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Old 07-23-2008, 12:53 PM   #1
pervitizm
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Default Worried about my Voltage


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I recently disconnected my postive cable off my battery to install a radio in my 07 Suburban. After reconnecting the battery my voltage readings stay at around 12.8 to 13.0 volts even when the truck in on and running. Why is this happening? Before the radio install my voltage was at a constant 14.3v wether the truck was at 800rpm or 2000rpm. Now its at 12volts. Why? The truck starts up fine, but I'm about to add this second battery and I'm worried that both won't get charged properly.
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Old 07-23-2008, 12:54 PM   #2
AZSCTACO
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Have you driven it?????

Drive 10-15 minutes and the voltage will go back to normal. The voltage will fluctuate
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Old 07-23-2008, 01:11 PM   #3
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Yeah, this has been going on for about two days now. Before I messed witht he battery when I drove the truck it would stay at 14v throughout the entire drive time. My commiute to and from work is about 30 minutes each way. Now when I drive it goes from 12 to 14v within the first 2 to 5 minutes of drive time then settles down to around 12v again. Last night when I left work I tested the voltage before I left work with my digital MultiMeter. The battery was resting at 12.84v, when I started the truck the voltage went up to 14.39 for about three minutes then it dropped down to 12.8v. When I drove home the meter never went to 14v. When I got home I shut the truck off tested the battery again, which was at 12.84v then I started the truck again, the voltage went up to 14.38v and then started dropping again all the way down to 12.78v. So I don't know what's going on but the fact that it started doing the after I had disconnected the battery. I'm thinking about taking the truck up to the dealer to have them look at it.

Last edited by pervitizm; 07-23-2008 at 01:15 PM. Reason: spelling and additonal drive info
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Old 07-23-2008, 01:13 PM   #4
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I would like to know what's going on thought. Because I plan on going with a bigger alternator here in like a months time and I don't want to spend the money on the Alt if it won't charge the truck properly after I disconnect the battery.
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Old 07-23-2008, 01:35 PM   #5
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So why did you disconnect the positive battery wire? What were you doing?
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Old 07-23-2008, 08:38 PM   #6
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If you have a 2007+ which I'm guessing you do since you are in that section of the forum, then the alternator is programmed to not charge when it's not needed in order to save gas.
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Old 07-23-2008, 09:40 PM   #7
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Quote:
Originally Posted by pervitizm View Post
I recently disconnected my postive cable off my battery to install a radio in my 07 Suburban. After reconnecting the battery my voltage readings stay at around 12.8 to 13.0 volts even when the truck in on and running. Why is this happening? Before the radio install my voltage was at a constant 14.3v wether the truck was at 800rpm or 2000rpm. Now its at 12volts. Why? The truck starts up fine, but I'm about to add this second battery and I'm worried that both won't get charged properly.
So to recap....I have 2007 Chevy Suburban LT and the reason why I disconnected the battery was to install my Pioneer Head Unit.

73Shark,
This doesn't make sense to me because before I messed with the battery the truck stayed at 14v whenever it was running. The voltage indicator never went below 14v until after I disconnected the battery. The truck drives fine and starts up, my only concern is that I'm about to install this second battery and if the alt is only pushing out 12v durning driving then I may fry my alt a lot quicker with the second battery. Or am I just going a bit overboard?
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Old 07-23-2008, 09:53 PM   #8
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Well if it truly never left 14V, then something was wrong before. Probably should take to dealer.
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Old 07-24-2008, 10:26 AM   #9
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Default how the system works......

this is right out of the GM "manual":

The electrical power management (EPM) system is designed to monitor and control the charging system and send diagnostic messages to alert the driver of possible problems with the battery and generator. This EPM system primarily utilizes existing on-board computer capability to maximize the effectiveness of the generator, to manage the load, improve battery state-of-charge and life, and minimize the system's impact on fuel economy. The EPM system performs 3 functions:

It monitors the battery voltage and estimates the battery condition.

It takes corrective actions by boosting idle speeds, and adjusting the regulated voltage.

It performs diagnostics and driver notification.

The battery condition is estimated during ignition-off and during ignition-on. During ignition-off the state-of-charge (SOC) of the battery is determined by measuring the open-circuit voltage. The SOC is a function of the acid concentration and the internal resistance of the battery, and is estimated by reading the battery open circuit voltage when the battery has been at rest for several hours.

The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the battery. Throughout ignition-on, the algorithm continuously estimates SOC based on adjusted net amp hours, battery capacity, initial SOC, and temperature.

While running, the battery degree of discharge is primarily determined by a battery current sensor, which is integrated to obtain net amp hours.

In addition, the EPM function is designed to perform regulated voltage control (RVC) to improve battery SOC, battery life, and fuel economy. This is accomplished by using knowledge of the battery SOC and temperature to set the charging voltage to an optimum battery voltage level for recharging without detriment to battery life.

The Charging System Description and Operation is divided into 3 sections. The first section describes the charging system components and their integration into the EPM. The second section describes charging system operation. The third section describes the instrument panel cluster (IPC) operation of the charge indicator, driver information center (DIC) messages, and voltmeter operation.

