1994 4Runner, 3.0

I've researched the FSM and found almost no info on the Knock Sensor.

I've also scoured the forum, but some of the info is contradicting. I found a great write up on how to replace the KS but not how to diagnose the issue before replacing and I've read that most of it is electrical rather then the sensor itself.

Looking for a good reference guide on diagnosing the lines to ECU and the pigtail coming out of the block.

 

There really isn't much to diagnose, if the wire looks at all compromised and not brand new you replace it..

It's a peizo electrical element, the "exact" same disc you will find in a telephone for example just in a different case.

They take sound waves and other vibration and turn it into an electrical (voltage) signal.

..
Hook your volt meter to its leads, tap near its mount, you get a voltage output. It still works fix the wire damage.

 

Well, the knock sensor is one of the least-well understood parts of our trucks. So you're in good company.

At the risk of being pedantic, here's why it's there. In the Model T, there was a control stalk that allowed you to directly adjust ignition timing. Whew! In a Toyota, the ECU constantly advances the timing until the engine just starts knocking, then backs off and repeats. As a result, the ignition timing is always set exactly correctly, varying with fuel, engine load, speed, etc. So how does it know the engine is knocking? The knock sensor is a piezo electric sensor (like a "crystal" phonograph cartridge), that is "tuned" to about 8.1khz. It generates an ac signal at around that frequency, which increases as the engine starts knocking. The signal is very small.

Because the ECU is swinging the timing back and forth, it expects to "hear" from the knock sensor each time it advances the timing far enough. If the sensor fails, there would be the risk that the ECU could advance the timing far enough to blow a hole in a piston. Not good. So if the ECU "stops hearing" from the sensor, it throws code 52. And since it doesn't know how close it is to dangerous knocking, it severely retards timing to avoid causing any damage. Retarded timing => runs like poop.

So testing the sensor is not easy. It's internal resistance is (supposed to be) greater than 1 Mohm. With the engine running at about 4,000 rpm, you can use an oscilloscope to check that the waveform has a period of about 120usec and an amplitude around 1v. Some have claimed that you can test them using an ac voltmeter and hitting the sensor with a hammer, but I have my doubts about that method. I doubt you have an oscilloscope. If you have a hammer, just put it away.

Your problem is almost certainly in the wiring, though. From the ECU, you can measure resistance to ground on KNK1 (which I show as pin 5 of the 16-pin ECU connector, with a solid black wire). If less than 1 Mohm, unplug the sensor and measure again. The resistance from that pin to the sensor should be "zero." Because the signal is so small, the sensor wire is shielded. The shield is connected to ground near the ECU, but NOT at the sensor itself. So the pigtail has a 2-pin connector at the harness end, but only a single connector to the sensor. If that shield is ungrounded or broken, engine electrical noise will drown out the knock signal.

Good luck!

 

You absolutely can check the sensor with an ac voltmeter and hammer. I have done it myself and proven it to get warranty work done on a recent engine rebuild. The bad sensor sent out no signal when I clamped threads in a vise and tapped on the vise. The good one sent out clear readings in the .125mv range ac when I tapped on the vise

 

Can be done with dc setting also, but this is a very rudimentary test. Just because it's outputting something doesn't mean it's working as it should be, it just tells you it's not completely dead..

Given most shops really aren't equipped to properly test these and its location in the 3vze, it's just due diligence to replace it if it's at all questionable so you don't have to go back in there later.

Kind of like replacing your 22re fuel filter while you have the engine on an engine stand.

 

Had this problem. I ran a new wire to the ecu and problem solved. The original stuff is shielded wire, which can get corroded inside. Im not using shielded wire now, just a piece of plain 18g wire. Its been working fine like that for years.

 

The shielded portion of the wire grounded is grounded.

There are 2 wires (black, direct to KNK on the ecu) and (brown, grounded on the shielded wire by the ecu. What did you do for a ground?

