Knock Sensor Installation

Last updated: February 25, 2000

Date: Wed, 12 May 1999 15:28:00 -0700
From: "James S. King" (

The following steps describes how I installed the J&S knock sensor on my '94 RX-7. I used the Mazda factory wiring diagram to verify the colors of wires in the factory harness to splice into. This is highly recommended. It is possible that for different model years the wire colors may be different. I haven't checked this. YMMV and all of that. What you do to your car is what YOU do.

The instructions included with the J&S suggest locating the J&S control unit in the passenger footwell and running several wires over to the driver's side footwell and then through the firewall to the engine compartment. This is not necessary. The only wires that need to go into the engine compartment are those that go to the knock sensor inself. These can be routed through a grommet on the passenger side. The J&S control unit can be mounted beside (on the outboard side) of the stock ECU.

Tools Required:


  1. Remove the door sill trim and kick panel trim on the passenger side to expose the stock ECU.

  2. Unbolt the stock ECU: Remove the 10mm bolt on the ECU bracket that secures two harness ground leads. Next, remove the three 10mm bolts that secure the ECU mounting bracket (2 on bottom, 1 above). Remove the connector from the small "black box" thats bolted to the ECU bracket. Remove the four ECU harness connectors. Push down on the tab on the top of the connectors to release the lock mechanism. Set the ECU aside.

  3. The ECU connector we will be tapping into is the 2nd largest (2 rows of 11 pins). Unwrap the tape from this connector to expose the wires at least 3" back from the connector.

  4. Get the J&S harness connector. Cut the GREEN, WHITE, RED and YELLOW wires to about 16" in length. Cut the BLACK (ground) wire to about 24". Don't cut the GRAY cable! This must be routed through the firewall to the knock sensor at the engine.

  5. (For soldering only) Strip about 1/4" of insulation from the ends of GREEN, WHITE and BLACK wires. Strip about 3/8" of insulation from the RED and YELLOW wires.

  6. Solder or crimp a bolt-down lug to the BLACK ground wire.

  7. Find the Light Green wire in the ECU connector. Make sure you have the right connector! It has 22 pins. The Light Green wire is 4th in from the end on the bottom row and should have a White/Black wire on one side of it and a Brown/Black wire on the other. Cut the Light Green wire about 1.5" back from the connector. This is the wire that goes to the leading coil ignitor.

  8. Connect the GREEN wire from the J&S harness to the end of the Light Green wire that is attached to the ECU connector. If soldering: Optionally slide 1/2" of heat shrink on one wire. Butt the two ends together interleaving the copper strands before soldering. Wrap with electrical tape and then slide heat shrink over and shrink with match carefully.

  9. Connect the WHITE wire from the J&S harness to the end of the Light Green wire that goes into the main wiring harness. Use end-to-end soldering technique described above.

  10. Connect the RED wire from the J&S harness to the Black/White wire coming out of the ECU connector. DO NOT CUT THE Black/White WIRE. It should be on the end in the same row as the Light Green wire. Use the splice method for soldering as discussed below. This ignition switched +12 volts. Splice method for soldering: Isolate the Black/White wire and carefully strip insulation from 1/4" long section about 1.5" from the ECU connnector, using X-Acto knife. Carefully wrap 3/8" stripped end of RED wire around the exposed copper strands in the Black/White wire. Solder the joint and wrap with electrical tape.

  11. Connect the YELLOW wire from the J&S harness to the Green/Yellow wire coming out of the ECU connector. It should be in the other (top) row, 4th from the end with a Green/Red wire next to it. This is the signal from the MAP sensor. This connection is not manditory. It disables the J&S retard function unless you are running positive manifold pressure (0 PSI or higher).

  12. Now it's time to route the knock sensor wiring into the engine compartment. Take your straightened clothes hanger and poke a hole in the main engine harness grommet, **working from the engine side**. I sharpened the end of my hanger to make the job easier. The grommet is located on the firewall near the ABS unit. The grommet is double walled. Punch through at either the 3 o'clock or 9 o'clock position to avoid the wires in the harness bundle. Go through as close to horizontally as possible or you will miss the 2nd wall in the grommet. I bent my hanger to a curved shape and had to run it near or through some of the brake lines near the ABS unit to get the right "angle of attack".

