In the first three parts we illustrated the most common battery dual ground cable using two separate ground cables connected to -BATT. One cable -BATT cable connects to the engine block and the other -BATT cable connects to the main sheet metal ground (1). The dual ground configuration is very common but now that batteries are mounted in out-of-the-way places outside of the engine department a single battery ground cable is used to ground the battery negative terminal to the engine block.
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A separate accessory/sheet metal ground circuit must be employed to connect to the major sheet metal ground (1). This is necessary because the generator/alternator -GEN (say minus GEN) is grounded to the engine block.
Notice there is a ground strap from the engine block to the accessory ground sheet metal (1). The ground strap appears on the left side of the engine block in the schematic and connects the engine block to the accessory ground sheet metal (1). This replaces the accessory ground that came from -BATT on the dual battery ground cable. The ground strap changes the channeling path of electron current through the engine block to supply electron current to the sheet metal.
On most vehicles the accessory ground will appear as a ground strap rather than a round battery cable. It may be covered with insulation or it may be an exposed metal ground strap.
Since electron current must channel through the engine block to get to the accessory ground strap, the location of the ground strap connection to the engine block is extremely important. The factory determines the safest place on the engine block to connect the ground strap so that channeling electron current does not pass through engine bearings or the water jacket.
If electrons are allowed to pass through the engine bearings, they would be damaged resulting in eventual engine bearing failure. If electrons were allowed to pass through the water jacket the resulting corrosion would cause water jacket failure and possibly coolant leakage into the engine oil.
A major issue
Always maintain the same location of the ground strap on the engine block. Never change the ground strap location on the engine block or arbitrarily add a ground strap to the engine block thinking it is a good ground for a particular circuit. The engine block is indeed a good ground as long as the electron current does not channel through engine bearings or the water jacket while supplying electron current to the circuit you have just added to the vehicle. To be safe never rely on the engine block for a ground connection unless the manufacturer has designated a location on the engine block as a safe ground terminal.
Figure 2 illustrates the electron current path when the battery is the voltage source, as happens when the engine is not running. The ground strap is used to connect the sheet metal to -BATT.
Since the battery is the voltage source when the engine is not running, electron current leaves the negative terminal of the battery and flows down the engine ground and enters the engine block. Electrons channel through the engine block to enter the connection of the ground strap to the engine block and travels through the ground strap to supply electrons to the major accessory ground sheet metal (1).
Any circuit grounded to the main sheet metal ground (1) by another ground strap allows current flow to the second sheet metal (2). Electron current then passes through all circuits grounded to either sheet metal. Lamp Circuit #1 and Lamp Circuit #2 each receive their necessary electron current which returns to the positive terminal of the battery +BATT. Next we explain the path of generator electron current.