A Logical Diagnostic Process Improves Charging System Diagnosis

By Albin Moore

This month, we feature a guest article, written by Mr. Albin Moore. Mr. Moore is owner of Big Wrench Repair, an ASE CMAT L1 technician, a frequent contributor to industry publications and forums and has 40 years of experience analyzing and fixing mechanical and electrical problems. We have highlighted his posts/articles in the past. Here, he provides an in depth look at the thought process he uses to diagnosis charging system issues.

Have you ever taken the time to think about the design of the starting and charging system on the vehicle you drive or the ones you see come through your bays? The systems are all designed unique to the vehicle. By this I mean the systems are designed to address the starting and charging requirements of that particular vehicle, with just a small amount of reserve capacity. As a result, issues such as unwanted resistance in either the starter or generator circuit are capable of lowering the efficiency of either system enough to cause problems.

The design of both the starting and charging systems seem, on the surface, to be the same as they were ten or fifteen years ago, but that is not the case on most vehicles. Most vehicles of today are using computer controlled smart charging systems and often the engine or powertrain control module manages the starter and/or generator. The practical result of the evolution of these systems is that, when a vehicle with a starting or charging system problem rolls into your bay, using the tried and proven testing of days gone by on these systems can leave the technician wondering where they went wrong. Things such as full fielding the generator are a thing of the past. Since most charging systems are controlled by an electronic module, it is likely that a scan tool is required to access the bidirectional controls of the generator. Because of the diversity of today’s systems, trying to use one single testing method for all makes and models is no longer possible.

In this article, I will use two different vehicles to compare today’s charging systems to older systems. A 2011 GMC Terrain will be the example of a current charging system, while a 1999 Ford Windstar will be used to represent older systems. Both vehicles use a “smart charging system” controlled by the Powertrain Control Module (PCM).

These two charging systems have the voltage regulator housed in the generator case and the regulator controlled by the PCM. This is all that is similar between the two, since the PCM inputs are different and the testing procedures are also different. There is more to analyzing a charging problem than hooking a voltmeter to the battery and starting the engine. If the generator is not charging, the question “why” should pass through your mind.

It is important to note that, since most smart charging systems are ultimately controlled by the PCM, it is possible for the PCM to turn the generator off or change the voltage under certain conditions. If you were to test the voltage when the PCM is fiddling with generator control, you could end up replacing a part only to have the new part exhibit the same symptoms. In my business, I see this all too often. A generator is replaced once, twice, or even three times and yet, they all exhibit the same symptoms. Knowing how a system works and how to properly test it will help you avoid throwing parts at the problem.

Start with the Basics

Before digging too deeply into a charging system issue, a few basic tests should always be performed. I guess you could call this basic testing, since all charging systems have a few things in common.

1. Confirm that the generator is receiving power from the engine. Today, most generators are supplied power from a serpentine belt that also powers other things. Don’t discount the possibility of belt slippage from a worn drive belt, pulley, or a slipping one way generator clutch as a problem source. A great way to test for these conditions is with a non-contact thermometer or an infrared camera. When something like this slips, it will produce heat.

2. Confirm that the generator has a proper connection to the battery at the B+ terminal with the engine stopped. Most charging systems will have a fuse or a fusible link between the generator and the battery. If these current limiting devices are open, the generator will never charge.

3. Proper battery condition must be confirmed.

The reason I mention these three seemingly simple things is because I have been blindsided by them in the past. It is very hard to wash egg from your face after a new generator is installed and you find the real problem is an open B+ circuit. Do the simple stuff first.

1999 Ford Windstar Charging System Design

First, we’ll start with our older charging system example. Please don’t assume, since this is a late 90’s Ford vehicle, that it has a simple “Ford” charging system. It is not like the charging system on a F-Series truck or even an Escort of that period. That era Windstar utilized a smart charging system. The point here is to be aware of the system you are working on. Knowing this information will make your problem analysis quicker, more accurate and more fun.

This Ford smart charging system is configured such that the PCM determines the required charging voltage and sends the command to the voltage regulator that is housed in the generator case (see the wiring diagram in Figure 1).

figure 1

Figure 1 – Wiring diagram of the charging system on 1999 Ford Windstar. The diagram shows the voltage regulator being housed in the generator case, and being controlled by the PCM.

This communication takes place on two dedicated communication lines between the PCM and the voltage regulator: generator command and generator monitor. This generator command signal wire transmits a pulse width modulated signal used to command the voltage regulator. The generator monitor sends a pulse width modulated signal back to the PCM to provide feedback as to what has happened. This action/reaction data can be seen in the scan tool capture (Figure 2) and the labscope capture (Figure 3). The voltage regulator has one more wire going to it. The orange/light blue wire is the voltage sense wire. This wire should have system voltage at all times.

