Signal noise can be a significant problem while tuning a MegaSquirt ECU. Inaccurate sensor readings due to signal noise are not only a nuisance, but a hazard. The best way to prevent unwanted signal noise is to ground the MegaSquirt ECU, sensors and actuators at the same spot. Doing so guarantees all ground points are at the same voltage relative to one another: 0V.
Cars exemplify a “floating ground” electrical system since everything is compared to the battery's negative terminal, not earth ground. These systems are where hazardous ground loops can occur.
An ideal “ground” has no resistance to the negative terminal. At first, it might seem like connecting your MegaSquirt ECU directly to the battery is the best solution. Remember, though, that long wires will introduce resistance into a system. To test a grounding point empirically, use a calibrated digital multimeter (DMM). With the car battery disconnected, less than 0.5 ohms should be measured between the grounding point and the negative battery terminal.
In practical terms, a good ground uses short, large gauge wires; preferably under 24”, ideally under 12”. The grounding point is a welded, threaded stud or a tapped hole in the vehicle body, not a removable bracket. The grounding point and immediate area should be bare metal free of all paint and rust. Once the ground connection is connected and tightly secured using a lock washer, it may be coated with a sealant or paint to protect against corrosion. Of course, if the ground connection is ever removed, the area needs to be cleaned again before reattachment.