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Basic Grounding Questions

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I'm looking for some help with a few basic grounding questions, please:

1. Is it necessary to individually connect the block and chassis to battery negative, or can they be daisy-chained? I was proposing to do the following:

Cable 1: Chassis-to-Block

Cable 2: Block-to-Battery (at same stud on block as #1)

2. Star-point earthing: should the ECU ground wires (which attach to the block) be on the exact same lug as the block-to-battery cable or is it acceptable to separate these smaller gauge ground wires from the high-current wire?

3. I've seen some folks recommend extra grounding cables for the starter and alternator. My starter and alternator are both grounded to the block through their metal bodies: there is no dedicated grounding lug. Surely this is sufficient, if they are fastened to the block correctly?

4. If I am placing all grounding cables on the block, is it recommended to bond the cylinder head to the block?

Thank you!

Here's my input on your questions :

1. The usual is : Battery to chassis, then block to chassis. Use the block as a star earthing point.

2. No, it doesn't need to be on the same lug. The block/head conducts electricity well enough.

3. I never had to add grounding cable to a alternator or a starter.

4. I never had to use a strap or a cable to bond the head to the block.

Important point for all the answers : You need to make sure your connection points are free of rust or paint or other contaminant.

Thanks! Concerning #1: if the block is to be the star point, shouldn’t the chassis connect to battery via block, instead of block going to battery via chassis?

Usually, both the vehicle chassis and the engine block are star points. But, there can only be 1 path for the current to flow back to the negative post of the battery, and it's your large AWG used for grounding.

The very important thing about grounding is that you never share 2 star points in one system (like grounding your ECU to the engine AND the chassis). This is what creates a grounding loop, or common grounding path. In the wiring fundamentals course, grounding module, Zac used the engine for the connection back to the battery, but if you use the chassis to battery negative, you achieve the same.

To sum it up : Keep your grounding point clean, free of rust or paint. Never connect one system to multiple star points to avoid common grounding path.

Darnit, I thought I'd commented.

Yes, as Francis said, using more than one earthing/ground point for a series of sensorrs, etc, connected to the same control/monitoring device can cause problems. In order for current to flow, there needs to be a difference in voltage/potential and different resistances of the ground return path(s) at different points means that the voltages between different points will vary slightly - this could be 1V or more, but with some electronics a milli-volt (0.001), or even micro-volts (0.000001V) may mean a difference in what the sensor is giving and what the control device is getting. This can really be a problem when high current draws occur, eg starting or switching high current devices like pumps and cooling fans,because the voltage drops at different points can be much bigger.

By using a common ground point for the same controller/ECU's connections, the same ground potential is seen by all. If you do feel the need to run higher current rated grounds (or power), either use heavier gauge wiring/cabling or double up using the same mounting points - I recall a case where adding a ground between the gearbox and chassis introduced all sorts of issues because just that addition changed the voltages for the transmission controller.

3/ Depends - some alternators are fitted, OEM, with isolators made of plastic/rubber to prevent vibration damaging the alt'r, and these NEED a ground between the alternator and chassis/bracket for a ground. less common, some alternators are made with the internals electrically insulated from the body, and will have a specific terminal to attach a ground wire/cable to. In both instances, make sure the ground/earth is rated for at least the maximum alt'r output - I've had an OEM setup where the ground had got hot enough to melt the insulation.

Frank & Gord: thank you for your comments! I've definitely absorbed the lesson on star point earthing from the HP course. I will send all my ECU device and power grounds to the block.

What do you think of the general wiring scheme drawn below? I've introduced a simple battery isolator which will cut the negative side of the battery when the car is in storage.


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