mholme

Gevo

I meant code 51 then, I never remember which is which I'm referring to the code that says something is wrong with the timing signal, not that it's missing. I would be more thorough if I wasn't at work

scope103

I don't think the knock sensor (is supposed to) works that way. It's not like an oil pressure light that comes on when the engine is knocking.

Just as the O2 sensor can't detect correct mixture (only too much oxygen, or none at all), no sensor can detect correct timing. So the ECU constantly advances timing until it "hears" knocking (through the knock sensor), then retards the timing until it stops. And repeat. The ECU knows the sensor is working by listening to the knocking starting and stopping. If it stops hearing that pattern, it has "lost" the knock sensor and dramatically retards timing to avoid blowing a hole in a piston.

So getting "some" signal to the ECU isn't enough; it has to start and stop with the timing. I don't know why Gevo's engine hook fix worked. Maybe he has just enough more carbon in #4 that it starts knocking before the rest, and it doesn't matter that the knock sensor can't hear the other cylinders.

RJR

Scope103 is exactly right. It's important to note the distinction between sensors whose primary purpose is to detect abnormal operation (i.e., your oil pressure or charge light), and sensors that are involved in the normal closed loop control of the engine (O2 sensor, ECT sensor, VAFM, TPS, etc.). The knock sensor is one of the latter. Its purpose is not to warn of impending doom due to detonation, but to continuously optimize timing to improve power and fuel economy. I'm not sure the knock sensor detection algorithms even know the difference between moderate knocking due to mildly over-advanced timing, and catastrophic detonation. There is no code for detonation or uncontrolled knocking, and certainly there is no message that comes up anywhere on your instrument panel that says "The engine is knocking uncontrollably. Shut down immediately to avoid damage!" All you get is the check engine light telling you the knock sensor circuit and/or control algorithm is not working, signaling you to investigate further at some point when it's convenient.

It's hard to know how much the engine hook location compromises the effectiveness of the KS. Metal is a pretty good sound conductor, so it's likely that anywhere on the block will pick up some kind of a knock signal from any piston. That being said, I'm sure Toyota instruments their test engines with multiple sensors to determine an optimum location for the KS, and it apparently wasn't the engine hook. I think the engine hook is a good, easy to get to, test position to establish that you have a working sensor and good wiring. Once that is verified, I'd put it back where it belongs.

mholme

scope103

Probably not. The knock sensor is a piezo sensor mechanically "tuned" to be most sensitive to a frequency (7kHz?) associated with knocking. http://www.cygnusx1.net/Media/Supra/...taTech/h38.pdf I suppose the sensor does produce SOME output with the engine running but NOT knocking, so I guess you could design a circuit to listen for that signal before changing the timing. I don't know what Toyota did (and I'm not taking my ECU apart to find out). But considering how small the signal is to begin with, I'm guessing the ECU is just looking for that peak indicating the onset of knocking. So I think it inches up the timing until it hears it.

RJR

I think the ECU is looking for a fairly specific signal pattern. The knock signal is heavily filtered, both by the sensor itself and by the ECU. Code 52 often appears due to defective shielding of the KS wiring, which leads me to believe that any old random noise is not adequate to satisfy the ECU that the KS is connected and working. Rather, it looks for the specific signature of impending knocking (what we hear as "pinging"). The characteristic knock signal is centered at around 7KHz, and is almost a bell-like tone. (I see while I was writing this that scope103 has said pretty much the same thing.)

Again, engine manufacturers didn't adopt the knock sensor technology primarily to prevent knock-related damage to the engine. If you read the original GM patents from the late 60's, you'll see that the use of a KS was always viewed as a way to more optimally control engine timing for better performance and economy. Before KS technology became prevalent, ignition timing was controlled by vacuum and centrifugal advance mechanisms in the distributor, and could only be an approximation. The manufacturers had to error on the side of retarded timing to protect the engine, which cost power and performance. As shade tree mechanics back in the day, we would advance timing, go out and drive the vehicle, listen for pinging, advance some more until the pinging got excessive, back off a bit, and assume we'd done the best we could. The KS and ECU simply do that many times/minute, keeping things optimized over a wide range of conditions. The ECU contains a "map" based on rpm, air flow, and throttle position to give the ECU a starting point for timing. It then uses the KS to tweak it from there, as scope103 has said.

I believe Saab was the first manufacturer to use a KS in a production vehicle, in the late 70's. Although one doesn't need a microprocessor to implement KS based timing, it makes the job a lot easier, which is why knock sensors became universal after the mid-80's microprocessor revolution.

mholme

I'm understanding what both of you have stated as to how the knock sensor functions as it relates to detecting knock and regulating timing, but my tiny brain can't wrap itself around how you say that applies to Code 52.

Here's an example of what I struggle with. So if I were to accelerate my truck in my driveway to 1600RPM, where the ECU starts looking for the knock signal, you're saying that the only thing keeping code 52 away at that point is the knock sensor experiencing an event to resonate at or near 7KHz, that event stopping and then repeating this signal pattern, indefinitely?

scope103

Yup.

Just like the O2 sensor. The only thing that keeps codes 25 and 26 at bay is the O2 sensor detecting "some" O2, then "no" O2, about once a second. If it fails to detect that transition, the ECU throws the code (depending on which way it was "stuck").