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GM charging system problems

Fix GM charging system problems

Late model GM charging systems are quite different that the standard alternator with internal regulator you’ve seen in prior years. If you have GM charging system problems, you must first understand how they work. Plus, you must use a scan tool to determine the root cause. Otherwise you’ll be replacing parts unnecessarily. The new GM charging system is actually called the electrical power management system. It’s designed to monitor vehicle voltage and only charge the battery when necessary. GM does this to improve gas mileage and reduce the need to generate power when it’s not needed. The system also monitors the battery to determine its condition and charge it in a way that prolongs its life.

The system:

• Monitors the battery voltage and estimates the battery condition.
• Takes corrective actions by boosting idle speeds, and adjusting the regulated voltage.
• Notifies the driver of any condition needing attention.

Battery condition is tested when the ignition is on and off. When off, the system waits until the vehicle is off for a prolonged period (several hours) before testing for battery condition. Then it measures open-circuit voltage to determine state of charge.

When the engine is running the battery rate of discharge is detected by a battery current sensor.

battery current sensor

Battery current sensor connected to negative battery terminal

The current sensor also tests temperature to determine state of charge and preferred charging rate.

The power management system also works with the Body Control Module (BCM) which is connected to the Engine Control Module (ECM) via a data bus. The BCM determines the output of the alternator and sends that information to the ECM so it can control the alternator turn on signal. The BCM monitors the battery sensor current, the battery positive voltage and battery temperature to calculate battery state of charge. If the charge rate is too low, the BCM performs an idle boost to correct the condition.

The battery current sensor is connected to the negative battery cable. It has 3-wires and creates a pulse width modulated 5-volt signal with a duty cycle of 0-100%. Normal duty cycle is considered between 5 and 95%.

When the engine is running, the ECM sends an alternator turn on signal to the alternator. The alternator’s internal regulator controls the current to the rotor by pulsing the current to obtain the correct output. If the voltage regulator detects a problem, it notifies the ECM by grounding the field current line. The ECM then checks with the BCM to obtain battery temperature and state of charge information.

If the system cannot correct the problem, it will notify the driver with one a charge indicator and a driver information center message of SERVICE BATTERY CHARGING SYSTEM (if equipped).

The ECM, BCM, battery, and alternator work as a system. The power management system has 6 modes of operation

Battery Sulfation Mode-determines the correct charge protocol to correct plate sulfation condition. The BCM enters this mode if the alternator output voltage is less than 13.2 V for 45 minutes. The BCM will enters Charge Mode for 2-3 minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Mode–The BCM will enter Charge Mode when it detects one of the following conditions:

The wipers are ON for than 3 seconds.

Climate Control Voltage Boost Mode Request) is true, as sensed by the HVAC control head. Ie, you’ve turned on the AC

High speed cooling fan, rear defogger and HVAC high speed blower operation are on.

The battery temperature is less than 0°C (32°F).

The BCM determines that the battery state of charge is less than 80 percent.

The vehicle speed is greater than 90 mph. (No need to save gas at that point)

The battery current sensor is showing a fault

The system voltage is below 12.56 V

When any one of these conditions are met, the system will set targeted alternator  output voltage to 13.9-15.5 V, depending on the battery state of charge and estimated battery temperature.

Fuel Economy Mode–The BCM will enter Fuel Economy Mode when the battery temperature is at least 32°F but less than or equal to 176°F, the calculated battery current is less than 15 amps but greater than -8 amps, and the battery state-of-charge is greater than or equal to 80 percent. At that point the BCM targets alternator output to 12.5-13.1 V. to save gas.

Headlamp Mode–The BCM boosts alternator output to 13.9-14.5 V whenever the headlights are turned on.

Start Up Mode–The BCM commands a voltage of 14.5 volts for 30-secs after startup.

Voltage Reduction Mode–The BCM enters Voltage Reduction Mode when the ambient air temperature is above 32°F, the battery current is less than 1 amp and greater than -7 amps, and the generator field duty cycle is less than 99 percent. The BCM targets output to 12.9 V. The BCM exits this mode once the criteria are met for Charge Mode.

Posted on by Rick Muscoplat


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