Power Ground I'm Confused

[Zchris]

So I'm wiring up my new Atom2 and the last thing I'm doing is the Power Ground. In the manual in section 17.3 is says to attach the 2 Black wires to the back of the head. Now isn't that about the noisiest place you can put a ground as the ignition current is travelling thru the cylinder head. Now I've only wired up 2 other brand of ECU and they both said to "not use the engine" and I always assumed that was because there was no current path at the engine other than the ignition. Please explain if you would. Thanks

[cj]

Most cars will run a rather large cable from the battery negative direct to the engine, and a smaller one to the body. This means your best "source" of ground is usually the engine block because its got the biggest path to battery negative. 

By the same logic, the engine is a big solid chunk of metal that conducts current pretty well so you get very low resistance from one side of it to the other. The sheet metal in the body is very thin, has numerous stich or spot welds, and is a relatively high resistance path back to battery negative.

[Zchris]

Its a tube chassis race car with a solid mounted engine and I have a ECU specific ground strap from the battery, which is in the rear. Can't get more clean and direct. Still doesn't jive well with a "not noisy" ground path using the head. Link must have a reason for wanting 2 separate lines to the engine. Before I connect anything, as a engineer, I need to know the why's. I have found most installers just do what there told without wanting to know why. Especially when counter intuitive. And the big reason to ground to the block is usually for starter motor current draw.

[cj]

Second bit of logic then - your most important sensors are those in the engine. Fuel level sensor a bit off, who cares, crank position a bit off = badness. 

If the ECU is using the same reference point for ground as the sensor, then any ground offset is minimized. There is always going to be some electical noise in the engine bay and any engine sensors + wiring are going to see it. If the ECU sees the same level of noise (on its ground, not directly on the chips etc inside it), from a relative standpoint, the signal is still "correct". You can shield individual wires but you cant shield the whole engine on the ground side.

The assumption here is always that a cable designed to flow current is going to be lower resistance and more consistent than a path through the body - even in your tube frame there are welds, possible different types or thicknesses of metal etc, and certainly no shielding. By the same logic, putting the ECU near the engine not only helps packaging (usually), but means any signal wires including ground have a shorter run, reducing losses in the cable and essentially the length of the antenna(wire) that can pick up EMI.

[Adamw]

I will add a little bit too:

In an automotive environment, the best grounding strategy to avoid ground offsets etc is generally star point grounding, this is where all grounds are tied to a single point.  If that was our only consideration then that star point could equally be the engine or chassis (provided there were no sensors that ground through the engine).  However a big part of the consideration of that star point location is that we also need to minimse magnetic loop noise from the ignition system.  To minimise this you need to make the magnetic loop as small as possible.  For most common ignition systems this means the ignition coil secondary ground needs the shortest path possible to the spark plug electrode.  Since we are using a star point ground and our coils need the shortest ground path possible then that means our star point needs to be on the engine.

[Andy P]

On 3/29/2018 at 4:59 AM, Adamw said:

I will add a little bit too:

In an automotive environment, the best grounding strategy to avoid ground offsets etc is generally star point grounding, this is where all grounds are tied to a single point.  If that was our only consideration then that star point could equally be the engine or chassis (provided there were no sensors that ground through the engine).  However a big part of the consideration of that star point location is that we also need to minimse magnetic loop noise from the ignition system.  To minimise this you need to make the magnetic loop as small as possible.  For most common ignition systems this means the ignition coil secondary ground needs the shortest path possible to the spark plug electrode.  Since we are using a star point ground and our coils need the shortest ground path possible then that means our star point needs to be on the engine.

Does this include 'Gnd Out' (D5 Thunder)? or is this treated differently and does it have a different purpose?

[Adamw]

Gnd out is the ground supply to sensors.  It should only be connected to sensors, and not the engine or chassis.

[Andy P]

From that I presume you mean the return path for the 5 Volt and 8 Volt supplied systems.

[Simon]

And Temp, VR sensors etc

[Fwdmn]

Again to avoid a fresh thread and add my most-likely-stupid question to this thread as it comes up immediately from Google on the ground subject...

So, the numerous individual power grounds...

No beef with going to the engine, makes sense to me, and I've done a couple of mates Link setups like that.

That said, why are 4 individual wires mandatory all the way to the engine needed? Say I went 2 x 18AWG from each A and B connector into one single 14AWG wire going to the engine? (Sizing is hypothetical, I've not checked amp ratings so if the size is ridiculous please disregard). Or say I ran all 4 to a BUS and then the BUS's main feed wire to the engine in an adequate size?

Is this purely, as I think Adam may have angled at in another thread, so that if a bolt comes loose or you otherwise lose a wire then you still have a safeguard in place via the remaining wire(s) until you are able to replace it? Is there some core electrical concept besides a precaution that I am missing with regard to grounding?

Thank you in advance for any clarification.

 

p.s. I have now found the help section in PC Link which answers masses of questions, I was unaware of this having not installed it previously so apologies for any prior questions that had been answered in the existing documents there.

[Adamw]

I dont think there is any specific electrical reason they need to be kept as seperate wires apart from redundancy coupled with the poor wiring skills we often see in this game.  There are some considerations, any connection adds resistance and a possible point of failure, so it should always be as direct as possible.  For this reason I would definately never connect the main ECU ground to a bus bar, it should go direct to the block/head.  Any splices must be done best practice with proper open or closed barrel splices, correct crimp tools, sealed from corrosion and strain releif etc.

Note you may not need all 4 grounds, You only need enough to carry the load you are low side switching.  For example if you only have 4 x 1A injectors and a couple of aux outputs connected then there is no need to connect all 4 ground pins.  

[Fwdmn]

That is excellent info thank you, weapons of choice here are usually Raychem closed barrell connectors with a decent crimp tool and adhesive shrink, so hopefully no trouble here besides the potential sleep deprived operator error, but I totally hear you on the often dicey connections seen in aftermarket automotive wiring.

Last question before leaving you all in peace and reading through the help files again, the Main Relay + and - terminals seen on an additional connector on some models, and I guess the Ign Sw terminal, are these mandatory when feeding the 14v+ terminals from a relay already and not wanting to use any hold power function?

Thanks again for your time and effort straightening out uneducated folks like myself.

[Fwdmn]

Appreciate the help so not trying to rush anybody, sure you have loads on your plate, but I'll forget about it until I really need to know if I don't find out soon haha, so time for a quick morning bump

[Adamw]

The only thing the Thunder needs to power up is 12V on pin A5.  The main relay functionality is only there to make it easier for installation in cars that already have an existing main relay circuit that you want to control.