Knock sensor low voltage

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Thread: Knock sensor low voltage

  1. #1
    Neutral Maqcro's Avatar
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    Hey guys I’m back and decided to make a fresh post now that I have a proper vag tool.

    2001 Passat 1.8l AWM

    knock sensor is reading low voltage.

    I’m on measuring block 26 and sure enough knock sensor is reading low voltage.

    Cyl 1: .23v
    Cyl 2: .23v
    Cyl 3: .35v
    Cyl 4: .35v

    When I rev the engine cyl 1 and 2 stay the same or not much change at all in voltage.

    cyl 3 and 4 increase according to throttle position moving anywhere from .35v to 10+ volts.

    Anyone got any ideas where to start with this?

    Thanks
    Last edited by Maqcro; 05-21-2019 at 12:51 PM.

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  3. #2
    Neutral Maqcro's Avatar
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    Also coming back is p0327 knock sensor low voltage

  4. #3
    Neutral Maqcro's Avatar
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    Bump

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  6. #4
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    refresh my memory did you check connector "bonnet" at (sensor) to connector at ECM for wire continuity between sensor and ecm ? apologize in advance for asking if redundant.....

  7. #5
    Neutral Maqcro's Avatar
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    No problem. I’ve checked all wiring for continuity and shorts from Ks bank 1 to pins on ECM and I’ve checked all wiring for ks bank 2 as well. No shorts or continuity at all which meant wiring harness is good.

    I’ve replaced the knock sensor on bank 1 with a brand new sensor. I’ve even pulled a used from from a junkyard and tried that.

    Last place I left off was trying to determine how I can pull Immo codes to swap an ECM but I figured now that I have a proper vag tool maybe we can narrow down to what the actual issue is here.

  8. #6
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    " No shorts or continuity at all"

    I think you mean "no shorts" BUT I have "continuity" correct

  9. #7
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    Yes that is correct no shorts in wiring and 3 pins on each connector are showing continuity as they are supposed to

  10. #8
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    Tested pins 106, 107, 108, and 99

  11. #9
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    I might try checking continuity again but this time moving the harness. Maybe there is a partial break in wiring and it’s intermittent?
    cchief22 likes this.

  12. #10
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    was thinking about this on way home....checked pinout numbers last night (shop manual) and OF course forgot to bring numbers with me, I am sure yours are correct......I thought well maybe the ECM ,but if that was the case (and I am not sure of its programmed logic) but if it was bad it (ECM) it probably would not recognize a low voltage/fault and throw a code.... make sense to you? As to measuring, see if to can rig up a like a small lamp 5 watts and test said wires in a series circuit to see if light bulb does indeed light up ,it is possible that you have continuity showing on your meter but wire is unable to carry the current that is drawn by the sensor..... definately a puzzler,hang in there

  13. #11
    Neutral Maqcro's Avatar
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    I don’t think the ECM is bad since it’s throwing the code and it’s communicating with vag tool and showing voltages.

    I’ll see what I can find but I may have to stop at a hardware store. Shouldn’t be too difficult.
    cchief22 likes this.

  14. #12
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    a side marker lamp holder from any auto store, cheapest you could find would probably work

  15. #13
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    Harness is good. Lights up 5w bulb no problem.
    Attached Images Attached Images
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  16. #14
    Neutral Maqcro's Avatar
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    If the picture isnt clear I have the circuit run like this.

    12v battery positive > wiring harness > 5w bulb > 12v battery negative

    I figured that would be the best way to test. If the harness is bad the bulb wouldn’t light up or might be dim.
    cchief22 likes this.

  17. #15
    Neutral Maqcro's Avatar
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    So ended up replacing the knock sensor again. Made sure to clean up the engine surface again just to be sure. Still same problem. Only getting .23v on cyl 1/2.

    I unbolt the sensor from the engine and I’m still getting .23v

    Bolt it back on and I get a small response around .23 - .49v

    Am I just not torquing the sensor enough?

    I’ve been going off 20nm / 14.75 ft/lbs for torque. I’ve been using a smaller torque stick that is measures in/lbs and torquing the bolt to 177 in/lbs

  18. #16
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    ugh..... that was quick.......positive (12 volt) to bonnet (knock sensor plug) thru the harness to brown wire (pinned out to ECM connector) to bulb, thru bulb to black wire to ground, makes perfect sense and confirmed on all three of sensors wires.... wow ,great job ! but still no answers.......long ride home for me and digest, hang in there.........

