In the last issue, we covered battery testing and the superior advantage of using a conductance tester in your preliminary inspections. All of us have had those times when we’ve gone through a lot of work, only to discover the root cause of the problem was simple. Something we overlooked. Who hasn’t seen driveability problems with a Dodge truck that, in the end, was caused by a bad battery or some electrical problem? I’m not picking on Dodge, per se. We’ve seen calamities with GM and Ford, too. For that matter, we can go down the list of imports that send technicians over the edge.
So, with that, let’s get back into the topic of electrical systems integrity. After checking the battery, let’s move to the alternator. It may be axiomatic to think that if the battery’s okay, the alternator must be too. That’s a good argument, but as we’ll soon see, it’s not that simple.
Not too long ago, the purpose and behavior of an alternator was “Once the car starts, produce 14.5 volts (or so), and send it to the battery. Figure one is an example of that. Here’s the alternator on my ’69 GMC Camper Special. My father-in-law gave it to me shortly before his death. It originally came with a generator, but he converted it to an alternator, rated at 60 amps. Notice it only has one system wire. And notice, too, the small 6-gauge wire. Today’s vehicles might operate with a capacity of 250 amps, maybe more. That 6-gauge wire wouldn’t do in today’s automotive world.
Today’s alternator systems have an array of features, including built-in processors that communicate with other CANBUS systems. They respond to things like turning on the rear window de-icer or turning down the charging voltage when a battery exceeds 80%. Imagine the thinking that included maximizing alternator voltage while going down a grade. It’s free energy and a brilliant conception.
With all this, let’s go through the rundown of systems and terminology. The basics of an alternator include three things:
- Rotor: The part that’s driven by the engine
- Stator: The windings surrounding the rotor.
- Regulator (including the rectifier): The output controller.
That part hasn’t changed. What has changed is the idea of having a regulator that’s always working and putting out 14.5 volts, regardless of the conditions. Today’s alternators will vary the voltage output based on certain conditions. These are referred to as modes. Figure 2 includes many of the modes used in late-model vehicles.
Getting back to our purpose, after checking the battery, you’ll want to check the condition of the alternator. The most common setup has three wires, one to the battery, a second to the stator (referred to as the field in most wire schematics), and a third to a computer that controls the regulator. However, you can run across alternators with only two wires or up to five, such as that with many Honda’s, as an example. The terminals you might find connected to an alternator include:
- B+: This main output terminal connects to the battery’s positive terminal. It carries the alternator’s charging current.
- IG: Ignit ion: Activates the alternator when the ignition is turned on.
- S: Sense Terminal: Monitors the system voltage and helps the alternator adjust its output to maintain the correct voltage. Ford refers to this as GEN MON.
- F: Field Terminal: Communicates the alternator’s field current or duty cycle to the vehicle’s control module for monitoring and diagnostics.
- L: Lamp Terminal: Connects to the charge indicator light on the dashboard, signaling issues with the charging system.
- P or W: Pulse Terminal: Provides an AC signal from the alternator’s stator, often used for tachometers or other monitoring systems.
- Communication Pins: These terminals allow digital communication with the vehicle’s control module for precise regulation and diagnostics. They include:
- LIN: Local Interconnect Network
- RC: Remote Control
- LI: Load Indicator
- GEN COM: General Communications
- DFM: Digital Field Monitor
- CAN Hi/Low: Some alternators include an onboard processor that shares information over the CAN Bus.
Given the flexibility of alternator output based on computer control, manufacturers have given names to their systems. Here are some examples, along with key features.
GM: Regulated Voltage Control (RVC). It is designed to dynamically manage the vehicle’s electrical system voltage based on battery temperature and state of charge. Unlike traditional charging systems that maintain a constant voltage, RVC adjusts the alternator’s output to optimize battery life, improve fuel efficiency, and extend the lifespan of electrical components.
Key Features:
- Dynamic Voltage Regulation: Adjusts voltage between 12V and 14.5V depending on battery needs.
- Battery State Monitoring: Uses sensors to estimate battery temperature and charge level.
- Fuel Economy Benefits: Reduces alternator load when full charging isn’t necessary, improving efficiency.
- Extended Component Life: Helps prolong battery lifespan.
