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#1 Old 03-02-2008, 08:30 PM
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Bypassing Knock Sensor

Now that I've found out that I have an oversensitive knock sensor I want to bypass it completely. I know what the purpose of it is and that it is not the best solution to eliminate it but given how hard it is to change I'd rather just do away with the thing. Can I simply leave it disconnected or will that cause the ECU to use less aggressive timing maps thus decreasing performance? If I can't just leave it disconnected how should I bypass it so that the ECU runs the most aggressive timing map? I was thinking of just using a 500kohm resistor (that is what the value of a working knock sensor is according to the FSM) or could I just ground the wire going to the knock sensor to the block?

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#2 Old 03-02-2008, 08:45 PM
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The best way to bypass a sensor is to first understand the sensor. As this happens to be my field.... The first thing to do is hook a normal one up to a 1000 dollar oscilloscope*lol* and watch what happens under normal conditions. **or you can look it up online and get the same results** After finding out what it is supposed to do, you could digitally recreate the signal that it is supposed to feed. After knowing how to recreate it, you can design and build a board to do what you need to do. But first I'd test it in programs like mulitsim and the output on Quartus. After all that is done, install it and test it in your car.
After it works and it would work, look at the cost and time it took for you to do all of this work and think this *Maybe I should have spent less time wasting time on how to bypass something and really just replaced the broken part*

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#3 Old 03-02-2008, 08:58 PM
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it just keeps you from blowing up your engine if you get bad detonation.

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#4 Old 03-02-2008, 11:50 PM
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I researched online how the sensor worked before I even unplugged it. My understanding is that when detonation occurs a piezoelectric crystal in the sensor vibrates and sends an AC signal to the ECU telling it to retard the timing. The worse the detonation the more it vibrates and the more the timing is pulled. I know that no detonation is occuring because I can't hear it and if by chance it starts to occur all I have to do is back off of the throttle. It has always been oversensitive and was detecting some vibration around 2500 RPM that was causing the ECU to pull timing thus making the engine hesitate for a second. Ever since I disconnected it the car has been running much smoother and I have yet to hear a ping/detonation.

All I want to do is trick the computer into thinking the sensor is there so I don't get a DTC. Also, I do not know if having the sensor disconnected will cause the ECU to run in a "safe" mode and use less aggressive timing maps. This is why I want it to think the sensor is there. After researching I'm thinking that if I replace the sensor with a resistor of the same resistance of a working knock sensor the computer will think everything is OK and the DTC will go away.

If the engine pings, big whoop de dooo as long as it's not constantly doing it I'm not worried. If it were constantly pinging something else would be wrong anyway. IMO its just a worthless piece of junk sensor. There are plently of engines out there that don't have knock sensors and run just fine. When pinging occurs all you have to do is back off the throttle. I'd rather not have this junk sensor and have a smoother running engine.

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#5 Old 03-03-2008, 02:41 AM
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Glad to hear you finally pinned down that 2500rpm problem. You still didn't mention whether you confirmed that the erratic timing on idle is gone as well.

Don't put a 500k ohms resistor in place of the knock sensor or ground the signal wire. Instead just leave the KS wiring disconnected. The ECU won't go into failsafe mode because of a missing KS. Yes, you'll get a DTC but not a CEL.

I have been running my programmable ECU for about 2 week now with the KS feedback signal turned off. In other words, I turned off the KS flag internally to the ECU ROM. My intentions were to see if I can detect any changes to ignition timing, fuel mileage, engine performance, or hear engine knock. My engine, like years ago when I ran two years without a KS, behaves the same. I would dare to say, it behaves even better. I am not advocating living without it, especially if you are into FI or doing aggressive engine tuning, but it is interesting how it can sometimes hinder performance if it becomes sensitive like yours.

If you ever reach the point that you detect knocking then first try going up in fuel octane and finally replace the KS if needed.



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Last edited by jserrano; 03-03-2008 at 02:43 AM.
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#6 Old 03-03-2008, 02:59 AM
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You know, I'm probably one of the most concerned people here with getting rid of trouble codes, but why not just leave it unplugged? Yes, it will register a DTC. However, this particular DTC does not cause the CEL to come on...

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#7 Old 03-03-2008, 03:31 AM
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Quote:
Originally Posted by jserrano View Post
Glad to hear you finally pinned down that 2500rpm problem. You still didn't mention whether you confirmed that the erratic timing on idle is gone as well.

Don't put a 500k ohms resistor in place of the knock sensor or ground the signal wire. Instead just leave the KS wiring disconnected. The ECU won't go into failsafe mode because of a missing KS. Yes, you'll get a DTC but not a CEL.

I have been running my programmable ECU for about 2 week now with the KS feedback signal turned off. In other words, I turned off the KS flag internally to the ECU ROM. My intentions were to see if I can detect any changes to ignition timing, fuel mileage, engine performance, or hear engine knock. My engine, like years ago when I ran two years without a KS, behaves the same. I would dare to say, it behaves even better. I am not advocating living without it, especially if you are into FI or doing aggressive engine tuning, but it is interesting how it can sometimes hinder performance if it becomes sensitive like yours.