  13. Once through, manuever the hanger around and push it through further till you can see it in the footwell. Tape the gray cable to it with electrical tape. Make sure the tape job is smooth so as to pull through the grommet without snagging. I lightly smeared some grease on the wrapped section and on the gray cable to make it easier. Pull the cable through the fire wall into the engine compartment.

  14. Route the cable across the firewall the the driver side of the engine compartment. I zip-tied it to the heater water pipe.

  15. Remove the intake elbow. Move the spark plug wires and dipstick handle out of the way. The threaded hole for the knock sensor is about 2-3" above the top (trailing) spark plug.

  16. Install the knock sensor on the rear rotor housing according to the J&S instructions. I used anti-seaze on the mounting stud. Use a 19mm deep-well socket for the mounting stud and a 13mm socket on the nylock nut to hold the actual sensor to the mounting stud. I faced the connector straight up (12 o'clock position).

  17. Cut the gray cable to length (I had about 2 feet extra), and assemble the connector as per the J&S instructions. I soldered the female connectors onto the ends of the wires.

  18. Plug the connector onto the knock sensor. I had to push really hard to get it to "click" on. Double check the orientation if you aren't sure. The connector is keyed.

  19. Back in the footwell, clean up the mess by using some of the big 5/8" loom to cover the wires between the J&S connector and where the wires tap into the ECU connector.

  20. Re-wrap the wire bundle coming out of the ECU connector with some electrical tape.

  21. Plug the connectors back into the ECU, and temporarily bolt down the two stock ground lugs and the J&S ground lug using one of the 10mm ECU bracket bolts. Now you can road test and adjust the sensitivity on the J&S before finishing the install.

    This write-up covers only the installation of the J&S unit. You are on your own as far as mounting the monitor unit. I plan on mounting the main J&S control unit directly on the back of the engine ECU. I have not done this since I'm still tuning the sensitivity adjustment. It looks like it will fit there ok, but the small "black box" control unit that is mounted there may be in the way. It could probably be relocated without much trouble.

    This is a first draft. Any criticisms, corrections, suggestions for better methods are welcome. I will be on vacation for few days, so can't answer any immediate questions.


    Date: Fri, 29 Jan 1999 15:14:43 +0000
    From: "David Lane"

    From memory, so don't take this as anything but a rough indication:

    The knock sensor control unit mounts away from heat--usually under the dash somewhere or in the glove box.

    Mount the microphone on your rotor housing. One of your housings already has the mic from the stock knock sensor. Use the analigous space on the other housing. The mic cable terminates in a plug which fits into the control unit.

    There are four wires to run: Two are used to interrupt (and route through the control unit) the low tension signal that fires the leading plugs--between the coil and igniter on my 1st gen. In other words remove the igniter wire from the coil. Attach it to one J&S wire. Attach the other J&S wire to the coil. The wires are color coded.

    The third wire is hot (switched). The 4th wire is ground. (Note: If your ground wire comes loose from the body, you will lose ALL SPARK to the leading plugs. Secure the ground wire well.) These four wires terminate in a multi plug that goes into the control unit. The unit comes with a shorting jack that fits into the end of the multi-plug. Use this if you are diagnosing a problem and want to take the J&S unit out of the picture. It simply turns the ignition signal around and sends it back without going through the box. Pretty nifty. I used it when my ground wire came loose one day when I was on the road. You should see what happens to exhaust temps when you are only firing the trailing plugs.

    There is an optional 5th (yellow) wire used if you notice the knock sensor activating during over-run conditions. This can happen if there is unburned fuel left in the combustion chamber. The knock sensor is sensitive enough to detect this stuff burning off in an uncontrolled way. Even though it won't harm the engine, some people don't like seeing the indication that the knock sensor has activated, so the yellow wire is provided to hook up to a voltage source like the TPS. This prevents the knock sensor from operating when your foot is off the throttle. Note that the knock sensor already knows not to operate below a certain RPM threshold. I don't remember what it is--probably somewhere between1200 and 2000 rpm.

