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Old 04-07-2021, 12:25 PM   #4
142 guy
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Join Date: May 2014
Location: Saskatchewan, Canada
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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.
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