I'm starting to get pissed off at myself that I don't just intuitively know some of this stuff, but I don't; I'm genuinely confused and it's embarrassing. I suck and I should be punched in the ***** for it...

Now that we've got that out of the way, what's the deal with the various ground points on a car? Why does it matter where you ground something?




Here's how I think it works: Ground is just the negative terminal of the battery, yet since the negative terminal of the battery is connected to the metal chassis, the entirety of the chassis is essentially turned into a "wire." This is logistically advantageous since you can effectively halve the number of wires you need running around the car. But the metal frame, like all conductors, will have some measure of resistance that is related to its dimensions and/ or the distance between where the chassis connects to the battery and where any given component is grounded, i.e., there is more resistance in a circuit that is grounded in the engine bay than in the trunk, right by the battery.

Am I right about any of that stuff? Is that why certain sensors need to be grounded at certain points? What makes those points suitable/ how is that determined?



Here's my problem. I am trying to wire in my NGK Powerdex AFX (simple enough, right) and the instructions regarding where to ground the signal vs. grounding the power to the unit are confusing.

Page 9 has the wiring diagram. Page 10 has the wiring instructions:
http://www.ngksparkplugs.com/pdf/AFX...ual_REV_06.pdf

Here's what it says:
AFX Wiring Notes
1. The AFX considers the point where the two BLACK wires connect to their
ground as the 0 (zero) volt reference point.
2. When utilizing the analog output feature of the AFX, always be sure to connect
the system ground (two BLACK wires) to the same location as the analog
SIGNAL GROUND (BROWN wire). The analog output wires (YELLOW,
BROWN) may be lengthened as long as the appropriate gauge wire is used and
the connection is solid.
3. It is advised to connect the BLACK wires directly to battery ground or as close to
this point as possible. Do not extend the two BLACK wires using a single wire or
else this can cause a shift in the ground level of the analog output SIGNAL
GROUND (BROWN wire) and any device linked to the analog output (data
acquisition or engine controller) will receive an incorrect signal.
4. The RED wire is for system power positive (+). It is acceptable to route this wire
through a fuse and/or relay where needed. The wire may be lengthened as long as
the appropriate gauge wire is used and the connection is solid.


So how should I interpret that? Should I ground the signal ground (brown wire) to the negative battery terminal along with the black wires? Or should I ground the signal ground and the system ground to ANY point so long as they go to the same point? Or since this is a "signal ground" does it need to be grounded to the same place other signal grounds for factory sensors are grounded to (like the throttle body)?


This stuff is really cooking my brain.

 

Also, I was taught in high school electronics class that electricity actually flows from negative to positive and that "conventional flow" is an outdated way of thinking. Is that accurate?

If accurate, do car electronics assume positive to negative?

 

Preface: the advice given in that manual is safe, but non-optimal. For the cleanest possible signal, the brown wire would be grounded directly at the ECU's sensor ground. The black wires can be grounded to the ECU's high-current ground, or any other convenient spot.


Because the longer the path that the current must travel, the greater the resistance. The greater the resistance, the higher the voltage drop, the greater the potential for offsets in analog sensor signals, etc.



Nope.

When the car is running, the alternator is the primary current source. Thus, the body of the alternator is the reference ground.

Since the alternator is bolted to the engine, the engine is effectively at "true" ground potential as well.

And since a ground strap connects the engine to the chassis, the chassis is fairly close to ground potential.


When it comes to sensors, they typically want to be grounded as close as possible to the device which is reading them (eg: the ECU) even if this device's ground is "further away" from engine ground than you could easily achieve by running a wire to engine or chassis.

The reason here is that you want the sensor and the ECU to be at the same ground potential, so that any offset from true ground is shared equally between them rather than showing up as a voltage offset.



I agree, that is your problem.



From a purely technical standpoint, this is correct.

It is pretty much totally irrelevant from a practical standpoint, since current must always flow in a loop. The electrons don't care which half of the trip is through chassis and which half is through a dedicated wire.

Older cars used positive ground wiring. The choice here was mostly arbitrary, and automakers switched to negative ground only because it caused less galvanic corrosion of the bodywork.

