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Grounding Engine to Battery

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I'm trying to get my grounding strategy straight for the car I'm working on. For background, it's a 2002 Mercedes-Benz CLK55 powered by a Holley Dominator EFI. The battery is mounted in the trunk (OEM location)

I have attached an image of the grounding layout as it currently stands. My main question is do I need to add a wire running from the engine block to the battery (red dotted line)? If so, what gauge should it be?

The current layout "feels" right to me, based on my limited understanding of the star pattern that was discussed in the EFI Wiring Fundamentals class, but Holley recommends running a ground from the engine to the battery. This would seem to deviate from the star pattern.

Attached Files

You are essentially replacing the OEM copper strap / chassis with the Holley ground wire. I would be tempted to add the Holley wire and remove the OEM copper strap. That leaves everything as a star to your grounding post.

Thanks for the feedback. Since that wire will be carrying the full current of the starter motor during cranking, I assume it'll need to match the size of the positive cable running from the battery to the starter. (I believe it's a metric equivalent to a 0 AWG).


feeding to this discussion, I was watching a Haltech video where Scott (aka Tuning Fork) made a quick comment about how "the chassis is not a wire"

To my knowledge the chassis is a fine conductor, yes steel conducts less then copper but the chassis has so much more surface area that its actually better. He was speaking to cars with the battery relocated, Im thinking the key issue might be install quality. Cant even begin to tell you how many times I've diagnosed a bad ground.

But anyway food for thought, if done properly is there anything wrong with using the chassis as you main ground? vs running a separate equal sized ground wire from the battery to the engine bay? I don't actually know what OEM's like Mercedes and BMW that trunk mount batteries do

My 2002 CLK55 came with the battery mounted in the trunk from the factory. Mercedes ran a short ground cable from the battery to a ground lug on the chassis in the trunk and a braided copper ground strap from the engine block to the chassis in the engine compartment.

I also saw Andre mention in a webinar that doing it this way should be sufficient - you don't necessarily need a separate ground cable running from the engine all the way back to the trunk.

My main concern, and the reason for this thread, is whether or not doing both would violate the star earthing principal. As far as I can tell, one or the other is fine, but both would create a ground loop.

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