    The knock/retard readout gauge simply plugs into the control unit. The A/F meter portion of the gauge has (I think) one wire to T into the O-2 sensor, and another wire. I don't have this feature so I don't recall where it goes.

    The following controls are on the front of the old model control unit. The new one may differ:

    There is a set of dip switches with which you can select options like retarding total timing according to the worst knocking rotor face, or retarding each rotor face independently. There is a sensitivity control which will probably end up open about a quarter turn from full counter-clockwise, with the screw slot somewhere around 11 and 5 o'clock.

    There is another adjustment for the rev limiter. No one mentions this because our cars under boost will over-rev on the trailing plugs alone (not controlled by the knock sensor). Nevertheless, there is a sophisticated rev limiter included. If I remember correctly, you tune it with a VOM and a screwdriver so that the VOM reading corresponds to the red line. The unit cuts spark intermittantly from that amount of RPM to 10% over, and totally cuts spark after that. If someone figures out how to make this work for both leading and trailing plugs on our cars (without buying a second knock sensor) it would be VERY handy.

    Once again, this is from memory after installing my J&S unit well over a year ago. The new model must have a switch to select the "10% max retard" mode Shiv mentioned. Take the above as a general indication of what is involved to install it, and not as specific instructions.


    Date: Thu, 26 Mar 1998 21:56:07 -0800
    From: "David H. Lane"

    I have a J&S unit on my car, which is an '85 GSL-SE with an after market turbo. I also have the read-out for the amount of retard being dialed in. The object is to set it up so that you get no reading under max boost. This is as opposed to setting it up to rely on it routinely to retard the spark under boost. You shouldn't see it activate unless you are messing with things, and have probably gone too far. You will know it is right if you have established a baseline where you know boost is about the max you can safely run. Set the J&S unit to activate at any point above that.

    Of course, if you hear detonation, you need to make it a little more sensitive. My unit is set so that the screw slot is on a line between 11 and 5 on a clock face, about a quarter turn clockwise from the least sensitive position. I have also discovered that if the unit is too sensitive, it will retard the leading plugs past where the trailing plugs fire. For some reason, this seems to cause pinging.

    I have not hooked up the yellow wire because I have not had a problem, however, I do occasionally see a flicker on the indicator when the car is off throttle--especially if I have just completed a high boost run. It isn't worth worrying about.

    If you don't already have it, you should get the read-out to tell you what is going on with the knock sensor. As an alternative you can use a volt meter hooked up to a mini jack and plugged into the front of the control box. J&S makes a version of the read-out with a parallel line of LED's that is an A/F meter. The combination meter takes up very little room. I wish I had gone that route. Twenty-twenty hindsight, eh?


    Date: Tue, 29 Dec 1998 13:32:27 -0600
    From: "Westbrook, Chuck"

    I have it in my 93 R1. The pickup is attached by a nut and bolt just above the rear trailing sparkplug. It uses an unused tapped hole there. My control unit is located under the carpet under the drivers seat. When doing initial adjustments, it was placed between the left side of the seat and the side rail, just behind the release of the seat back. My retard monitor is attached with Velcro to the dash, just left of the steering column. The control wires connect to the ignition igniter on the inner left fender. Requires splicing and cutting! I also had to connect the yellow arming wire due to low rpm engine/trans noise. This can be connected to the map sensor or the TPS. The TPS connection works best for me. My unit's sensitivity had to be set to only about 1/16 turn past minimum due to picking up noise around 4500 and 5500 rpms.


    Date: Mon, 1 Mar 1999 08:02:20 -0600
    From: "Westbrook, Chuck"

    Update on my original comments: since my new engine is fully broken in, my final setting on the J&S sensitivity is 1/4 turn from minimum. The retard monitor will momentarily light up (about 1 to 2 seconds) sometimes when initially accelerating. After the turbos are fully heated up, this retards stops. Either the turbos are noisy when heating up to max HP, or maybe it's caused by loose carbon hot spots that quickly burn off. There isn't any pinging at these times even if the unit is switched off. Another J&S user also once mentioned this same oddity.


