Shielded wire earths

[Richie-Van-GTi 70 2 Cars]

Where do people normally earth the shielding from knock sensor and crank sensor wires?

Everything i can find says at the ecu end only, but does that mean through the ecu earth point that the sensor grounds connect to or just through the ecu mounting points etc? I assume not with the ECU earth as that is tied to the engine so makes no sense.

[wicked 103 3 Cars]

The shielding is intended to shield from electromagnetic waves. The effect of emc is that it induces current in wires that have closed loop. If you would not have shielding around the knock sensor for instance , this current would show as voltage on the high impedant input of the ecu, which is of course unwanted. To avoid this you shield it with metal that has a constant voltage and NO closed loop.

 

That's why you connect the shield only on 1 end. It does not matter which end, but never on both ends to ground, because you create a new current loop then and there will be current flowing again in the shield, close to your signal wires and the shield has become your enemy.

 

Second thing, the constant voltage. The shield should have a connection to a low impedant voltage supply. This could in theory be the battery, but that is not so stable and low impedant as the ground and for safety not the best choice. Therefore they connect it on the ecu end to chassis ground. The signal wires inside the shielded cable also have a connection to ground, but that does not mean they are having the same voltage at the sensor side, because this ground wire is part of a closed loop running current. 

 

So in short, connect the shield only on ecu side to chassis ground an leave it open on sensor side and don't use it as ground on sensor side. Consider the grounds of sensor as signal wires and don't use them for actuators like stepper motors. They should have their own ground wire. 

 

Hope this helps to understand why... 

[Richie-Van-GTi 70 2 Cars]

Thanks for the in depth reply, i understand the reasoning for the shield and the effects of a closed loop so only grounding one end.

All my sensors that require a ground go to a specific ground through the ecu on a dedicated pin. The ecu then has its own ground which is on the engine as star point earth.

What i wasnt sure on was if i tied it into the ECU earth then its effectively tied to the engine again so does that matter, or as my ecu is under the dash can i just earth it to the steering column earth block?

[wicked 103 3 Cars]

Does not really matter, as long as it remains a star. Should be no cross connections between the legs. 

Nowadays they put that star connection on the chassis iso the engine and all sensors nowadays have their ground isolated from the sensor fitting. 

Edited January 30, 2022 by wicked