensor. It is not intended to catch out of control conditions, but rather to help keep things in control, much like the O2 sensor. It is a closed loop feedback device. The ECU reads, responds if necessary, then reads again to see if the response brought conditions back into range. It continues to adjust and compare until conditions are back in the acceptable range. With the boost pressure sensor, on the other hand, the ECU only responds when the condition (boost level) is "out of control", then simply shuts down.

The knock sensor is a piezo-electric "listening" device that listens for vibration of a certain range and intensity. It will hear knock beginning long before there is any audible ping, and before it is significant enough to do any damage. While it does convert those vibrations into a voltage signal, the ECU actually looks for frequency, not voltage or current values. That also means that it cannot be monitored with a voltmeter/ampmeter, etc.

Since detonation is normally the result of a combination of variables, making a correction to any one of them will often correct the condition. In this case, a correction is made to the ignition timing curve. and again, unlike the "overboost response" knock response varies with the type and intensity of the sensor input. As soon as any out of range signal is sensed, the ECU will retard timing by an amount determined by the intensity of the input. If that corrects the condition, the ECU will then advance the timing again, until knock is heard, the back it off again. This process continues until the maximum advance possible without detectable detonation is achieved. These small corrections take place frequently, and in most cases are so small the driver is never aware that it was happening. Again, like the O2 sensor, you normally aren't aware that it is working.

When the knock sensor input indicates a more intense knock input, the corrections are of greater magnitude, and this is often felt by the driver as "stuttering", "hesitation", "missing" etc. It is simply the cyclical retard/advance sequence that is being felt, and it will stop as soon as the ECU finds the optimum advance that does not cause knock..

On rare occasions, the magnitude of the knock input will be so great, and it will come on so quickly, the ECU simply makes one major adjustment AND simultaneously de energizes the T/VSV, lowering boost, to further insure no damage will occur. This is the "safe mode" response.

Or, if the retard/advance cycle does not correct the knock, the ECU will also initiate this "safe mode" response.

Knock is most likely to occur at low to mid RPM, (3000-5000) in upper gears, at WOT, under boost and under increased load. It is most common in cool weather ( mid-30s and below) or in hot weather (90s and above, but can sometimes occur at moderate temps as well. The response cycle will typically be felt for only a few seconds, and the engine will again feel "normal" after the correction has been completed.

This "self-correcting" characteristic is the most distinctive symptom of the knock response. The other common causes of a rough engine, limited fuel delivery or weak ignition, are not likely to improve at increasing RPM and/or increasing boost.

And a knock input/knock response DOES NOT mean something is "wrong".; it simply means that the ECU is adjusting the engines operating parameters in response to the environmental conditions that it sees.

Here is a brief description of the concept of knock response from a Toyota-specific training manual:

When the ECU judges that detonation is taking place, it retards ignition timing until the knocking stops. Timing is then advanced back to calculated value or, if detonation happens, retarded again until detonation is stopped. In this manner. the ignition system can be operated at maximum efficiency, on the borderline of detonation, while avoiding the audible "ping."

More detailed info can be found HERE, see articles 24 and 25.

Questions, comments and criticisms can be directed to: Jeff