3sgte rev3 knock sensor replacement

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MR2T_Jake
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3sgte rev3 knock sensor replacement

Post by MR2T_Jake »

Hi all,

Anyone know of the best knock sensor replacement for the rev3 3sgte in the UK?

Don't fancy spending over £150 on the genuine one.


I can't figure out how to put in an image here, so this is my cry for help. :confused1:
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Re: 3sgte rev3 knock sensor replacement

Post by SonicSW20 »

OEM.

The cheap / third party ones will "fix" a knock sensor code but are utterly useless at detecting knock. You can see this if you scope the sensor or log it with a standalone ECU.
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Re: 3sgte rev3 knock sensor replacement

Post by MR2T_Jake »

SonicSW20 wrote: 26/09/21 0:44 The cheap / third party ones will "fix" a knock sensor code but are utterly useless at detecting knock.
ideally i am trying to see if i am being thrown into a safe map by a faulty knock sensor. It is the one from my previous engine that i spun a bearing on and if they are as fragile as others say could it cause me to be in a safe map where it dumps fuel and pulls timing?
I can't figure out how to put in an image here, so this is my cry for help. :confused1:
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Re: 3sgte rev3 knock sensor replacement

Post by MR2DI4 »

Would be interesting to know why these fail.

They must be a Piezo ceramic element as this produces a voltage with movement and visa versa you get a movement with Voltage applied to it, its basically the same setup as what makes the beep in a digital watch, you have a piezo disk on the rear cover of the watch. Some cheap speakers use a Piezo element as the tweeter.

I'm thinking age and heat and constant "Normal" vibration must be a factor but I'm also thinking that making changes to the engine that are not good suddenly brings it into doing some hard work and it just instantly dies. I suspect that if the engine is running well then its not doing anything much at all but knock is pretty nasty and it just instantly kills your 25-30 year old sensor.

May of course not just be knock, it could be a problem with the pistons or the bottom end or anything that produces a sudden shockwave through the engine block. Obviously quite a few mechanical problems could exist that the ECU cannot fix with changes in the timing and these will just go on and on and finally kill the sensor.

The OEM sensor would be pretty finely tuned. Its like a microphone, it will be putting out a certain level low voltage signal when the engine is running normally so you don't get an engine check light and then some relatively high level pulses if knock occurs. If the ECU sees nothing coming from the sensor at all the checklight comes on.

The ceramic used in the sensor is very brittle, they are pretty fragile. Dropping the sensor on the ground will kill it.

I guess I would also be looking to reason why it had failed and not necessarily put the failure down to the sensor itself as its possible something else changed and killed it.

The stock ECU in the MR2 is pretty old so I doubt it is constantly trying to trim the limits in the ignition, it will be more of a "Set and Forget" after you disconnect and reconnect the battery unless something nasty happens after that point which will be seen as a sudden change. The knock sensor will sit there 99.99% of the time in a safe zone just monitoring. The sensor is designed to see knock but then be able to correct the problem very quickly so its probably not designed to be continuously "hit by a hammer" for long periods of time or even shocks worse than the knock itself.
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Re: 3sgte rev3 knock sensor replacement

Post by SonicSW20 »

Correct MR2DI4 - they fail due to excessive vibration within the resonant frequency band (this could be knock or something else), or being hit by something. Piezo mics are delicate. The OEM knock sensor is not known to produce false signals, the usual failure modes are the sensor itself or issues with the signal wire or the shielding of the signal wire.

The stock sensor is a narrowband sensor, it is tuned to be resonant at around the frequency that knock on that specific engine will produce. This is common on older vehicles because it makes it easy for the ECU to interpret that data - any signal above the general noise floor is likely to be knock so there is no need for complicated filtering logic. The downside is any signal produced by the sensor will likely be considered knock. More modern control systems use a wideband sensor and its down to the ECU to process the signal. This is what Toyota use on their newer stuff, for example check the range of engines 89615-20090 is used on - https://www.japan-parts.eu/?parttt=&par ... i=&cars=ok

Coming back from that tangent, the OEM knock sensor is tuned to respond to the knock frequency of the 3S engine. The stock ECU is programmed to respond to the signals that knock sensor produces. As such I would not use anything other than a stock sensor with the stock ECU. I have seen testing of the cheap pattern ones, and they do not respond anything like the OEM one. While they will "fix" a knock sensor code, they just don't work as a knock sensor - fit one of those cheap pattern ones and you don't have knock control anymore.
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Re: 3sgte rev3 knock sensor replacement

Post by MR2DI4 »

Thanks for the post SonicSW20 its not often you learn some informative new stuff on here.
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