Corroded Battery Terminal

Testing a Vehicle’s Charging System

Testing a Vehicle’s Charging System:


The charging system on most modern vehicle’s is a fairly easy to understand and easy to diagnose system. At times the problem can be tricky, but for the most part it is a very straightforward process to find and diagnose it. As long as you have a few tools and some common sense you can figure it out. Read below and you will learn the proper procedure for testing a charging system.


Basic 12V Charging System Diagram

Basic 12V Charging System



Vehicle electronics, including the starting system and almost all of the accessories run off of the 12V battery power supply. Since the battery usually has some sort of power demand it requires a powerful charging source when needed, this charging source is the Alternator.

The alternator gets a request from the engine control module when it is in need of a charge, and then the alternator engages and sends produced voltage to be stored at the battery. As seen in the picture above, the charging system is pretty straightforward and easy to trace out.


If the battery is not charging it can be narrowed down to a few possibilities:


  • Bad alternator
  • Bad battery
  • Wiring issue
  • PCM/ECM signal issue
  • Power/Ground cable issue


The usual charging system diagnosis works through a simple process of elimination. I like to check the charging voltage first thing, it should be around 14V, if it is low and obviously not charging then I head to the alternator and check the request signal and the alternator output.

If the alternator is putting out the proper voltage then I know there is a voltage loss between the alternator and battery, but if the alternator does not have the proper output then either it is not getting a charging/request signal, or the alternator has failed and is no longer able to maintain system requirements.

The alternator will have a power cable that runs to the battery, as well as a small connector, that is the signal connector and it is there will you will test for the proper on/off signal.


Alternator Drive Belt

Directional cycle of the alternator drive belt

Modern vehicles have a lot more electronic accessories than they ever have before, and being set up this way means the battery will always have a higher power supply demand, so it is not uncommon now days to see batteries going bad within the first one to two years.


If you are having a charging issue and cannot seem to figure it out then be sure you inspect the terminals/connections and load test the battery, a lot of times a vehicle will be attempting to charge but the battery has failed, or the connection is bad, therefore the charge is immediately lost, so the charging system may seem inefficient when it really is just a faulty battery or connection.


If everything tests out and you still cannot find the issue, then next you will want to load test the system and put the alternator to work, if there is a fault present it will be easier to find when the alternator is loaded and attempting to charge.


Have fun, and stay safe!!



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