  19. #17
    Neutral Maqcro's Avatar
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    Yes tested all 3 wires on harness. All light up bright.

  20. #18
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    To be specific tested pins 106, 108, and 99 which are all for bank 1 knock sensor.

  21. #19
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    Is there any chance the wires are long enough to swap the connectors between the two knock sensors? That would positively determine if it's ECM or sensor.

  22. #20
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    Not sure what you mean by that. You mean swap the sensors around? That’s not an option sense the connectors are slotted differently. I do have a spare bank 2 sensor and I could also unplug sensor 2 to see if sensor 1 responds differently. The only way to swap sensors would be to make jumpers and and cross the wiring at the connectors.

  23. #21
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    108 and 99 are the supply voltage and the ground for each of the sensors and should have continuity between each of the bonnet connectors as they are spliced somewhere in the harness/setup at least that is what is shownIMG_20190523_224917.jpg

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  25. #23
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    I don’t have a lot of time today to check this out on the car. But it seems like pin 2 should have continuity between all 3 sensors and same with pin 3. I also haven’t checked any wiring from the speed sensor... could be a short coming from that.

  26. #24
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    Also what is the line and box inside the connectors between pin 1 and 2? Is it saying they are connected at the plug?

  27. #25
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    the little line inside the box is an internal connection ,possibly could get an ohm reading accross those for reference....the dotted oval line designates shielding from electrical "noise" usually and as it is there connected to ground

  28. #26
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    the shaded area (box) is a representation of the actual sensor itself

  29. #27
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    I’m back. Sorry for delay. I’m gonna run through continuity tests again between connectors. I’ll update when I get it all finished.

  30. #28
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    Ok finished. These were the ohm readings of each pin combination. If a pinout isn’t listed then consider it open. Here are my results:

    With ECM unplugged,

    G61 pin 2 - pin 2 G66; .3 ohm
    G61 pin 3 - pin 3 G66; .3 ohm
    G61 pin 3 - pin 3 G28; .3 ohm
    G66 pin 3 - pin 3 G28; .3 ohm

    With ECM plugged in,

    G61 pin 2 - pin 3 G61; .3 ohm
    G61 pin 2 - pin 2 G66; .3 ohm
    G61 pin 2 - pin 3 G66; .3 ohm
    G61 pin 2 - pin 3 G28; .3 ohm
    G61 pin 3 - pin 2 G66; .3 ohm
    G61 pin 3 - pin 3 G66; .3 ohm
    G61 pin 3 - pin 3 G28; .3 ohm
    G66 pin 2 - pin 3 G66; .3 ohm
    G66 pin 2 - pin 3 G28; .3 ohm
    G66 pin 3 - pin 3 G28; .3 ohm

  31. #29
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    OK, digested again, first I believe that the wiring is all functioning as intended, you have checked and confirmed this. Since sensor two is operational it also follows that sensor one is recieving a ground connection and a power source ,the +12v. and the ground,since they share (splice)in wiring diagram. Now I believe this is how they (two sensors) work, a "potential" is set up within the sensor itself +12v and ground. as vibration occurs an electronic "gate" allows voltage to increase/decrease (which you have observed as engine rpms increase/decrease, without an ocilloscope cannot tell if it is digital pulsed/modulated or purely analog, it matters not. The sensors are different in their "tuned" frequency so if there is knocking (which would occur in all four cylinders) the two would get different responces thus signalling ECM....you could have two bad #1 sensors which is highly doubtful or bad input "receiver" circuit in the ECM . When you unplug sensor # two you should get "low voltage" code there .....correct? so I would go back to junkyard and get "bonnets" off knock sensors and put your good sensor #2 and "pin in" to sensor #1 circuit, that will tell you if it is the ECM once and for all. Even if a hot +12v wire were to touch/short to #1 sensor "input" wire it would not cause it to fail since it gets that voltage at some point in the rpm range......

  32. #30
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    edit
    "as vibration occurs an electronic "gate" allows voltage to increase/decrease (which you have observed as engine rpms increase/decrease" this is the inputs to pin outs #106 and #107

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