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-   -   Spartan2 Lambda controller wiring to MicroSquirt V3 (https://forums.turbobricks.com/showthread.php?t=359818)

Msi 04-05-2021 07:06 PM

Spartan2 Lambda controller wiring to MicroSquirt V3
My question is at the bottom, but I need to set the scene first, so that you could feel my pain.

Looking at the MicroSquirt V3 grounding and sensor grounding docs -

This is a dedicated sensor ground wire.

Pin22 and Pin23
The ground wires from pins 22 and 23 should be run to the same spot on the engine.

In this diagram -


We can see that the Wideband controller is shown to have 2 grounds. One goes to Pin20 (like other sensors do) and the other goes to "Power Ground".
We also see that the MS unit is grounded via Pin22-23 to a "Good Engine Ground".

Now, looking at the Spartan2 Lambda controller documentation (this one is a PDF, sorry) -


It also has 2 ground wires.


Black wire, Electronics Ground, Ground where interfacing device is grounded.

White wire, Heater Ground, Ground to chassis or engine block.

Also, some more detailed explanation is available here -



Grounding is very important, Spartan 2's black wire (electronics ground) should be grounded where the MegaSquirt ECU is grounded, Spartan 2's white wire (Wideband Sensor Heater Ground) should be grounded to the chassis far away from where the black wire is grounded. The white wire carries upto 3 Amps and if grounds are not properly chosen, it can cause very large ground offset errors.

Now, there are several contradictions here -

If I follow the Spartan docs, they specifically say one ground on engine, another ground where MS is grounded (which is on the engine as well via Pins 22-23) but they say that these grounds should be far away from each other. WTH?

If I follow the MS docs, I do not understand what is "Power Ground" as opposed to "Good Engine Ground" and is Pin20 far away enough from it. WTH?

I'm in pain...

Could some one please tell me how to wire the Spartan2 Lambda controller to MS? And the rational behind it? Cause I really want to understand.

Thanks in advance!!

Dirty Rick 04-05-2021 10:44 PM

Power ground is a good engine ground you can't get much closer to the battery negative than the motor.

The power source for the O2 controller should be separate from the MS as well because the O2 controller uses PWM to control the sensor heater and are inherently noisey. Plus the O2 heater should not be on unless the motor is running, so I grab power from the fuel pump relay as that shuts off when the motor does.

Msi 04-06-2021 04:36 AM


Originally Posted by Dirty Rick (Post 6173113)
Power ground is a good engine ground you can't get much closer to the battery negative than the motor.

This is where I'm confused.

In this diagram -

There is a clear distinction between:
"Good Engine Ground" - where the MS unit is grounded via Pin22-23.
"Power Ground" - where the fuel pump is grounded and where the Wideband is grounded (that's the heatsink ground, the sensor is grounded at Pin20).

So "Engine Ground" and "Power Ground" must be different. Right?

I think they want the high current devices grounded far away from where the electronics are grounded.

So, if MS is grounded at the Engine via Pin22-23 and sensors are grounded to MS Pin20 (or alternatively at the Engine as well) then "Power Ground" for high current devices should be far away from the Engine?

Or am I completely wrong here?

142 guy 04-07-2021 12:25 PM

The most important factor is that you do not want to ground the heater circuit for the wideband controller at the Microsquirt controller. This will couple the heater current which can include a lot of high frequency noise (if the heater uses PWM control) into the MS ground plane which can lead to other problems.

Engine ground and power ground are meaningless distinctions. Grounding the Spartan heater circuit at the same point on the engine block as you ground the MS should be perfectly acceptable - as long as it is a well made connection. Once the engine is running the alternator becomes the controlling source for all voltages in the vehicle power system and the alternator's ground becomes the reference ground for the vehicle's power system. Normally, the alternator is solidly grounded through its case to the engine block by its mounting so the alternator ground and engine ground are the same and the preferred point for making your 'power' ground connections for critical engine functions.

Note that the terminals of the battery are generally not a good location for making power or ground connections for critical functions. Once the engine is running, the battery generally does nothing and there is no current flowing into or out of the battery. The exception is if the battery is being recharged after an engine start condition (or your battery is approaching its end of life) in which case current is flowing into the battery and the voltage on the battery terminals is actually slightly less than the running voltage of the car's power system. Make your critical power ground connections to the engine block.

The MSExtra diagram implies that the +12v supply comes off of the battery + terminal. The better choice for connection would be the +12v distribution point closest to the alternator. On my car, the alternator has a short electrical connection to a bus bar through a dedicated alternator fuse. That bus bar splits two ways, one connection to a fuse / relay panel with all the critical engine functions and a second connection to a splitter bus with all the other electrical stuff including the charging connection to the battery. Having all the critical engine functions; ignition coils, MS controller, wideband controller, Hall sensors, idle air valves .... on a common + supply is highly desirable.

Dirty Rick 04-07-2021 01:11 PM

142 nailed it ^^^^^^^^^^^^^^

Msi 04-08-2021 10:29 AM


Originally Posted by 142 guy (Post 6173576)
The most important factor is that you do not want to ground the heater circuit for the wideband controller at the Microsquirt controller...

Thank you for the detailed explanation. Much appreciated.

Shinchan 05-04-2021 09:28 PM

I just went through a lot of misery trying to figure this out myself, my AFR was bouncing all over the place as I had the black wire connected to the MS sensor ground and the white wire running to a ground near the main MS ground. This is one of the rare occasions where you need to ignore the MS documentation mantra of grounding all of the sensors to the MS sensor ground.

Black wire to where the MS is grounded, white wire to some separate chassis ground point. Do NOT use the MS sensor ground wire.

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