 

I think I follow you. My ECU's sensor ground has be rewired per the instructions from my old Flyin' Miata Link ECU install guide, which says:

Ground Wire Modification
We have found better overall operation of the FM ECU when the following modification is made
to the ECU ground wire circuit.
4) Locate the two Black/Green wires coming from the 2C and 2D pins and follow them back
to where they are spliced into one wire. These wires are signal ground wires that carry small
amplitude signals to a ground point. Cut this single wire after the junction.
5) Crimp the ECU side of this wire into one end of the yellow butt connector. Take the 16
gauge wire included with the ECU and crimp it into the other side of the butt connector. The
remaining chassis side of the wire can be taped off to the wire harness. It will be unused.
6) Run the new 16 gage wire through the firewall. Behind the windscreen washer bottle there
is a plugged hole that is perfect for running this new wire. There is a photo of this hole on the
next page.
7) Run the new ground wire behind the engine to the bolt that holds the ground strap connected
to driver’s side rear of the cylinder head. Refer to the photo on the following page.
Note: Do not add 2A and 2B to this connection, as these are power grounds and they will negatively
affect the signal grounds.
8) Clean the points where the ground strap contacts the body and the head with fine sand
paper.

So I should connect the brown wire (from the O2 sensor setup) to this point, right?

It is also my understanding that the ECU's power ground runs to junction 3 (where ever the hell that is), so I should connect the black wire (from the O2 sensor setup) to that point, right?

 

Also, I found an old thread that sort of addressed some of these issues:
https://www.miataturbo.net/general-m...ine-bay-19342/


In that thread, you said, "Ideally, I'd think you'd want to take your wideband power off of the white/red wire which comes from the Main Relay supplies the injectors, the ECU, the CAS, the IAC valve, etc. I spliced into the white/red wire at position 1B of the old ECU harness, and that's where I take power for both my MS and my WBO2. Fortunately, the current draw of the wideband is relatively small."

Should I hook the power from my Powerdex unit to the white/red wire at position 1B?

 

And just in case you haven't gotten your due and proper often enough, Joe, you sir are awesome. In every thread I've found whole searching, you pretty much saved the day in every single one.

I really appreciate your help and suggestions and you truly are an asset to this forum.

Have you ever considered a blog (or maybe even a book) where you explain all this electrical stuff from start to finish?

 

That's pretty much the gist of it. Splicing the brown wire into the Blk/Grn ECU grounds puts the sensor's analog ground as near as possible to the ECU's analog ground, and thus minimizes the potential for offset and noise.

The grounding the of sensor's black wire is less critical, as it's just the heater ground. That can go anywhere you like. Since you're already up in the ECU harness, the ECU black ground wires would be a convenient spot.




Where you take the power supply from is not terribly critical.

The Innovate products don't like to be powered on while the starter is turning, so I sometimes hook them up to the radio's power lead (which is interrupted while the key is in the START position.) The AEM sensors don't care, so the white/red wire is a fine place for them. I have no idea how persnickety the NGK sensor is to its supply voltage.



Nope.

The minute I write a book then suddenly I'm claiming to be an "expert," and I have no desire to bear that cross. I just contribute a few words around MT from time to time, mostly to fill the empty space between the cat pictures.

 

One last question...

On my MAP sensor and other sensors required to get my AEM setup running properly, the AEM instructions recommend, for instance:
With a mind toward the sensors all working to the best of their ability, should I actually tap the sensor ground to the ECU's wiring harness at 2C or should I run the wire from the sensor to the place where 2C terminates, i.e., the ground point on the block?

It seems that there would theoretically be some difference between the two points, but would it be negligible?

 

Ahh, that's a crock of ****. Everyone on here already considers you to be an expert. Your modesty is telling of your character, and admirable to boot, but your expertise is apparent and very much so appreciated.

 

They should be connected as close to the ECU itself as possible. Running new wire out to the ground point on the engine would be worse, as the ground path between the sensors and the ECU would be longer.

You're correct in assuming that the current path from the sensors does indeed terminate at the engine, but it's desirable for it to follow the same path as the ECU to get there, so that any minute offsets between ECU ground and engine ground are also "seen" by the sensor.

 

Oh, hell. I COMPLETELY misunderstood what you said earlier. I thought you were saying that I should hook the brown wire (signal ground) from the Powerdex to the point where the signal ground wires for the ECU are, uhh, grounded (at the engine), but in actuality you want the sensors to be grounded right by the ECU, as in, where the ECU's signal ground wires enter the ECU.

Man, I got entirely the opposite understanding from your earlier posts. Thanks for clarifying. You mentioned something about me potentially being insane in another thread. I think you may be onto something. (Poops self and sits in a fetal position in the corner).