Search Autoparts/Motorage/Electrical/

Ground Circuits: Part 5

Generator Electron Current - Single Battery Ground Cable
Wednesday, July 5, 2017 - 06:00
Print Article

When the engine is running there are significant differences in ground electron current which comes from the generator, the voltage supplier of the electrical system during engine run. Figure 1 below illustrates electron current from the -GEN terminal (say minus Gen) entering the engine block through the generator ground connection and channeling in two different directions through the engine block.

Motor Age Magazine Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.

SAVE 20%

On Automotive Electric Training Videos , ASE study guides and more.


There is a small amount of electron current, usually about 5 - 10 Amps, that channels through the engine block and up Ground G100 to provide electron current to recharge the battery. The battery is “off-line” during engine run meaning it is not providing any electrical energy to vehicle circuits. In fact, the battery becomes an electrical load on the generator as it draws electron current from the generator.

Figure 1

This points out something about batteries that is often overlooked. During battery discharge, as we have already illustrated back in Part 2, Figure 1 and Part 4, Figure 2, electrons leave the battery negative terminal flow through the circuit and return to the battery positive terminal. The exact opposite occurs during engine run. Electrons enter the battery negative terminal, flow through the battery and leave the battery positive terminal and return to +GEN.

Trace the electron current through the battery in Figure 6 to be sure you understand what is happening in the battery during engine run. We call this electron current “battery recharge current.” The amount of recharge current depends on the internal resistance of the battery. If battery internal resistance is low, the recharge current is high and puts a burden on the generator. It battery internal resistance is high, the recharge current is low and the generator is very happy. We will explain in the next section how to measure battery recharge current.

The vast majority of generator electron current is used to operate all vehicle electrical and electronic circuits during engine run. The performance of all vehicle circuits depends on proper operation of the generator circuit providing the proper charging voltage and sufficient electron current to meet the needs of the battery to recharge and at the same time supply all vehicle circuits so they can perform properly.

Generator electron current must get to the accessory ground/sheet metal through a ground strap that connects the engine block to the major sheet metal (1). The ground strap is located at the lower left in Figure 6. The location of the ground strap on the engine block is determined by the manufacturer and can vary with different makes and models. Electron current channels through the engine block in a different path from battery electron current and enters the ground strap to provide electron current for the primary sheet metal ground (1) and continues on to any other ground straps, such as sheet metal ground (2) to supply electrons to the other ground circuits.

Current clamp

Tracing electron current through a circuit is a great way to understand how a circuit works. By tracing the path of electron current from the negative terminal of the voltage source, which could be -BATT or -GEN, you are able to identify all components in the circuit. You also can determine if the control switch is on the voltage side or the ground side of the load. By the way, when I use the term “load” I am referring to the component that the circuit turns “ON” or “OFF.”

The circuit we use in our illustrations is a simple circuit but it does illuminate the uniqueness of ground circuits on automobiles, trucks, and heavy equipment which contain a battery ground circuit and a generator ground circuit.

Some of you are thinking this circuit is so easy and wish more complex circuits were being used. If you can understand the electron current through the ground circuit of this simple vehicle electrical circuit you can understand electron current through the ground path of more complex vehicle circuits because all circuits work the same way regardless of the complexity of the ground circuit. So why not learn the principles of ground circuit electron current so that we can thoroughly understand what’s happening on the ground side of the circuit? This understanding is crucial if we are to measure electron current and understand what the reading is telling us.

To measure electron current, you must have a current clamp. The illustration above shows a typical current clamp connected to a voltmeter. A single wire is placed inside the jaws of the current clamp to measure electron current in amps. The current clamp used here generates 1.0 mV for each amp it senses. Note the readings.

If you don’t have a current clamp-GET ONE! In the next article, we begin to discuss measuring electron current in the ground circuit. If you want detailed information about Current Clamps and how to use them you can find full details in our book "Vehicle Electrical Troubleshooting SHORTCUTS." Current Clamps are explained in Sections 2, and applications of their use in Sections 4, 5 and 6.

Article Categorization
Article Details

< Previous
Next >
blog comments powered by Disqus