Proper Grounding

Signal noise can be a significant problem while tuning any engine management system. 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 sensors at the MegaSquirt ECU, and ground the MegaSquirt ECU directly at the engine block. Doing so guarantees the MegaSquirt ECU and sensors are using the same ground reference.

Ideal Grounding Systems

Cars exemplify a “floating ground” electrical system since everything is compared to the battery's negative terminal, not earth ground. Even though all points on the chassis and engine block are theoretically at the same potential, in reality they're often not. Floating ground 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.

Practical Grounding Systems

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 on the engine block or cylinder head, 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.

All sensors should be grounded directly at the MegaSquirt connector. SymTech Labs wiring harnesses have a special “sensor return” wire for this purpose. The wire is black/white, and any number of sensors can be connected to it.

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