    Date: Mon, 15 Mar 1999 17:04:56 -0600
    From: "Westbrook, Chuck"

    I set mine by adjusting the sensitivity to just barely retard at normal boost (10PSI) when the turbos were not fully heated. You will notice that when first getting on the boost the unit will retard for a couple of seconds but will stop after the turbos and engine are fully heated up. Evidentially it is picking up the noise of the various turbo parts creaking as they heat up. More sensitivity than this will cause the unit to retard for every gear instead of the first one when boost was fully engaged.

    This is accomplished by starting with too much sensitivity and gradually reducing it for the above mention affect.


    Date: Wed, 13 Oct 1999 15:04:18 +0000
    From: Wael El-Dasher (

    I just installed my J&S, and wanted to verify the final settings.

    switches 1,3,4 up
    switch   2 down

    Sensitivity is dependant on mods, I guess, but I ended up setting it 1/4 turn from minimum (9'O'Clock position).

    Are all the above settings about right?


    Date: Wed, 13 Oct 1999 16:49:09 -0400
    From: "Jason Baughman" (

    Make sure the actual sensor is in tight. I found mine to come lose after a bit of driving.

    Set the sensitivity to maximum and go for a drive. Give it throttle and decrease sensitivity till you get one or two red lights on the meter. I have mine set to 1-2 red lights when under throttle up to redline.

    When driving it is hard to tell if it is actually retarding the timing. The best way to test is to turn on the car, and put the meter on the dash where you can see it from the outside of the car. From the throttle cable give the car some gas and tap on the sensor with a wrench; you can actually hear the RPMs decrease as the timing is retarded. This way you know if it is actually working.

    There really isnt a need to hook up the Yellow wire to the TPS.


    Date: Wed, 13 Oct 1999 20:51:50 +0000
    From: Wael El-Dasher (

    I called Brian of M2 and je told me they do not use the yellow wire. They say there is no need to hook it up to the TPS.

    > Also, during the test drive, I cranked the sensitivity to maximum and got no
    > indication of retard under cruising conditions.  When I went full throttle,
    > I did get some retard, as much as 8 degrees on one chamber.  I felt no
    > butt-dyno loss in power.  Is this common?

    The instructions for the RX-7 install specifically say not to do that. In fact they say that it is a common mistake to set it on maximum sensitivity. Start at mid point and work down.

    I also had a very interesting conversation with Ari of RP-Online, and he mentioned that he found that the stock knock sensor sometimes interfers too much, he found in those specific cases that cutting the single wire going to the stock knock sensor and using a 400-600 ohm resistor a ground would remedy that problem. You have to remember that the ECU recieves a signal from the stock knock sensor, not the J&S. The J&S only goes between the ECU and the igniter, ie it's on the output side.


    Date: Wed, 13 Oct 1999 20:52:57 -0500
    From: Steven Kan (

    Actually, it's 400k-600k ohm resistor. There's a big difference!!!!


    Date: Wed, 13 Sep 2000 08:51:42 -0500
    From: "Westbrook, Chuck E." (

    >I have a question about my newly installed knock sensor. Last Sunday I was
    > messing around with it prior to an autox and left it with the rev limiter at the
    > minimum setting... (snip) I know it was reducing power. I understand the rev limiter
    >doesn't work as well because it only affects the leading ignition, but now I have doubts
    > as to how I have it set up. I'm surprised to have reached the stock rev limiter
    > in this condition.
    > The main reason I question my installation is that I can only get it to read
    > "retard" when it's set to maximum sensitivity and I rapidly floor it from
    > highway speeds when I've already gotten the EGT up to 1400F or so from closed
    > loop operation. I have the car setup to be as far from detonation as possible
    > (duh :) but I'm surprised to be getting none when I'm really getting on it.
    > I suppose some description of the car is appropriate.
    > '87 TII with 8K on rebuilt motor and rebuilt Series 5 turbo with ported
    > wastegate (plumbed to the manifold to allow 8 psi boost), UPRD reprogrammed N332
    > ECU, open exhaust and TID.

    Try connecting the arming wire to the TPS or MAP sensor and see if the problem goes away. Without an active arming wire, mine will retard around 2000rpms even if slowing revving. My sensitivity is now set at about the 10 o'clock position.

    Knock sensor install on Miata.

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