Engine light, Code says Knock Sensor

[cadman]

(Background Information)
I have a 1988 Toyota Supra Turbo. It had no fan shroud when I got it, and a clogged radiator. So it had an overheating problem. The Waste Gate hose was also hooked to the steering pump when I bought it. Overboost and overheating problems were fixed, dual electric fans are ordered. The car runs and drives great, dyno runs put out perfectly stock numbers. The car has had no mods. It is totally stock. There are no body kits, I only have aftermarket wheels on it that came with the car.

(Problem)
The engine light comes on if the car is not totally cooled down after running at normal temp. For instance, in the morning, after starting and running the car, there is no problem. If, however, I turn the car off to go into a store, and come back out 15 minutes later, after I have put any type of load on the engine, the engine light will come on. After I let the car cool all the way down, when I start the car and run it, the engine light stays off.

The code it flashes is knock sensor. It's the same thing every time. I was told it could be the knock sensor wire along with heat soak that causes this problem. I also was told it is a very common problem with that year of Supra.

(Question)
Is this really the problem? If so, what type of wire would I use to fix this problem? Do I need a new knock sensor, or just the wire? What else could be the problem?

Thank you for taking the time to read this.
Allen.

 

[jdub]

Damn...now that's the right way to ask a question :thumbup:

I agree...it is likely your wiring to the knock sensors. You do need to fix it...this problem (and it is common) retards your timing making the car run rich. Do a search for "knock sensor rewire" and it will give you the info you need to fix it. Shaeff used to sell a kit...you could PM him to see if he still does.

 

[JustAnotherVictim]

It's like an essay.

 

[cadman]

Thank you for your quick reply.

I will look into the wiring then, that seems to be the cheapest fix. And if that's the only cause to this problem, it will be nice to get rid of it.

I heard it was a pain to get to this wire. Am I going to need to do any major tool spinning? This might be answered when I do the search for "knock sensor rewire".

Again, thank you for the quick answer.

 

[JustAnotherVictim]

If I'm not mistaken it's only difficult due to the fact that the knock sensor wire is actually three in one so to speak.

 

[jdub]

Nah...basic wiring stuff. The knock sensors are under the intake manifold...on a turbo, there's 2 of them. It is a bit of a pain to get to them...jack stands or better yet, a lift will make it easier. What you're doing on the rewire is giving the sensors a good ground so the ECU gets a clean signal.

 

[JustAnotherVictim]

I always heard the knock sensor wire was three wires that were intertwined.
Can you use just one?

 

[jdub]

I always heard the knock sensor wire was three wires that were intertwined.
Can you use just one?

The knock sensors have just one wire...that's part of the problem. Shaeff's kit provides a ground and a shielded wire (coaxial cable) to the ECU. Clean signal...that's what you want

Here's Shaeff's link:
http://www.supramania.com/forums/showthread.php?t=2943

 

[lagged]

I always heard the knock sensor wire was three wires that were intertwined.
Can you use just one?

who the hell told you that crap?

the knock sensor wires are SHEILDED.

be sure to use sheileded wire when you make youre repair.

also be sure not to splice in the shielding with actual KS wire as that will cause a short and will throw a KS code aswell.

 

[JustAnotherVictim]

I see. Well now I know if my sensor ever goes out on me.

 

[jdub]

the knock sensor wires are SHEILDED.

Just looked at a stock harness I have in the garage...the stock wires are not shielded

 

[supra90turbo]

not ENTIRELY true, Lagged.
Mine were spliced in to the stock wires ~>1" before each terminal as I couldn't source ones that would replace them. Never had a problem here, but I could see once I removed them that there would be a problem within the next two years.
Actually, if anyone has anywhere that they can buy the ECU and KS terminals, that would be amazing... I could make some ridiculously good KS wire kits along with shaeff.

::edit:: JDUB: therein lies the problem!!!! they are shielded, however, but not well. you need to go further into the harness to see what I'm talking about.

 

[jdub]

edit:: JDUB: therein lies the problem!!!! they are shielded, however, but not well. you need to go further into the harness to see what I'm talking about.

Ohhh...you mean in the main harness bundle. Well that's goofy! Why in the heck would Toyota not shield the wires leading to the KS connector :3d_frown:

 

[bigaaron]

Just looked at a stock harness I have in the garage...the stock wires are not shielded

They are shielded, but the shielding stops about 4" before the connector on the end. The shield is grounded near the ecu. The knock sensor is basically a microphone. The ecu input is high gain, so the shielding is very important. It is not coax cable, it is shielded wire. Coax would probably work, but that is just because it is shielded, not because it is coax. The rated impedance of coax only comes into play when using it for high frequency RF signals.

The shielding is needed because the wire goes through the engine harness right next to all sorts of noisy electrical wiring and it also comes within about 4" of the #6 spark plug cable. There is not much electrical noise on the intake side of the block under the manifold, and the the knock sensor output is from ground anyway, which is why the sensor didn't need to be a 2 pin shielded connection.

 

[lagged]

Just looked at a stock harness I have in the garage...the stock wires are not shielded

wrong. check again or you have a hacked up harness.

 

[jdub]

You guys are right...cut open the bundle (it's an old harness) and yep, it's a shielded wire.

I still think it's goofy that Toyota would have the exposed wire unshielded :wtf:

 

[cadman]

It sounds like the best way to fix this problem is to bypass the harness, run the wire through the firewall seperately, and run seperate ground, using a well insulated wire. Thank you for all the great information here. I will speak with the guy that does the wire harness you spoke of, and see what we can work out.

Thanks again,
Allen.

 

[lagged]

You guys are right...cut open the bundle (it's an old harness) and yep, it's a shielded wire.

I still think it's goofy that Toyota would have the exposed wire unshielded :wtf:

not really. when its in the harness close to all the other wires, and when it passes past the coil packs is where the most noise is going to come from, and theres where its sheilded.

under the intake manifold a few inches before the plug for the knock sensors there should be that much electrical noise.

 

[lagged]

It sounds like the best way to fix this problem is to bypass the harness, run the wire through the firewall seperately, and run seperate ground, using a well insulated wire. Thank you for all the great information here. I will speak with the guy that does the wire harness you spoke of, and see what we can work out.

Thanks again,
Allen.

waste of time.

simply replacing your old possibly damaged knock sensor wires is the solution.

while doing so it is of course a good idea to use wiring with better shielding.

 

[88MK3]

what would cause a knok sensor to melt in half??? And does anyone have an extra one?