Grounding Q

[lukej]

Can someone please explain the difference between these two scenarios...

Relay board earthed to the engine.
All sensors earthed to the engine (same point)
Engine earthed to -ve terminal of the battery via gearbox earth strap.

Relay board earthed to an m6 stud.
All sensors earthed to the same point next to the relay board (m6 stud)
Stud earthed to -ve terminal of the battery direct with 1 wire.

Many thanks

Luke

[DaveEFI]

None of those is correct, because it means MS will share the power and sensor grounds for a longer distance than ideal. The power and sensor grounds should only be joined together inside the MS. If you can unplug MS and a sensor still has a ground, it's not correctly wired.
Thing is that every installation is different. You might get away with sharing grounds. But you may not. So if having any issues, best to do it the recommended way.

[lukej]

Ah ok, thank you for your response Dave.
The first one was my interpretation of the Megamanual and the 2nd was how I have run my wires before consulting the manual... The MS ground when using the relay board is the ground soldered onto the pad above the two lives yes?

[DaveEFI]

First, I'd not use the Megamanual unless using B&G code. And even then, I'd refer to the manuals on here as they have been updated with experience of various problems like grounding, as MS gets more and more versatile.

I've never used the relay board, since I've never seen the point in it. However, it does show a separate ground for the TPS return, so assuming the link cable between it and MS is correctly made, it would be possible to keep the sensor grounds separate from the power ones. Thing being the relay board design dates back to the days of MS being mainly used fuel only - ie early MS1 with a V2.2 board.

[Matt Cramer]

Dave is correct - the relay board has sensor return ground terminals already present.

[rickb794]

Section 3.2 of the manual lays it out for you.

Use only the MSExtra manuals http://www.msextra.com/manuals/ms2manuals/

For best results the sensors should have no ground connection with the harness unplugged at the MS.

Your first scenario is best except the sensors (they should ground to the MS)
The motor is a better (quieter) ground than the battery.

I would consider not using the relay board and relay harness.

Even breaking out the grounds from the MS creates a potential ground loop if the relay board is retained.
Also the relay board is not a "weather tite" and is really not designed for under-hood use.

That said, (there are many successful relay board installs out there)
If retaining the relay board, do ground it directly to the motor.
I like to use "Monster" wire 8-10ga (car stereo) for this.
If you use a screw and nut to connect to the board, solder it to seal the connection.
Solder any crimp connections as well to seal them.

[Targa44]

Luke,
All the above is from very knowledgeable people with great advice. I have a bit of a similar dilemma as you with the relay board. My car is so old it didn't have a fuel pump relay or other convenient connections. But the concern I have for you is how you've connected the Relay Board to the MS and what parts you plan to use. Are you using the DIY relay cable or did you create your own?

One problem is that the pins for the db37 Main Plug on the MS do not match all the pin descriptions on the Relay Board with regard to grounding (sensor vs. power grounds). The Relay Board schematic has the terminal (screws) sensors grounds at location #14 (TPS return), #17 (Air temp return), and #19 (Coolant return) - all linked to db37 pin #19 "sensor ground". If you have connected pin 19RB to pin 19 on the MS, the MS description is for power ground. The MS sensor grounds (I'm MS3) show pin 1 as crank sensor ground and pin 2 as crank sensor shield. All other "sensors" are at pin #7.

I'm in the middle of a work around (had a long delay) that essentially only uses parts of the Relay Board with other connections going direct to the MS. Get into the hardware guide for your MS2 and you'll see the issues.

good luck,

kj

[DaveEFI]

I'm not certain which pins on the MS DB37 you use for power grounds and which for sensor grounds makes any real difference. Provided they are kept separate, and only join together inside MS. But am willing to be proved wrong.

But when MS supplies an external wiring digram, best to just stick to it unless you have a good reason not to.

[rukavina]

Even though ground is ground .... they have split all the high current,noisy stuff in one area and bring the low current, clean grounds away in another section of the board. Electrons flow to ground and they eliminate "cross traffic" and isolate the different paths

[lukej]

It would seem most of the confusion is around the relay board, I have decided not to fit it and wire everything as per the schematic on pg 12 (or13 somewhere around there) That way I can run shielded wire for my VR and also the 2nd IGN output on IAC2B as opposed to having to run those external to my harness between the MS and the RB.

Shout out if I'm talking rubbish

[rickb794]

Even though ground is ground .... they have split all the high current,noisy stuff in one area and bring the low current, clean grounds away in another section of the board. Electrons flow to ground and they eliminate "cross traffic" and isolate the different paths

Exactly, you don't want all the traffic from the noisy neighborhood driving past or through the quiet neighborhood.

The high number pins 14-19 are the noisy side of teh board.

Adding a ground connection to pin 14 will help sink noise from low impedance injectors and or extra ignition or other outputs.
You want the lowest resistance path away from the quiet side as possible.