If you ever reach the point that you detect knocking then first try going up in fuel octane and finally replace the KS if needed.
Thanks for the advice on the resistor! I'll just leave it disconnected. It feels great to no longer have that annoying bog at 2500 RPM. Power over the entire RPM range feels somewhat better as well. I can't believe this thing was the issue all along. As far as the erratic timing on idle I can't check it because I loaned my timing light out to my buddy and have yet to get it back. As soon as I do I'll let you know if the problem is still there. I'm hoping that it's not. If it is I'll try your suggestion of moving the distributor with the TPS connected and post the results.

1999 GXE Auto
Stillen Header
2.25" mandrel bent piping
Dynomax SuperTurbo muffler
Accel 8.5mm wires w/NGK BKR5E-11 plugs
SilverStar headlamps
American Racing AR-157 wheels with Cooper Viper 205/55/R16 tires - Ruined due to a collision with a curb
Warm Air Intake w/AEM Dryflow filter
Taz's Street Series motor mount inserts.
Crimestopper CS-2016 Alarm/Remote Starter.
5% tint in the back, 35% tint in the front.

Last edited by Racincc85; 03-03-2008 at 03:37 AM.
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#8 Old 05-30-2011, 10:18 PM
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Quote:
Originally Posted by Racincc85 View Post
Thanks for the advice on the resistor! I'll just leave it disconnected. It feels great to no longer have that annoying bog at 2500 RPM. Power over the entire RPM range feels somewhat better as well. I can't believe this thing was the issue all along.
I know this is a super old thread - but I've been trying to figure out why my car bogs between 2000-2500. New plugs, wires, cap, rotor, O2 sensor, cleaned the MAF, new fuel filter, checked every vacuum line... everything looked fine.

It's had a P0325 stored since I've owned it (August). Crawled under and unplugged the knock sensor sub-harness today and reset the ECU - seems like the bogging is gone now (though I've only put about 15 miles on so far).

I did try to get to the sensor first.. damn thing is almost as hard to get at as the PCV valve. I run stock timing and don't drive it hard, so I think I'm pretty safe leaving it unplugged.
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#9 Old 05-31-2011, 01:56 AM
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I wouldnt drive unplugged but yes you should be good if the ecu is getting adaquete info from the other sensors since other sensors have similar enough data collection. I wouldn't worry though as long as the engine and timing are good your definitely fine.
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#10 Old 05-31-2011, 03:12 AM
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if you one day find the need to have the sensor functional you can always relocate it.its very common to relocate the sensor in nissans because its installed in spots where it picks up false signals aswell as knock.

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#11 Old 06-02-2011, 01:53 AM
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I've run mine disconnected in the past with no issues, and that was while driving and datalogging with Datascan...I'm about to do it again just as a test. I replaced my KS a couple of years back (have fun!) and found the OEM KS, cracked in 3 places.

If overtightened, timing could be pulled unnecessarily or the sensor could be damaged.

It's nice IMO that you can disconnect the KS without having automatic timing retard....then again, I'm fairly consistent and careful about where I fill up...

I noticed that my rear motor mount is cracked badly and have noticed vibration in the steering wheel. I'm curious as to whether the excessive vibration could be sending false signals to the KS at all. And though the high heat/humidity is no doubt a factor, at times the engine feels sluggish as if timing is being pulled so I'm going to find out...

Last edited by niceguy; 06-02-2011 at 01:59 AM.
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#12 Old 06-13-2011, 06:42 AM
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Quote:
Originally Posted by 1STGENON20Z View Post
if you one day find the need to have the sensor functional you can always relocate it.its very common to relocate the sensor in nissans because its installed in spots where it picks up false signals aswell as knock.
Where's a good place to relocate it to? I can see mine between the #2 and #3 intake manifold runners and can see cracks in it, but damned if I can actually reach the thing. I do plan to get some ramps soon, I might try to tackle it then.

I substituted a 460k resister for the K/S to get rid of the DTC - I know the sensor is supposed to be 500k ohm, but RatShack didn't have 500k. I've been driving it like that for a week and a half (about 300 miles) and it hasn't logged any DTCs in that time, so I guess 460k is close enough. I also have emissions coming up soon, and I'm not sure if they would fail it over the P0325 or not.

niceguy, the motor mounts shouldn't cause anything with the knock sensor unless the mounts are to the point where there's no rubber, only metal - in theory the sensor might pick up the metal to metal contact. If the engine is running properly, it vibrates the same regardless of worn motor mounts. The mounts just help to isolate the vibration from the cabin.
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#13 Old 06-13-2011, 02:20 PM
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Relocating the KS to get rid of the DTC is one thing but relocating it to where it cannot detect engine knock defeats its purpose.



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#14 Old 06-13-2011, 08:18 PM
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Which is why I'd like to relocate it. I want a working knock sensor, but damned if I want to even try to get to it where it's at now.
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#15 Old 07-12-2011, 03:38 AM
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any pics of where the knock sensor or the harness is or where to start looking? from the top or bottom?
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