GM uses two types of RVC Systems:
- Integrated RVC: Uses a Battery Current Sensor to communicate with the Body Control Module (BCM), which then regulates the alternator.
- Stand-Alone RVC (SARVC): Operates independently with an Alternator Battery Control Module mounted on the battery cable.
This system ensures the battery remains at 80% charge or higher while supporting vehicle electrical loads. These systems have three terminals (Figure 3).
Ford: Smart Charge. This system optimizes the performance of a vehicle’s alternator and battery. It’s similar to GM’s RVC. Its key features are:
- Dynamic Voltage Control: The system adjusts the alternator’s output based on the vehicle’s electrical demands and the battery’s state of charge. This ensures efficient energy use and reduces unnecessary load on the engine.
- Battery Health: By charging the battery only when needed, the system helps extend its lifespan.
- Fuel Efficiency: Reducing the alternator’s load on the engine improves fuel economy. This is especially true when going down a grade. In this case, it’ll go to high output. It’s free energy since you’re off the throttle.
Ford uses four terminals with the Smart Charge system (Figure 4).
Chrysler refers to its modern alternator system as a PCM-Controlled Charging System (No fancy name here). Like GM and Ford, the Chrysler system uses the Powertrain Control Module (PCM) to regulate the alternator’s output based on battery condition and vehicle load, ensuring optimal charging efficiency.
Chrysler includes many key features that have already been discussed with GM and Ford, but it also includes two field circuits in some alternators. Chrysler also uses three terminals in most of their system (Figure 5).
Now that I’ve put you to sleep let’s get to the nub of this. A customer comes in with a complaint. It’s likely intermittent because so many electrical failures are… intermittent. You have your receiving tech do his or her preliminary checks, including the battery test highlighted in last month’s issue, and it fails the alternator test; it only reports 12.83 volts. You’re scratching your head because the battery passed. You’re thinking, “How could the battery pass with a bad alternator?” The answer, it can’t. What you likely have is a dynamic function with the alternator where the manufacturer programmed no alternator output when the engine starts, and then, “We’ll see.” While this might seem odd at first, it reduces wear and tear on the drive belt and uses less fuel to operate the alternator, so why do it if it’s not needed?
If the battery is sufficient to start the car and support the demand of the other systems, then why force a high output from the alternator? All of this makes sense once you think about it as an engineer trying to keep up with the strict CAFE standards we have today.
So, what do you do? Do the battery tests outlined in my article in the April 2025 issue of Gears magazine. When you get to the alternator tests, make sure you load the system. Otherwise, you might fail the test. Turn on everything you can, like the rear window de-icer, headlights, and air conditioning. That’ll put any system into high-output mode, and you’ll likely pass the test.
Now, then, the capacitance tester has its limitations. It may pass a vehicle that has underlying problems. After all, it checks the system voltage as it exists “right now” as you’re measuring it. If your car measures 14.1 volts as an output, it’ll measure fine. But you could be in a high-load situation where the alternator isn’t sufficiently grounded and causes intermittent complaints.
The third part of your preliminary test is the voltage drop. I know this is boring and, to some extent, overplayed, but follow along. You’ve hired an entry-level tech. He or she is 19 (my age when I entered this incredible field). In this role, the auto attendant (or valet) may be a valuable position for your shop and find problems that could result in intermittent electrical problems. After all, how many transmissions or solenoid packs do you need to replace to fix a loose or corroded wire at the battery?
Here’s the process for an adequate voltage-drop test.
- Start the car
- Turn on everything you can (e.g., rear window de-icer, headlights, air conditioning, etc.)
- Put the negative lead of your DVOM on the battery post (not the cable connection) and the positive lead on the alternator housing. The Maximum you should read is 0.25 volts. 0.1v is optimal
If you exceed the limit, look at the cables and other sources where you might have a bad connection.
Note: You might see a negative voltage value, such as -0.023V. If you do, don’t be alarmed. Today’s DVOMs can pick up traces of electricity and display it. That low value, whether positive or negative, is immaterial for the test.
While you’re at it, check the positive lead to the alternator in the same way.
These are a simple set of procedures you can use for your entry-level tech to help you ensure every car gets the attention your customer deserves.
I’ll be at ATRA’s Powertrain Expo in San Antonio, September 3rd-7th. I look forward to seeing you there.
