[Supras] Factory shielding of knock sensors

[berniek at technicaldevelop.com]

Jon:

    You are absolutely correct about ground loops.  Grounding in this 
instance is a conundrum of sorts, because of ground noise elevation of 
the engine block with respect to the ECU.  A major factor in the 
equation is internal capacitance of the sensor, which if low would 
indicate grounding to be better at the ECU.  (wait)  I just grabbed a C 
meter and found the capacitance to be about 8000 pF, far from low, but 
characteristic of piezo devices generally.  Also, I lightly clamped the 
sensor in a machinist's vise by the threaded end and tapped both the 
vise and the body of the sensor with the plastic handle of a #1 Phillips 
screwdriver.  This is just a qualitative test, obviously.  The sensor 
developed about 1V-5V peak to peak on the oscilloscope, with a ringing 
frequency of about 7 KHz.  That frequency may be mechanical resonance 
arising from the sensor mass and cantilevered mounting arrangement from 
the threaded end. 

    Maybe use of substantial ferrite beads on the coax from the ECU 
would serve for assuredness of high frequency common mode decoupling 
with the shield grounded at the ECU.  I've ready of many instances where 
grounding at the ECU is better, but if so, why did the factory get it 
wrong for so many years? 

       BernieK

Hacker J wrote:
> Bernie,
>
> The knock sensors shielding should be grounded from the ECU end only to avoid ground loops. The actual ground used by the factory is the main ECU ground (so-called B ground in the wiring diagrams) which ultimately connects to the bunch of ground wires that get bolted to the intake runners. I would not deviate from this arrangement.
>
> Jon
> 90T
>
> ----- Original Message ----
>
> The '91-up manual set I have here shows the knock sensor cabling to be 
> shielded in much the same manner as mods which have been posted on this 
> same group.  I inspected the forward most sensor wire and found it to be 
> shielded about 2" back from where the overall protective tubing starts 
> at the sensor.  According the the wiring schematic, the shields are 
> grounded to a common point near the AFM ground.  However, I have seen 
> mods involving shield grounding at both the block and the ECU case (with 
> soldering to the ECU case whch can be done to aluminum with SAL-Met flux 
> with difficulty).  So the factory shield grounding point does not seem 
> that outrageous.  Yet I had instances with my '90 where knock sensor 
> codes were set.  I have a brand new '90 harness in the original plastic 
> bag (which will be for sale) which shows the same shielding.  Maybe it 
> was an upgrade from the original. since I did have knock codes set a 
> couple of times on the '90 before the BHG.  There were no codes set on 
> the '90 when it was received from the dealer. 
>
> Were the list modification comments intended for older harnesses only?  
> Both the 91 and '90 manuals show the shielding (referred to as 
> sealing").  Any comments would be appreciated.  Thanks.
>
>        BernieK
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