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Coil and ECU Ground Best Practices

Professional Motorsport Wiring Harness Construction

Discussion and questions related to the course Professional Motorsport Wiring Harness Construction


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I've followed the grounding layout of a harness built by one a well known motorsport harness constructor. This had 4x 18ga coil ground wires and 4x 22ga ECU ground wires combined and grounded to the chassis on the firewall next to the ECU (inside the cabin).

Having followed some of the recent discussions on star point earthing, this seems to go against what is being recommended. As I understand, running the coil grounds back inside the cabin to the firewall is creating large ground loop for the coils with the potential for EM induced noise.

I'm now planning to ground the 4 coils to the cylinder head at the OE grounding point. I'm looking for some input on whether or not to run the ECU ground wires through the bulkhead connector and engine harness to the cylinder head (star point)?

Also, now that I have a grounding point on the engine, what are the pro's and con's of combining the shielded cable screen's and grounding them to the cylinder head vs running them back through the bulkhead connector to an ECU 0v?

Like many things in the automotive world, there's the correct way to do it which in this case is star point earthing with a common earth point on the engine block. That doesn't necessarily mean that doing it the wrong way will always cause issues and there's undoubtedly many cars out there with some kind of weird earthing strategy that still operate. Often this will be causing ground offsets that might not be immediate obvious and the errors they cause have been baked into the tune. You're correct that grounding the coils to the chassis will create a large loop area which can cause EMI. Again, this doesn't immediately mean that you WILL have an issue but it raises your chances of problems.

Personally I always recommend using industry best practice to limit my exposure to weird problems that are going to be a hassle to rectify later.