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Ground Circuits: Part 9

Measuring generator electron current engine RUN
Tuesday, December 19, 2017 - 09:00
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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.

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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
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