This makes four wires to the generator: the B+ wire, voltage sense wire, generator command wire and generator monitor wire. It doesn’t take long to check the voltage and/or waveform on these wires to verify correct inputs and outputs from the generator.

figure 2

Figure 2 – Scan data of the generator command and generator monitor as electrical loads are being turned on and off and watching how the PCM is commanding the voltage regulator.

Figure 3

Figure 3 – B+ wire, voltage sense wire, generator command wire and generator monitor wire. It doesn’t take long to check the voltage and/or waveform on these wires to verify correct inputs and outputs from the generator.

2011 GMC Terrain Charging System Design

The charging system used on this Terrain is a little different from the Ford Windstar. Although both charging systems supply power to run the electrical systems of the vehicle, the later model GM system does its job a little differently.

The generator regulator is again controlled by the PCM. The GMC charging system uses a battery current sensor, which monitors the current going in and out of the battery (see wiring diagram in Figure 4). This charging system has six different charging modes controlled by the PCM.

– Battery sulfation mode
– Battery charging mode
– Fuel economy mode
– Headlight mode
– Startup mode
– Voltage reduction mode

Each of these different charging modes have a specific purpose, with the ultimate goals of increasing battery life, improving fuel efficiency and improving cold weather engine starting.

Figure 4

Figure 4 – Wiring diagram of the charging system on a 2011 GMC Terrain. The generator regulator, which is housed in the generator case, is controlled by the powertrain control module.

Testing the Charging System

When it comes to diagnosing any charging system issue, it is always a good thing to take a look at a wiring diagram of the system (see Figure 4) and reference the theory of operation to brush up on how the system works. Now that we know how it works and how the wiring is laid out, we can test the system using a voltmeter, battery tester, scan tool and labscope. A digital voltmeter can be substituted for the labscope steps, though testing accuracy is greatly limited.

First, use your battery tester to check the open circuit voltage (OCV) and test the condition of the battery. If the battery is compromised, replace it with a battery that meets the specifications for the vehicle and proceed with your testing. This is important because it is critical to have a properly charged and functioning battery in the vehicle in order to accurately assess the other system components. Don’t consider substituting a battery change as tossing a part at the problem. It is a necessary step. Before you exit this phase of your test, use your positive voltmeter lead to test for B+ voltage at the B+ terminal of the generator.  

With a good battery in the vehicle and proper voltage at the B+ generator terminal, it’s time to use your scan tool to check for any diagnostic trouble codes (DTCs). On all smart computer controlled charging systems, checking for any stored diagnostic trouble codes is a critical next step because you can get some very good clues from these codes. For instance, the vehicle is in with a starting complaint and the battery light is not illuminated, do not assume the generator is charging. It is best not to trust the gauges and indicator lights in the instrument cluster. I trust the scanner or labscope data taken from the vehicle much more. If there are DTCs stored, research them and follow them up to see if you can get a direction for further testing.

If you have a known good battery, any diagnostic trouble codes applicable to the charging system have been addressed and proper voltage to the generator, yet the generator still is not charging, it’s time to analyze scan data. By selecting the generator charging command and charging feedback data, the technician will know what the generator is being commanded to do.

Your testing procedure needs to include both the inputs to the generator and the output from the generator. This is where it is critical to understand the way the particular system you are diagnosing is intended to perform. A scan tool is needed to see what the PCM is telling the generator to do. Without proper inputs to the generator, the generator will not output the correct voltage or amperage. A labscope should be connected to verify the outputs from the generator.

Figure 5 - Scan data of the charging system command from the PCM & the feedback from the generator regulator.

Figure 5 – Scan data of the charging system command from the PCM & the feedback from the generator regulator.

Figure 6

Figure 6 – Labscope capture of the two communication lines between the generator regulator and the PCM, and the generator output current. This is an easy way to verify proper generator control and operation.

Figure 5 shows scan tool graphing data, including engine speed, engine at idle and electrical loads being turned on and off. Notice how the engine speed remains constant, but the throttle is being opened by the PCM, which is causing the engine load to increase. On the right side of Figure 5, the engine, generator load and command is displayed.

The generator installed on this vehicle is rated at 125 amps output. The scan data is showing the generator to be close to its maximum output and the labscope data backs up the scan data. If the scan tool on your tested vehicle is telling the generator to charge, the proper signals are getting to the generator (test these with your labscope), and there is still no charge, then it is time to replace the generator.

Everyone Benefits from a Methodical Approach

By using a thorough, logical diagnostic process, technicians can avoid unnecessarily tossing parts at a problem in hope the problem will be fixed. Our parts suppliers and our customers both benefit from a methodical approach, which will result in fewer warranty claims and a more reliable vehicle for our customer, with a significantly reduced likelihood of a comeback.




  1. Philip Levin says:

    I was a licensed Airframe and Powerplant mechanic – 50 years ago. I haven’t done my own auto repairs in many years, so this article was a real revelation! Many thanks!

  2. jimohara says:

    Mr. Levin, Thanks for your comment. We’re glad you enjoyed it.

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