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.
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.
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.”
Current Clamp #2
This reading only indicates the electron current through the circuit represented by Lamp Circuit #1 based on battery voltage. The reading indicates the condition in this individual circuit. Expect this reading to be higher when the engine is running because the charging voltage is higher than battery voltage resulting in higher electron current. If troubleshooting the circuit take the reading when the engine is running and the charging voltage is operating the circuit. Ambient temperature must also be considered because in cold weather the charging voltage is higher than it is in warm weather and this will affect the electron current in the circuit. (Higher voltage produces higher amps.)
(The topic of charging voltage changing with ambient temperature is covered at length in our book “Electrical SHORTCUTS” in Section 6, which discusses the operation of generators and charging voltage analysis according to ambient temperature and other factors that affect the value of charging voltage.)
Current Clamp #3
This reading indicates the electron current through the circuit represented by Lamp Circuit #2. The reading only indicates the condition in this individual circuit. Expect this reading to be higher when the engine is running because the charging voltage is higher than battery voltage resulting in higher electron current. If troubleshooting the circuit take the reading when the engine is running and the charging voltage is operating the circuit.
Ambient temperature must also be considered because in cold weather the charging voltage is higher than it is in warm weather and this will affect the electron current in the circuit.
Current Clamp #4
This electron current reading is not a very conclusive indication of all vehicle circuit activity. It Indicates the amount of electron current being supplied to sheet metal (2) based on battery voltage. It will not include the amount of electron current flowing through circuits connected to sheet metal (1). This ground strap measurement is only helpful if a normal reading has been established using a known good vehicle of the same make and model and is in the same exact mode of operation at the time the measurement is taken.
The electron current reading will be higher in cold weather when the charging voltage is higher and lower in warm weather when the charging voltage is lower. If using this current clamp reading in troubleshooting of circuits connected to sheet metal (2), established the reading when the engine is running and the higher charging voltage is operating the circuit. Compare the reading to readings taken in a similar known good vehicle.
Current Clamp #5
Notice electron current channels through the engine block to supply electrons to the ground strap connection on the engine block. Always maintain the same location of the ground strap connection to the engine block to avoid electron current channeling through engine bearings which would damage them overtime.
This reading can be both helpful or not so helpful. If this is the only ground strap between the engine block and the sheet metal, then total vehicle circuit electron current must flow through this ground strap and the measurement can be useful in determining total amps needed to operate the vehicle. The problem is there may be more than one ground strap between the engine block and the sheet metal which may not be known at the time this measurement is taken. If more than one ground strap is used this reading would not be reliable since it doesn’t have total vehicle electron current flowing through it. All ground strap electrical currents would have to be added together to determine total electron current.
In Summary
Current clamp #1 offers us the most information but only when it indicates cranking amps. The reading we get when we first obtain an electron current reading with the KON doesn’t have much meaning.
Current clamps #2 and #3 readings are both useful to indicate the electron current flowing through an individual circuit. This can be helpful sometimes in troubleshooting a circuit problem if the engine is running. We will cover some of those test procedures in future parts of this series.
Current clamps #4 and #5 readings have little use unless you work on these vehicles every day and have a chance to take these measurements in similar vehicles and establish normal readings to compare with the reading taken in a vehicle with an electrical problem. A future part of this series will discuss checking electron current on the voltage side.
In the next part we will review these measurements when the generator is online (RUN) and this will bring up a very interesting fact that most technicians do not know about. Significant changes occur in these readings when the engine is running. Stay tuned in to this electrical series for the specific details.
