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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

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Old 09-08-2007, 06:23 PM   #1
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How to test knock sensor

I just finished installing a rebuilt 3VZE in my truck, and of course I got error code 52. Might have been nice if I'd known ahead of time of the habit of the knock sensor wire to go bad. Anyhow...

Is there any procedure for testing the sensor/wire? Before diving back into the engine - I am very not interested in going back in there for a while - I'd prefer to test if I can.

I've also got code 51, I'll deal with that later.
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Old 09-08-2007, 06:31 PM   #2
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You might be able to check the Ohm's if you have a meter and someone knows what the base is supposed to be.
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Old 09-10-2007, 04:56 PM   #3
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Since a knock sensor is basically a microphone, I'm guessing I should see some frequency spikes if I tap on the block. Right? If only I had a functioning one to test!

Does the ECU pull timing if the knock sensor is throwing an error?
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Old 09-10-2007, 05:41 PM   #4
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ECU drops the timing to ZERO degrees when the KS is malfunctioning, so your power will SUCK. 90% chance the problem is the wire, which only costs about $7.

The knock sensor measures for a VERY SPECIFIC frequency of sound.
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Old 09-10-2007, 06:49 PM   #5
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the only real way to test a KS is with an o-scope.

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Old 09-11-2007, 07:47 AM   #6
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Fair enough, and I think I have access to one. I'll have to check. Part of what I do is deal with aftermarket engine management such as the Hydra Nemesis. So I'm on the right track, tap the block and look for a frequency spike. Anybody know where the sensor is sensitive, frequency-wise?

I've never driven the truck with all six cylinders firing before. I bought it sick, and there were piston bits and at least one ring in the oil pan when I tore the old engine down. Given the fact that I live at 5000', have 32" tires and the engine is nicknamed the 3.slow, I figured the amount of power on tap right now was probably all I could expect. A power bump from getting the sensor working would be nice
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Old 09-11-2007, 07:59 AM   #7
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As stated, the knock sensor will retard the timing if activated. Since the crank has to turn at total of 720 degrees to fire all 6 cylinders, the frequency is half the rpms. Not sure if it has a resistive value that you can measure with a VOM? The wire usually fails where it exits the head/intake gap. The sensors have never given me any problems. I will say that an open circuit will trigger a code.

My $0.02
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Old 09-11-2007, 08:08 AM   #8
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The sensitive frequency of the knock sensor won't have anything to do with the number of cylinders, it'll be related to the audible frequency of a knock event. Knowing where the usual failure is will be useful, though. I'll be digging back in there soon enough from the sounds of things.
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Old 09-11-2007, 09:34 AM   #9
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I agree, it's very unusual for these to fail.
If your truck is running, just hook up the scope. You should see all kinds of noise. The output of these things is not an exact science and can be difficult to deal with in terms of isolating noise from knock.
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Old 09-12-2007, 01:32 PM   #10
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I'll have the new wire tomorrow, I'll be underneath the intake manifold tomorrow night. Sigh. New oil pressure sender and new harmonic balancer going on at the same time.

Thanks for the help.
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Old 09-12-2007, 05:22 PM   #11
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Quote:
Originally Posted by SEAIRESCUE View Post
As stated, the knock sensor will retard the timing if activated. Since the crank has to turn at total of 720 degrees to fire all 6 cylinders, the frequency is half the rpms. ....
Whiskey
Tango
Foxtrot??

Im no expert in engines, but you do know what knock/ping is right? It's flame fronts colliding in the tight confines of the combustion space. When this happens it generates a whole hell of a lot of sound that is radically different from the rest of the engine sounds and has not a damn thing to do with the number of cylinders.

Besides, rotation in degrees/radians, what-have-you, has nothing to do with sound waves (hz) generated as a by product of premature explosions in the combustion chamber...
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Old 09-12-2007, 05:47 PM   #12
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Quote:
Originally Posted by shaeff View Post
the only real way to test a KS is with an o-scope.
Quote:
Originally Posted by 89pickup View Post
Fair enough, and I think I have access to one. I'll have to check.
Replace the knock wire first, it always breaks. Every time we have an intake manifold off, we always replace the wire.

With the oscilloscope, you check the knock sensor output with the engine running and watch for the waveform. No waveform = bad wire, or bad sensor. Pick one and replace it. Since the wire is less than $10 and the sensor is over $100, most people will change the wire and put the engine back together.

The waveform is in any toyota service manual since 1990. It's a similar waveform for all Toyota knock sensors.

HTH.
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Old 09-12-2007, 05:54 PM   #13
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would a 1988 22RE have a knock sensor?
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Old 09-12-2007, 10:12 PM   #14
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Quote:
Originally Posted by 86tuning View Post
Replace the knock wire first, it always breaks. Every time we have an intake manifold off, we always replace the wire.

With the oscilloscope, you check the knock sensor output with the engine running and watch for the waveform. No waveform = bad wire, or bad sensor. Pick one and replace it. Since the wire is less than $10 and the sensor is over $100, most people will change the wire and put the engine back together.

The waveform is in any toyota service manual since 1990. It's a similar waveform for all Toyota knock sensors.

HTH.
i would have written the exact same thing except im to drunk to be as gramatically accurate as you
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Old 09-13-2007, 08:07 AM   #15
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I pulled the intake manifold off last night and the wire does look a little haggard. But only the shielding, the signal wire checks out just fine for continuity and the insulation isn't broken. The shielding was torn up enough to be grounding out on the block. I'll know tonight if the problem is solved.
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Old 09-13-2007, 04:07 PM   #16
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maybe im wrong, but isnt grounding the shield on the block kinda the idea? lol
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Old 09-13-2007, 04:21 PM   #17
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^ yes, the sheild should be grounded on one end.

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Old 09-13-2007, 07:04 PM   #18
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It'll be grounded at the ECU, that's the usual way to do it. There's only one connector at the sensor end so I expect that's what is going on. Regardless, you're not supposed to ground the shield by fraying it. I might not know Toyotas, but I know this.

I sure hope you guys are right about the wire being the usual problem. I would be much happier if I didn't have continuity through the old one. At least I'm avoiding a future problem by changing this frayed wire now.

Naturally, the dealer ordered the wrong wire. So I'll have a dead truck in the garage all weekend. This was not the plan.
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Old 09-19-2007, 10:04 AM   #19
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The new wire's in, and the truck is much happier. It's all ate up with motor now!

Now I just need to sort out the TPS (easy enough) and I'll have a happy, happy truck. Thanks for the help.
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Old 09-21-2007, 06:11 PM   #20
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I replaced my wire and the sensor itself ($240) and i still get the light come on and error code 52, and retarded timming.

What can be done to stop this????
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Old 09-21-2007, 06:11 PM
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