canucklehe

my 93 es300 throws knock sensor CEL. Replaced the sensors, the wiring harness under the intake manifold, swapped out ECU. still get the CEL and still runs like a slug.

mechanic also tried to completely bypass the knock sensors by hooking up a dummy sensor in an open, accessable area but I still get CEL.

Wells knock sensor techie told me something about the shielded wiring. Another Lexus expert made a similar comment. any idea where this wiring runs from/to ? where can I get new shielded wiring?

Can I somehow bypass from the connection at side of motor (where my new harness plugs in), run it to ECU and splice in at that the ECU connection?

I'm desperate. been trying to sell car for a few months and this CEL and sluggish performance is a killer.

LeX2K

The knock sensor is a piezoelectric device, the short of it is the signal going to the ECU is susceptible to electrical interference. Because of this, the wiring going to the ECU is shielded. All that means is a braided ground that covers the center wire completely, keeping out unwanted interference. If you've ever seen a TV coax cable, you know what this type of wire looks like, your car has essentially the same thing only a smaller diameter.

If you want to try and replace this all the way to the ECU, I would suggest going to a salvage yard and stripping out that length of cable from the knock sensor all the way to the ECU and put that in your car.

canucklehe

in this link, this person seems pretty knowledgeable. he talks about the Lexus and the "resonance frequency" style knock sensors they use.



I just replaced the wiring from under intake to the first place where it connects. So if I could figure out where to run the rest of the wiring, I might have a chance to fix it (wishful thinking??). It looks to feed into the main harness soon after, and then it's lost to me from there. Not sure what I'd need to remove from donor car. They have a couple of them at our local wrecker.

i was picturing what you described....something like a cablevision wire. I know those can cause interference if the shielding touches the copper cable.

LeX2K

I've sliced apart these harnesses before, generally the shielded wire has a grey sheathing. If you want to grab this from a donor car, disconnect the harness from the ECU and pull it through the firewall. Then with a sharp knife remove the tape and whatever else you encounter and pull the wire back all the way to where it goes to the knock sensors.

Ideally, you would want to pull the pins out of the connector at the ECU, then remove the entire section of wire that goes all the way to the knock sensors. If you can't do this, then you will have to do some soldering.

The tricky part is splicing the wires together, you want to maintain the shielding effect so to do this, you cut the center wire shorter, then push back the outer braid, solder and heat shrink the center wire, then pull back the outer braid over the join and carefully solder the braid in a few places.

canucklehe

okay, thanks. Not sure how many connections there might be on the wiring harness from ECU, but maybe i could swap out the entire section and keep plugs the same/simple...??

I'm guessing the knock sensor portion splices into main somewhere along the ways, and doesn't get a clean straight run to the ECU??

LeX2K

It's possible but honestly it does seem unlikely, although you seem to have eliminated other possible causes. You can swap the entire harness if you want, but it is a lot of work.