MSefi.com read only forum (Jan.15,2018)

I have noticed a few posts on this topic while searching but didn't see a good explanation for why the pins are different numbers on the DIYAutoTune pigtail vs the megamanual. The answer I saw said to take one wire and run to the sensor grounds and ground the rest to the engine block. I'm fine with that as long as it works, just would like to know the reason for the difference in diagrams.

My guess is it doesn't much matter since all the ground pins on the MS go to the ground plain in the PCB. The important bit would be to use just the one pin for all the sensor grounds. Since I made my own loom I've stuck with the MS layout. But used a short length of 1.5mm cable to bring the sensor ground outside the DB 37 and made all the sensor ground connections to that as I've found real difficulty getting a connector cover to fit over the loom due to the size of it.

amrcnv8 wrote:I have noticed a few posts on this topic while searching but didn't see a good explanation for why the pins are different numbers on the DIYAutoTune pigtail vs the megamanual. The answer I saw said to take one wire and run to the sensor grounds and ground the rest to the engine block. I'm fine with that as long as it works, just would like to know the reason for the difference in diagrams.

Pins 1, 2, and 7 through 19 are all tied to the same point on the board. Consequently, even official MS wiring diagrams do not always show the same ground pins used for the same functions.

Matt Cramer wrote:

amrcnv8 wrote:I have noticed a few posts on this topic while searching but didn't see a good explanation for why the pins are different numbers on the DIYAutoTune pigtail vs the megamanual. The answer I saw said to take one wire and run to the sensor grounds and ground the rest to the engine block. I'm fine with that as long as it works, just would like to know the reason for the difference in diagrams.

Pins 1, 2, and 7 through 19 are all tied to the same point on the board. Consequently, even official MS wiring diagrams do not always show the same ground pins used for the same functions.

Would it make any difference if you used a number of pins all grouped together (say 19-16) for the sensor grounds with one of them going to the same block ground as the main grounds? Would make wiring the DB37 a deal easier, where you're not using a relay board.

trakkies wrote: Would it make any difference if you used a number of pins all grouped together (say 19-16) for the sensor grounds with one of them going to the same block ground as the main grounds? Would make wiring the DB37 a deal easier, where you're not using a relay board.

I'd use at least 5 ground wires from the MS to the block. You can give the sensors their own separate return grounds if you want, just don't try wiring a ground wire to both the sensors and the block at once.

Matt Cramer wrote:Pins 1, 2, and 7 through 19 are all tied to the same point on the board. Consequently, even official MS wiring diagrams do not always show the same ground pins used for the same functions.

Perfect. Thank you very much, that was very helpful. I am better at mechanicals than electronics, so I appreciate the help.

Matt Cramer wrote:I'd use at least 5 ground wires from the MS to the block. You can give the sensors their own separate return grounds if you want, just don't try wiring a ground wire to both the sensors and the block at once.

Silly questions I'm sure, but would like to learn if I can: Why do 5 different ground wires run to the same spot? Larger current carrying capacity when used together? Less noise? Also, why does running a separate wire to the sensors matter if they all run back to the same spot on the board?

amrcnv8 wrote:

Matt Cramer wrote:I'd use at least 5 ground wires from the MS to the block. You can give the sensors their own separate return grounds if you want, just don't try wiring a ground wire to both the sensors and the block at once.

Silly questions I'm sure, but would like to learn if I can: Why do 5 different ground wires run to the same spot? Larger current carrying capacity when used together? Less noise? Also, why does running a separate wire to the sensors matter if they all run back to the same spot on the board?

As regards the main grounds, my guess is it's because the DB37 has too low a power handling per pin. And because of the pin spacing, there is a limit on the size of cable that can be used to each one.
It's best practice to run all sensor grounds to the same spot as it prevents tiny differential currents flowing in them. But how much it would matter if you used several pins for these sensor grounds, I'm not sure. However, Matt says using individual pins for those grounds is ok. It would make wiring your own loom easier. I've taken a short length of 1.5mm wire from pin 19 to just outside the connector shell and made off all the sensor grounds to that, which seems to work just fine.

Why multiple wires to the ground point? Bruce said this in a post about grounding the Microsquirt. He was talking about the high-power wires, but I suspect the statements are valid for all grounds.

"It is important to run all three wires because it will reduce both the resistance and the overall inductance of the ground return path. Each wire has a resistance, and using three of them in parallel reduces the overall resistance. Equally important, each wire has an inductance, and inductances do not "like" fast-changing signals (like a pulse from a spark) and can cause very brief voltage offsets in the ground path. By having multiple wires it is the same as having multiple inductors in parallel, resulting in an overall lower inductance. "

Brian

cmonref wrote:Why multiple wires to the ground point? Bruce said this in a post about grounding the Microsquirt. He was talking about the high-power wires, but I suspect the statements are valid for all grounds.

"It is important to run all three wires because it will reduce both the resistance and the overall inductance of the ground return path. Each wire has a resistance, and using three of them in parallel reduces the overall resistance. Equally important, each wire has an inductance, and inductances do not "like" fast-changing signals (like a pulse from a spark) and can cause very brief voltage offsets in the ground path. By having multiple wires it is the same as having multiple inductors in parallel, resulting in an overall lower inductance. "

Brian

The V3 diagram shows 5 main ground wires, but only one sensor ground.

cmonref wrote:Why multiple wires to the ground point? Bruce said this in a post about grounding the Microsquirt. He was talking about the high-power wires, but I suspect the statements are valid for all grounds.

"It is important to run all three wires because it will reduce both the resistance and the overall inductance of the ground return path. Each wire has a resistance, and using three of them in parallel reduces the overall resistance. Equally important, each wire has an inductance, and inductances do not "like" fast-changing signals (like a pulse from a spark) and can cause very brief voltage offsets in the ground path. By having multiple wires it is the same as having multiple inductors in parallel, resulting in an overall lower inductance. "

Brian

Yes, that also applies to the MegaSquirt. The MegaSquirt uses more ground wires because the D-sub pins have a lower current capacity than the AMPSEAL pins.