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Ground Circuits: Part 6

Measuring single battery ground electron current
Wednesday, August 2, 2017 - 07:00
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Now that we have a general understanding of electron current through a vehicle ground circuit from the first five parts in this series on ground circuits, we continue begin measuring electron current and understand what each reading is telling us about vehicle electrical circuits during Key ON (KON) and engine run (RUN). We started off this series with a typical dual battery ground circuit which is the most common ground circuit configuration. One battery cable connects from -BATT to the engine block and is called the engine ground. A second battery cable connects from -BATT to the primary sheet metal (1) and is called the accessory or sheet metal ground.

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As vehicle manufacturers began to mount the battery outside of the engine compartment (away from the generator location) it is more practical to use a single battery ground cable and a separate ground strap to supply electron current to the sheet metal. This exercise in measuring vehicle ground circuit electron current will be with the single battery ground shown in Figure 1.

Figure 1

Current clamps are shown placed at critical points in the ground circuit to measure ground electron current. We must state what conditions exist in the vehicle’s electrical system to establish an explanation of each current clamp reading. For the circuit in Figure 7 the ignition switch is in KON but the engine is not cranking or the engine running in this first vehicle condition. This establishes that the battery is the voltage source for all electrical circuits and will have a major effect on the electron current readings decreasing as the battery discharges. The issue of battery voltage will be discussed in a later session.

Keep in mind that the battery discharge current will begin to decrease as the electron current drain lowers the battery state of charge. This will result in a continued decrease in electron current in all the current clamp readings as the battery discharges.

Current Clamp #1
This current clamp reading indicates total electron current for all vehicle circuits that operate when the ignition switch is placed in KON and the engine is not running. The reading is not very useful because it depends on the state of battery charge and the number of circuits that are operating with the KON and engine off. All these factors vary greatly depending on make and model of vehicle and the number of circuits that are active during KON.

Cranking Amps (During engine cranking only)
Current Clamp #1 reading is only essential information when cranking the engine. The reading would indicate the cranking amps while the starter motor is cranking the engine. The cranking amps will vary with battery state of charge, number of cylinders, make and model of vehicle, etc. Always take into account ambient temperature when testing cranking amps. Expect higher readings in cold weather and lower readings in warm weather.

(The topic of DC motors, such as starter motors and how to troubleshoot circuits with DC cranking motors, is covered at length in Section 5 of our book “Electrical SHORTCUTS.”

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