ECU Shielded Cable Questions

[50RTD]

Hi all,

Im currently in the process of sussing out some wiring stuff and need some insight into the theory behind shielding critical wiring for the ECU.

The big question i have is, i want to run all the wires for an aftermarket ECU through a multipin plug, to simplify the relationship between engine and computer, and also allow for easy engine removal/refitment in the future.

I note that most ECU's, both factory and aftermarket,have shielding for crank angle sensor and coil wiring (BMW's go to the length of earthing all the shields too...).

Is this to protect from interference from other wiring or just the electrical components themselves or both? I mean, i understand that coils would throw all sorts of EMF action around, considering the voltage they run at, but does close proximity to other wires cause the same issues?

Long and short, can i run shielded and earthed cables to a multipin plug, in the corner of the engine bay, then shielded and earthed cables from the plug to the critical components, with the only 'unsheilded' wires, those that are going through the multipin plug, in close proximity to other wires?

Cheers

-Dan

[oldcorollas]

group the higher current (ie, injector) wires at one end of the plug, put a few earths next to them, put the low voltage and critical signal wires at the other end.

i thought shielding should always be earthed at one end (not both, to prevent ground loops or something). not grounding the shield partially defeats the purpose of the shield.
toyota shields are earthed too, and drawn on the wiring diagrams as such

the other option is to have two plugs.
high current and injector wires on one, and signal wires on the other.

[thechuckster]

'shielding should always be earthed at one end' <- definitely do this... and don't cheat and try to use the shield as an earth return for your VR sensors.

[50RTD]

Great stuff guys, thank you greatly!!

A further question though on a point you made Oldcorollas, i was under the impression that (at least in the case of many later Toyota's and others) the injectors were 'negatively switched', so shouldnt be too many worries with the injector wires from the ECU? (specific case is an EMS Stinger on 1JZ). I could see the wires from the ignitor to the coilpacks being an issue though... Is this theory correct?

[TERRA Operative]

For earthing, the shields should also always be grounded at the controls side of the circuit, not the field side (Ie in a car, at the ECU, not the sensor).
If you have a connector, you can continue the shields through the connector, but this may end up using a lot of extra pins depending on how much of your wiring is shielded. The other option is to have all the shields connected at the connector (in a star arrangement) and then have one drain wire connected to the chassis near the ECU or the battery or whatever.

The big no-no, as oldcorollas stated, is to NEVER connect the shield at both ends. This introduces ground loops and can be worse than not having a shield at all. You want to avoid having any closed loops in the shields.
Also, not connecting the shield at all can be worse too, as the shield acts as an antenna and actually amplifies the effects at the open ends. (This is especially true with metallic duct work).

[RT40Corona4AGTE]

TERRA speaks the truth!

[thechuckster]

only needs shielding on stuff like VR sensors (e.g. CAS) and narrow-band O2. Igniter signal is fairly high amperage in comparison.

[oldcorollas]

i was under the impression that (at least in the case of many later Toyota's and others) the injectors were 'negatively switched', so shouldnt be too many worries with the injector wires from the ECU? (specific case is an EMS Stinger on 1JZ). I could see the wires from the ignitor to the coilpacks being an issue though... Is this theory correct?

ignitor to coil packs.. they might carry a couple of amps, but the ignitor signal wire from ECU to ignitor (if it is in engine bay) will have very little current.

regardless if injectors are positively or negatively switched, they will still flow the same amount of current.
for high impedance at full duty thats about an amp or so.
for low impedance they can have peak of 3-4 amps, but again, average at around an amp or so over the whole cycle..

so if you had say, 3 injectors on a bank, thats 3amps of pulsing current to help mess up your signal wires from sensors

[50RTD]

Thank you guys, you've answered my questions excellently!

+1000 Rep for everyone!!!!

[thechuckster]

was thinking about stu's last post ... that wires to ignitor would benefit from shielding.

Given that the ignitor(s) are essentially a 5+ switch, as long as you're not inducing a massive current surge, they 'should' (and i say that advisedly) cope with a mild amount of electrical noise. If anything, they'd be more prone to interference if the return earth from the ignitor to ECU was somehow floating.

But anything you do to reduce noise is a good thing.

... and a smart ignitor that triggers off an inverted signal (start dwell when IGN is pulled from 5 to 0v) or a dumb ignitor (ECU determines dwell and the duration of time the signal changes state represents dwell time) both might be more sensitive if you can't pull that signal to 0V.

So now that I've defeated my own argument, shield anything you can

[50RTD]

Ive pulled the guts out of the BMW loom and the bavarians seem to shield everything, all the sensors have their own shield & earth, same with coil and injector wiring etc, and im going to try and use most of the factory wiring as possible, so hopefully at will be sweet.

Here are some work in progress pics of the multipin plug im using... The car already has one of these plugs that interface the engine bay-chassis stuff, and ive pinched another one for the engine to ECU connection.

[Hen]

I can only agree with what's been said above.

And add that I ran all engine wiring through a couple of aviation plugs (similar to the plug pictured above, just smaller). I ran the shields through the plugs too (each shield got it's own pin) and earthed them back at the ECU.

This wiring has never given me any issues.

[Flex]

My thoughts,

The reason for shielding circuits is to stop unwanted emission, EMI, noise from interfering with low voltage (EGO sensor) or edged triggered (crank sensor) circuits. Shielding works by the electrical skin effect, where any noise affecting a shielded cable will travel along the cable shield on the outside surface and be dissipated / "shorted out" into the module's ground. Therefore any sensitive circuit which are routed or are near by any large current ( > 0.3A) devices such as solenoids, injectors, etc should be shielded. Another thing to consider is that connectors in between sensors and modules cause problems because they break the shield (unless you are using a fully shield connector, which is big $$$$$) as you must pigtail the shield to pass is through the connector.