Monday, September 25, 2023 Detailed Auto Topics
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For many years, the automotive alternator changed very little. The output increased to help keep up with growing demand for current, but the alternator worked much the same. That has now changed, with electrical power management. Current GM alternators may look similar to past models, but they cost a lot more and are far more difficult to diagnose.

Alternator basics

A partial view of an automotive alternator

When the engine is running, a magnetic armature inside the alternator spins within coils of wire producing an alternating current. The battery and electrical system of an automobile need a direct current to work. Diodes convert the flow of electricity to a direct current useful by the vehicle. The alternator provides electricity to operate the electrical devices on the vehicle and replaces power taken from the battery.

A storage battery converts chemical energy into electrical energy. We use this current to turn the starter motor and to crank the engine. An alternator does not produce power until we start the engine, using the battery. After starting, the engine turns the alternator, producing current flow to replace power taken from the battery.

We also use power from the battery when the electrical load exceeds the output of the alternator. How much power an alternator produces depends on several factors. One factor is the physical design of the alternator and another is the speed we turn it. When the engine is at an idle, alternators only produce a fraction of their capacity. If we turn on the headlamps and windshield wipers at an idle, the draw may exceed the capability of our alternator. The battery supplies needed power until we increase the engine speed.

GM Electrical Power Management

The normal charge range of a GM Power controlled alternator

An alternator can produce more voltage than needed and we must regulate the output. Too much voltage will damage system components. Many vehicles use a voltage regulator to turn off the alternator when voltage exceeds a limit. In 2005 General Motors began using Electrical Power Management or EPM on many SUV and truck models.

Instead of only off and on, with EPM the alternator output is variable from five to 95%. Charge rate and system voltage varies depending on system need. EPM uses several components to estimate battery condition and decide which charging strategy to use. Six different charge modes are available and the vehicle switches between them, depending on conditions.

Startup mode

After starting the engine, the body control module attempts to charge at 14.5 volts for 30 seconds. This quickly replaces power used by starting. It can also confuse drivers. When startup mode ends, the system voltage will change, depending on the next mode selected by the body control module. This may cause a drop on the voltmeter, which some drivers mistake as a problem.

Charge mode

When we use the HVAC system, we need additional current. The LAN reports this to the body control module which decides the vehicle needs the charge mode. Running the inside fan on high, switching the rear window defroster on or cooling fans running, causes charge mode to start. In charge mode, output is between 13.9 and 15.5 volts, depending on the state of charge and temperature of the battery.

  • Other things that cause charge mode include:
  • The battery temperature is at or below freezing.
  • The state of battery charge is below 80%.
  • We turn the windshield wipers on for three seconds or more.
  • A problem causes voltage to drop below 12.56 volts.
  • Driving in excess of 90 MPH or 140 km/h.


Headlamp mode

Turning on headlamps can cause the voltmeter to rise

If we turn the headlamps to either high or low beam, the BCM switches to headlamp mode. In headlamp mode they regulate voltage between 13.9 and 14.5 volts.

Voltage reduction mode

Normal operation in voltage reduction or fuel economy modes

To increase battery life, the BCM will reduce system voltage under certain conditions. This alarms many drivers that think the alternator has a problem. In reality, the target output will only be 12.9 volts in voltage reduction mode.

They use this mode when the ambient temperature is above freezing. The system measures current flow and reduces system voltage under certain conditions. The current draw must be less than seven amps, the charge-rate less than one amp and the alternator duty cycle less than 99%. Voltage reduction will continue until any of these conditions change.

Fuel economy mode

When the system draw is less than eight amps and alternator output is less than 15 amps, battery condition is considered. A calculated state of charge of 80% or more and ambient temperature above freezing sets fuel economy mode. The voltmeter will read as open battery voltage in this mode. A reading of 12.5 to 13.1 volts is normal in the fuel economy mode.

Fuel-economy mode remains selected until the conditions change or the BCM calls another mode as described above.

Battery sulfation mode

When a battery discharges, sulphur can cling to the plates. We call this sulfation and it drastically shortens the life of the battery. When we get a battery voltage below 13.2 volts for 45 minutes, the BCM switches to sulfation mode. The BCM enters charge mode for 2-3 minutes and then retests the system. Depending on the battery state, the BCM decides the next mode to use.

System problems

With all the complexity of the EPM system, they have not extended battery life. Failure at three years is common, as with other less complex systems. Diagnosis can be very involved and bidirectional communication with the BCM and power control module is necessary to diagnose problems. The system stores codes, much like a check-engine light and these provide a starting point for diagnosis.

Potential problem shown by a warning message

Warning messages, such as BATTERY NOT CHARGING SERVICE CHARGING SYSTEM, also alert the driver of possible problems. These messages will remain on, until the condition causing them is corrected.

A common example may be allowing the vehicle to warm up in cold weather, by idling. With the blower on high, headlamps on and rear the defroster on, the alternator may not keep up. High load at low engine speed may cause a warning light. If the light goes out after driving, the system is likely okay. A light that continues to come on or that lights under normal conditions, means a problem.

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