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Old 01-05-2016, 12:11 PM   #1
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Default Knock sensor ground

I'm installing a vvt motor in my na. Currently butchering the vvt engine harness, removing unnecessary wiring and splicing into the na chassis wiring. Should I take the knock sensor ground to the megasquirt analog ground or would the back of the head work well for a good ground?
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Old 01-05-2016, 12:22 PM   #2
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I had mine grounded at the head and it was too noisy to use. I would say with megasquirt, you should locate a free sensor ground and just wire it to that. I'm going to explore shielding as well.
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Old 01-05-2016, 12:30 PM   #3
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the original knock sensor wire is twice as thick as all the other ones. I think I will run the ground to the MS then and a stand alone shielded wire to the ms by itself.
In the MS instructions it says the DB 37 pin 2 should be used for the wideband and pin 3 is "analog ground", should I ground it to pin 3 then? the knock sensor signal should be analog
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Old 01-05-2016, 06:15 PM   #4
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I had my knock sensor grounded at the ECU end (ECU power ground), picked up a ton of alternator whine below 3krpm.

Its now grounded at the block side (ring terminal onto bolt), and is better. Still not perfect, but much better.



EDIT: Knock sensor always grounded at ecu, knock sensor SHIELD grounded at ecu, then moved to block. I has the stupid.

Last edited by sparkybean; 01-06-2016 at 05:49 PM.
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Old 01-05-2016, 08:30 PM   #5
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On my MS3 I used the sensor ground on the ECU side. I also uses shielded cable that was grounded to the block.

Wire - 2 Core Shielded 20 Gauge - 10&#39 DIYAutoTune.com

My knock logs were pretty clean.
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Old 01-05-2016, 09:01 PM   #6
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Quote:
Originally Posted by btabor View Post
the original knock sensor wire is twice as thick as all the other ones. I think I will run the ground to the MS then and a stand alone shielded wire to the ms by itself.
In the MS instructions it says the DB 37 pin 2 should be used for the wideband and pin 3 is "analog ground", should I ground it to pin 3 then? the knock sensor signal should be analog
Running 1 unshielded and one shielded ruins the point of running a shielded ground. You are still going to get noise in the unshielded ground.
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Old 01-05-2016, 10:33 PM   #7
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Broadly speaking:

Sensor ground should be located at the ECU itself for all analog sensors (pressure, temperature, airflow, EGO, etc.) Where external signal-conditioning electronics are used (as with many knock sensor installations), the sensor should be grounded at the conditioning circuit, which should in turn be grounded at the ECU.

Shields, where present, should be grounded at one end only, and at the end nearest likely sources of EMI/RFI which, for most purposes, is the engine block.


These are general rules, and not every install is the same. But this is how the "professionals" do it, whether the application is automotive, industrial, broadcast / recording, etc.
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Old 01-06-2016, 09:58 AM   #8
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I ended up using a shielded 3 strand ( 1 signal, 1 ground and extra non insulated wire -may hook this one up to the block- ) and 4 strand shielded for the extra 2 injector wires and vvt signal. They all end in the DB37 at their respective pins.
I think this should work fairly well.
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Old 01-06-2016, 10:51 AM   #9
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Quote:
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I ended up using a shielded 3 strand ( 1 signal, 1 ground and extra non insulated wire
Does the overall arrangement of the cable look like this?



If so, the "extra non insulated wire" is the drain wire, and it's electrically connected to the shield. This is how you ground the shield (at one end only) to provide a drain for any energy which the shield absorbs.
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Old 01-06-2016, 11:11 AM   #10
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That's exactly how it looks plus tin foil around the cables. Since the shielded cable doesn't run all the way to the knock sensor itself but to the knock sensor plug, could I wire the shield to be grounded to the back of the head?
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Old 01-06-2016, 11:40 AM   #11
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That's exactly how it looks plus tin foil around the cables.
Yeah, that particular photo was of Belden 9451, in which the foil is bonded to the jacket such that it breaks away when the jacket is stripped (makes prepping the cables easier.) It's there, you just can't see it.

This is Belden 8451, which lacks the foil bond, so you can see that the foil remains in place after the jacket is stripped:



Both cables function the same, it's just a matter of preference. Some old-timers prefer the original design where you have to trim the foil away manually, I like the newer stuff which takes care of itself.


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Quote:
Originally Posted by btabor View Post
Since the shielded cable doesn't run all the way to the knock sensor itself but to the knock sensor plug, could I wire the shield to be grounded to the back of the head?
Yeah, you can bond the drain wire (the uninsulated wire that's in contact with the shield) to whatever point is convenient. In the OEM design, I believe it went to ground somewhere near the front of the intake manifold, but any spot on the engine will be fine.

Last edited by Joe Perez; 02-21-2016 at 02:48 PM.
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