Charging System Components
Generator
The generator is a serviceable component. If there is a diagnosed failure of the generator it must be replaced as an assembly. The engine drive belt drives the generator. When the rotor is spun it induces an alternating current (AC) into the stator windings. The AC voltage is then sent through a series of diodes for rectification. The rectified voltage has been converted into a direct current (DC) for use by the vehicles electrical system to maintain electrical loads and the battery charge. The voltage regulator integral to the generator controls the output of the generator. It is not serviceable. The voltage regulator controls the amount of current provided to the rotor. If the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts.

Body Control Module (BCM)
The body control module (BCM) is a GMLAN device. It communicates with the engine control module (ECM) and the instrument panel cluster (IPC) for electrical power management (EPM) operation. The BCM determines the output of the generator and sends the information to the ECM for control of the generator field control circuit. It monitors the generator field duty cycle signal circuit information sent from the ECM for control of the generator. It monitors a battery current sensor, the battery positive voltage circuit, and estimated battery temperature to determine battery state of charge (SOC). The BCM sends idle boost requests to the ECM.

Battery Current Sensor
The battery current sensor is a serviceable component that is connected to the negative battery cable at the battery. The battery current sensor is a 3-wire hall effect current sensor. The battery current sensor monitors the battery current. It directly inputs to the BCM. It creates a 5-volt pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes.

Engine Control Module (ECM)
The ECM directly controls the generator field control circuit input to the generator. The ECM receives control decisions based on messages from the BCM. It monitors the generators generator field duty cycle signal circuit and sends the information to the BCM.

Instrument Panel Cluster (IPC)
The IPC provides a means of customer notification in case of a failure and a voltmeter. There are 2 means of notification, a charge indicator and a driver information center (DIC) message of SERVICE BATTERY CHARGING SYSTEM.

Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 6 modes of operation and they include:

Battery Sulfation Mode

Charge Mode

Fuel Economy Mode

Headlamp Mode

Start Up Mode

Voltage Reduction Mode

The engine control module (ECM) controls the generator through the generator turn on signal. It monitors the generator performance though the generator field duty cycle signal circuit. The signal is a 5-volt pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:

Commanded Duty Cycle
Generator Output Voltage

10%
11 V

20%
11.56 V

30%
12.12 V

40%
12.68 V

50%
13.25 V

60%
13.81 V

70%
14.37 V

80%
14.94 V

90%
15.5 V


The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the ECM. This information is sent to the body control module (BCM). The signal is a 5-volt PWM signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic purposes.

Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator output voltage is less than 13.2 volts for 45 minutes. When this condition exists the BCM will enter 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 ever one of the following conditions are met.

The wipers are ON for more than 3 seconds.

GMLAN (Climate Control Voltage Boost Mode Request) is true, as sensed by the HVAC control head. High speed cooling fan, rear defogger and HVAC high speed blower operation can cause the BCM to enter the Charge Mode.

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

Battery State of Charge is less than 80 percent.

Vehicle Speed is greater than 145 kph (90 mph)

Current Sensor Fault Exists

System Voltage was determined to be below 12.56 volts

Tow/Haul Mode is enabled

When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9-15.5 volts, depending on the battery state of charge and estimated battery temperature.

Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the ambient air temperature is at least 0C (32F) but less than or equal to 80C (176F), the calculated battery current is less than 15 amps and greater than -8 amps, and the battery state of charge (SOC) is greater than or equal to 80 percent. Its targeted generator output voltage is the open circuit voltage of the battery and can be between 12.5-13.1 volts. The BCM will exit this mode and enter Charge Mode when any of the conditions described above are present.

Headlamp Mode
The BCM will enter Headlamp Mode when ever the headlamps are ON (high or low beams). Voltage will be regulated between 13.9-14.5 volts

Start Up Mode
When the engine is started the BCM sets a targeted generator output voltage of 14.3 volts for 30 seconds.

Voltage Reduction Mode
The BCM will enter Voltage Reduction Mode when the calculated battery temperature is above 0C (32F). The calculated battery current is less than 1 amp and greater than -7 amps, and the generator field duty cycle is less than 99 percent. Its targeted generator output voltage is 13 volts. The BCM will exit this mode once the criteria are met for Charge Mode.

Instrument Panel Cluster (IPC) Operation
Charge Indicator Operation
The instrument panel cluster (IPC) illuminates the charge indicator and displays a warning message in the driver information center (DIC) when the one or more of the following occurs:

The engine control module (ECM) detects that the generator output is less than 11 volts or greater than 16 volts. The IPC receives a GMLAN message from the ECM requesting illumination.

The BCM determines that the system voltage is less than 11 volts or greater than 16 volts.

The IPC receives a GMLAN message from the body control module (BCM) indicating there is a system voltage range concern.

The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.

The ignition is ON, with the engine OFF.

Battery Voltage Gauge Operation
The IPC displays the system voltage as received from the BCM over the GMLAN serial data circuit. If there is no communication with the BCM then the gauge will indicate minimum.

Utilities and light duty full size pickups are equipped with a new regulated voltage control (RVC) system. This system turns off the alternator when it is not required in order to improve fuel economy. The generator will turn back on when additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems which usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

SERVICE BATTERY CHARGING SYSTEM
The BCM and the ECM will send a GMLAN message to the DIC for the SERVICE BATTERY CHARGING SYSTEM message to be displayed. It is commanded ON when a charging system DTC is a current DTC. The message is turned OFF when the conditions for clearing the DTC have been met.
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Old 07-24-2008, 01:04 PM   #10
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Thanks for the information GMTech.
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