Ground Circuits: Part 9

Dec. 19, 2017
In electrical system design, testing and evaluation of operating conditions there are two circuit parameters of utmost importance to consider - voltage and electron current.

In electrical system design, testing and evaluation of operating conditions there are two circuit parameters of utmost importance to consider - voltage and electron current. Both of these must be measured under all conditions to validate the proper operation of an electrical circuit. Anytime an electrical circuit is suspected of subpar performance, measuring the voltage and electron current parameters evaluates the circuit’s performance and permits analysis as to what is wrong. In other words, is it a voltage problem or an electron current problem? It could be both.

A generator produces two electrical parameters that have to be evaluated:

  1. sufficient charging voltage (in volts) to drive sufficient electron current through all vehicle circuits including the battery during engine run.
  2. electron current (in amps) meeting requirements demanded by all circuits that are operating during engine run (while maintaining proper charging voltage).

To properly test a vehicle generator charging circuit, two measurements are necessary. One is to measure the charging voltage while the generator is being operated through minimum and maximum electrical load conditions because the charging voltage changes with electrical load and engine RPM. Therefore, engine RPM must also be considered so that the generator circuit is evaluated under all possible “worst case scenario” conditions.

Our two electrical flipcharts: “FIRST THINGS FIRST-Pro” for a 14 V single battery electrical system and “FIRST THINGS FIRST-2” for a 14 V dual battery electrical system do just that. They show a technician how to perform a charging system voltage test procedure in less than five minutes. Confirming proper charging voltage under all conditions should always be performed to assure the vehicle’s charging system is functioning properly under all conditions. And it doesn’t take long at all.

So far in this series we have been dealing strictly with electron current because we can learn a lot about a circuit by understanding the paths of electron current flow through it. Would you have ever thought about measuring battery recharge electron current, or understood the reasons why you should without this series of articles?

I have received some comments from techs about measuring generator electron current flowing through the generator (output/red) cable connected to the +GEN terminal. This practice is widely regarded throughout the automotive and truck service/repair industry and taught in many auto and truck technology classes as a valid test of the generator’s output. Previously, I have written “why would you want to do that” and that resulted in a few comments that deserve a more in-depth answer.

In Figure 1 below, Current Clamp #7 is connected on the +GEN cable. Of course, there is a current reading obtained during the measurement, but what does it tell us about the vehicle’s electrical system?

Figure 1

Let’s say for purposes of discussion the reading obtained by Current Clamp #7 in Figure 11 is 48 A. (DMM indicates .048) That represents a total of 48 A to operate all vehicle circuits during engine run. It’s an aggregate number but does not afford the opportunity to isolate electron current levels drawn by individual circuits. The reading from Current Clamp #7 includes the battery’s recharge electron current. But we are unable to determine how much of the 48 A is battery recharge electron current. Neither can we determine how much current is being drawn by each individual circuit in the electrical system? That is the reason for Current Clamps #1 and #3.

But the question comes up: “What would happen if one of the circuits in the vehicle began to draw too much electron current? Wouldn’t that cause the generator to burn up?” No!

In our schematic diagram notice that each individual leg of the electrical system consisting of Lamp Circuit #1 and Lamp Circuit #2 (which represents the vehicle’s total electrical system) has its own individual circuit fuse. If a circuit were to fail and draw excessive electron current because the load developed a short circuit or the voltage side of the circuit developed a short-to-ground, the generator reading of 48 A would begin to increase. But what happens at that point? The circuit fuse blows and shuts down that electrical circuit to prevent excessive electron current from overheating the generator and melting the wiring insulation.

I have received some comments from techs about measuring total generator electron current flowing through the generator (output/red) wire connected to +GEN.
Here’s an example.

Hi Vince ………….. The reason I check generator output is because many of the buses I maintain have two generators. And to decipher which generator is not contributing fully even though the voltage outputs are within spec. If there is a better option that I'm missing? Thanks again, Glen B.

I have redrawn the circuit below showing two generators on this vehicle. Both generators are connected to the same point on the B+ (voltage) side. There is a connection to the engine block on the B- (negative or ground) side. Trace the wires. Both generators are connected in parallel.

Figure 2

Electrically speaking, since the generators are connected in parallel, the laws of parallel circuits apply.

The law for voltage in parallel states in so many words: “Two voltage sources connected in parallel have the same (charging) voltage between their B+ and B- terminals.”  

The law for electron current in parallel states in so many words: “The electron current produced by both generators combines into one total electron current for vehicle circuits,” as illustrated in Figure 2. Electron current from each generator channels through the engine block and combines as it goes up the Engine Ground cable. Trace the path of electron current.

The only place available to measure the electron current contributed by each generator is on the B+ (positive) cable as shown by Current Clamps #7 and #8. The reading obtained indicates how many amps each generator is providing. The method that Glen uses to test generator output is valid for its intended purpose of finding which generator in a dual generator set-up is not contributing to the electrical system. Ideally, the two readings should be within a few amps of each other indicating both generators are contributing almost equally. If one of the generators is defective the reading on that current clamp will be very low or zero identifying the defective generator.

Next time we begin discussing ground voltage.

About the Author

Vince Fischelli | Founder & Director of Training, Veejer Enterprises, Inc.

Vince Fischelli is the founder and Director of Training for Veejer Enterprises, Inc., in Garland, Texas. Over a 10-year military career, VInce graduated from the first electronics school in 1959. He has over 50 years of experience in servicing electronics, with the last 49 years devoted exclusively to the vehicle service industry. He managed both GM's Engine Computer and Digital Dash Remanufacturing Centers in Dallas, Texas, and is the former electronics editor of Import Service Magazine. He founded Veeer Enterprises in 1985 to provide vehicle electrical and electronics troubleshooting training to all segments of the vehicle service industry. He is the author of more than 20 books including workbooks and instructor guides covering vehicle electrical-electronics troubleshooting. He also conducts hands-on training and workshops. He is a featured speaker at industry trade shows and conventions presenting electrical electronics training for all segments of the vehicle service industry.

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