 

They said they used plain old 18g unshielded wire, this is highly not recommended.

Again the knock sensor is essentially a microphone, and a very low powered device, it is very susceptible to interference which is why it's wrapped in a coaxial ground wire.

You can actually plug that knock sensor into say your car stereo's aux input for example and you're going to hear lots of crackle and popping if you use an unshielded line.

Listening to the output of a knock sensor is a bit of an art-science hybrid, there are probably still some tuners who do this with varying success. Again part art part science.. The computer has the benefit of accurate timers and precise data of when the ignition is fired.

 

Looking online I saw that people use CoAx cable. But, I also read that, that kind of cable may not have the same impedance and still cause a bad read. That being said has anyone successfully replaced the line and what did you use?

Coax ax is certainly the most readily available shielded wire....

 

One of them dozens of threads is going to have a link for the appropriate (coaxially shielded! Twisted wire), yeah your typical cable coax is the wrong stuff solid core wire should be avoided due to its failure mode (only one conducting/signal wire, this breaks and you start all over).

Sorry I don't have the link on hand.

EDIT:
Ok that was easier to find than you might think..
82219-89103, 89-91
82219-35010, 92-95
Still available from the dealer in 2017.

 

Ok maybe a bit of miscommunication... No prob

So, I have the pig tail and sensor (OEM, Toyota) sensor is $214!

I'm speaking of the actual wire in the harness from the plug to the ecu.

After doing research

90%wiring
9% sensor
1% ecu

My wiring harness is pretty chopped up and I wanted to rewire direct to the ecu and make sure I had no grounds, kink, or damage to the line direct to the ecu.

I will be replacing the sensor and pigtail. But, also wanted to rewire the plug to ensure the job was done 100%.

That where the coach cable came to play. But after you mentioning the one conducting wire, I'm looking for a comparable replacement.

 

The KNK wire IN the harness isn't really a transmission line (the sensor is "Hi-Z"), so getting an exact impedance match probably won't matter. I would try to avoid common cable tv wire (RG59) with solid core just because it doesn't tolerate vibration well. (and it's pretty fat) I would try microphone cable; it has a stranded core and is pretty tough. (Regular XLR microphone cable has two cores in the shield ("balanced") but you only need one. But if you're just down at your ordinary guitar store, XLR is probably what they have, and it should work fine. Rather than hunt down unbalanced line, I would be tempted to just pick up their cheapest cable and cut off the connectors. E.g., https://www.parts-express.com/talent...10-ft--240-902 )

Having said all this, I'm not aware of anyone systematically replacing cables and determining what replacement works best. I, too, am surprised by the171's successful use of plain ole wire, but if he says it works, it does.

 

This weekend consisted of me ripping apart the engine again and replacing the knock sensor, pigtail, timing belt , and water pump.

Code 52 is gone, hopefully for good. Once I got to it, the pigtail was completely destroyed...it was an ahhhaaa moment. Lol.

Timing belt and water pump...what a mess..(it's all about having the right tools). I did some research before starting and bought 3 ft of angle iron and made the harmonizer tool... Made the job pretty easy.

I will say though... The thermostat housing having no gasket concerns me... The thermostat itself has a gasket but its basically metal to plastic.

Clamps and a belt with marks helps....my belt didn't have marks, but a saint on this site did a write up counting every tooth on the belt...193.

I will say that I know the innards of this truck probably better than any other vehicle I've owned, don't know if that's a good thing or a bad thing.

Next weekend I'm replacing the solenoids in the transmission (code 64, or 74) I think.

I'm concerned with the status of the trans fluid... I've actually changed the trans fluid on an older vehicle and the new oil actually put the nail in the coffin. It slipped and basically was junk.

Might save the fluid and do a 50/50 mix old and new to keep continuity.
New filter as well. (Any thoughts on this?)


BTW, Code 52's suck but they can be fixed with a weekend!