Knock sensor questions

[CyFi6]

Just some questions about the factory knock sensors...

First off, how well do they work as far as effectively reducing knock? Supposedly the 7m creates a ton of noise on the knock sensors under completely normal operation, so how does the stock ECU differentiate noise from knock? Does it look at crank position etc and disregard signals it knows must be noise?

How important are the positions of the knock sensors? I know pre 89 and post 89 7mgte's had the rear sensor in a different location. Will having the sensors in the wrong location (there are 3 holes in the block) throw the ECU off or make the car perform differently? In order to replace my rear sensor the starter is going to have to come off because the sensor is so tight that I need to get a socket on it... I would rather just put both sensors in the 2 front locations if nothing will be affected.

I am running more than stock horsepower (about 375) and a lot more boost than stock, are these sensors going to cause a negative affect on my engine? In other words, since the engine is now running more power will the knock sensors falsely sense knock and pull timing when it isn't even needed? With my meter on AC volts, the knock sensor output increases in voltage as rpm and boost increase, so is the computer pulling timing gradually as this output goes up?

Just things I am curious about and haven't seen answered anywhere.

 

[IndigoMKII]

Not sure if this would help answer your question or not.. But I did find a link regarding how knock sensors work in general. http://www.engineblox.com/?p=65

 

[jetjock]

This stuff has been answered but I'll go over it again.

1) The system is effective. Knock is sensed at three levels, not simply a matter of knock versus no knock. On-the-fly feedback that reduces/advances timing in stages depending on the level of knock sensed and the max retard angle of 10 degrees is more than adequate to stop knock generated under normal conditions.

2) Knock produces a specific frequency with inline blocks and the sensors are tuned to this frequency. In our case it's 7K hertz with amplitude denoting knock severity. That's one way the system differentiates knock from other vibrations.

3) The other (as you alluded to) is since the ECU knows which cylinder it's going to fire it knows when to listen. Just before ignition occurs it opens a window, called the knock control gate, and begins sampling the signal. It does this not only to listen for knock but also for normal ignition events. This is how the ECU checks sensor and wiring integrity (the reason behind code 52) without the internal resistor commonly found in other sensors.

4) I can't say for sure say if not having a sensor in the original location will be a problem but for reasons I won't go into I think not.

5) As long as the tuned frequency doesn't occur at the programmed amplitude timing will not be retarded. The knock signature is specific and one your engine, regardless of horsepower, should not normally generate.

There's more to how the knock control system works but that should be enough to answer your questions...

 

[CyFi6]

Ok thanks guys I think that clears everything up. Looks like the TCCS is more complicated that I would expect it to be for its age as usual (the thing is older than I am!). These are things I would expect out of a modern vehicle, never seizes to amaze me how in depth the guys at Toyota took it.

 

[jetjock]

Yep, before you were born everyone drove Flintstones cars powered by their feet. :icon_razz

If you think this car is sophisticated try learning the newest ones. I know almost as much about my Infiniti G as I do the Supra but it took some effort.

 

[CyFi6]

Considering carburetors were still in production mid to late 80's, I would say the Supra ecu is pretty damn sophisticated for its age, but considering I wasn't around to see it I guess my argument is pretty invalid

...and I can only imagine delving into the workings of a late model PCM...it's like you almost have to be an engineer just to fix the car!

 

[mk3_7m]

Ok thanks guys I think that clears everything up. Looks like the TCCS is more complicated that I would expect it to be for its age as usual (the thing is older than I am!). These are things I would expect out of a modern vehicle, never seizes to amaze me how in depth the guys at Toyota took it.

It's japan what do you expect XD.

Very informitive post Jet. Learnt quite a bit.

 

[3p141592654]

I posted some additional info on the knock sensor system here: http://www.supramania.com/forums/showthread.php?73379-3P-s-TCCS-Disassembly-Analysis&p=1724332&viewfull=1#post1724332