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12 Posts
Discussion Starter · #1 ·
Found out the hard way that many modern alternator stators are controlled by the body control module. The system is called ROC or rate of charge and take in consideration inside/outside temperature, electrical load, battery condition......EVERYTHING. I am use to a voltage range of 13.5 to 14.5 on most everything with an alternator. My saturn often dips down below 13Volts. I know it shouldn't be at anything under 12.6 so when this happened, I decided to rebuild that alternator. At 200k+ miles, I knew it could use a set of brushes. It was extremely hard finding the parts for it. I got the front and rear bearings from a company in texas and voltage regulator and brushes on ebay for $40. If you don't need to replace the inner bearings, replacing the brushes and voltage regulator is easy as you do not need to split the alternator, just three nuts on the back cover. Also, the front one way bearing, is THREADED onto the rotor shaft, not pressed. Took me awhile to figure that one out. I put everything back together and I did get different voltages, but, my battery cable connectors were loose and it quite charging the battery (smart system). The battery cable ends look a bit whimpy to me. When the vehicle didn't start, I checked them and they were just a little loose. There is not much surface area on these cable ends so I replaced them and the battery (AGM battery). Everything went well for 6 weeks, then it started to go down to voltages around or below 12.8. That, I know is not acceptable. I just purchased another regulator and plan on installing that within a couple of days.

My question are;
1) what is the voltage range that the system should be at?
2) Where is the module that controls the stator/alternator and how do I test it without a diagnostic tool?
3) What program can I buy for a laptop that will fully interface with all functions of this vehicle?

4 Posts
As to question 1:
Your alternator may be working properly. The Engine control module (ECM) along with the BCM measure system electrical requirements then sets the alternator output to one of five modes. Things that trigger the different modes are A/C on, lights on, state of battery charge and things like that. When the battery is fully charged and electrical load minimal, the voltage may go down to 12.4 volts. As conditions change the voltage may vary from the low stated up to 15 v or so. If your voltage is changing within the range stated above things are working properly. Your owners manual even states that voltage variation is normal.

I run a ScanGuage II OBD2 diagnostic monitor on my Acadia (and 09 Traverse before) on a regular basis and have observed actual voltage levels while driving real time. The voltage constantly varies within the range above depending upon the electrical load.

Question 3:
The ScanGuageII I use is about $160.00. Lap top apps are available but I'm not familiar with any. Cheapest way to go is with a Bluetooth OBDII reader that plugs into the OBDII port and a smartphone diagnostic app that are available. Readers as low as $20.00 are available on EBay and the app is also not too expensive. One app is called "Torque" I believe. This has been discussed on the Traverse forum so do a search.

12 Posts
Discussion Starter · #3 ·
Thanks for the reply, I really appreciate the good information.

I watch the voltmeter gauge on the dash, but I have a plug in cigarette style voltmeter that works great. I noticed that when I start the vehicle, the voltage runs up to 14.5 volts. It settles down within 2 to 10 minutes to 12.8 to 13.1. I feel comfortable at 13 volts, but below 12.6, its pulling from the battery I assume. It frequently runs between 12.4 to 12.6 and that really worries me. The electrical load has no effect on it either which seems odd to me. I can turn everything on (seat heaters, stereo, ac, defrost ,etc.) and then off and I voltage dose not change. The original battery lasted 7 years and I replaced it when the voltage began to be erratic. I rebuilt the alternator at the same time and found that the brushes were definitely shot. What I found interesting was that before I rebuilt the alternator I installed the new battery and the charging characteristics wildly changed. It always displayed a voltage between 13.5 and 14.5. It does make sense that the new battery requires less voltage potential to maintain a proper charge, but I really wish I could find out what the minimum voltage is. I also spent a little more money and purchased an AGM battery so I a not sure if that makes a difference or not. I have an extra voltage regulator and I am on the fence here as to install it or not. If this is how the system works, then I will leave it alone. If I find something different out or more information, then I will add to this post. I think this is a bit important because people like me are not use to this system and can panic when battery voltage consistently falls below 13 volts.

I would like to ask is anyone can tell me what module directly controls the alternator on these vehicles and possibly the location on the vehicle.

Also, I am looking at an ELM327 device to read my vehicles OBD2 diagnostics. If anyone has had success with a good laptop program, let me know. I have tried to research this a bit and found out there is alot to it.

thanks for the help
Brian Storm

214 Posts
Download and look at the factory service manual. It should have the answers you need to learn more. This is a free download for the Enclave up to I believe 2010 model year. It will be the same for the Outlook, or very similar, if it doesn't include the Outlook. Make sure you paste or copy the entire string into your browser in order to see the Enclave download, not just the home page.!zlFDEIJZ!lhEvdcmJL895CqO3HHkWfb1_8lGKFRajYnKT9Uac1i8

4 Posts
Some additional info for you to mull over. I wouldn't change anything else until you drive for a while to see how well it works in every day use.

The ECM controls the alternator output using information from the BCM. The BCM is located under the dash to the left of the steering column while the ECM is bolted to the front cross member in the engine bay to the left of the radiator where it can get some cooling air.

Here is what my 2009 repair manual says about the charging system. Quite complicated but it appears to work well.

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 performs idle boost.

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 Operation
2009 ENGINE Electrical - Acadia, Enclave, Outlook & Traverse

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 field control circuit. It monitors
the generator performance though the generator field duty cycle signal circuit. The ECM controls the generator
through the generator field control 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

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 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 0°C (32°F).
 -Battery State of Charge is less than 80 percent.
 -Vehicle Speed is greater than 145 kmh (90 mph)
 -Current Sensor Fault Exists
 -System Voltage was determined to be below 12.56 Volts

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

Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the estimated battery temperature is at least 0°C (32°F) but less
than or equal to 80°C (176°F), the calculated battery current is less than 15 amperes and greater than -8
amperes, and the battery 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 and 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 and 14.5 volts.

Start Up Mode
When the engine is started the BCM sets a targeted generator output voltage of 14.5 volts for 30 seconds.

Voltage Reduction Mode
The BCM will enter Voltage Reduction Mode when the calculated ambient air temperature is above 0°C (32°F).
The calculated battery current is less than 1 ampere and greater than -7 amperes, and the generator field duty
cycle is less than 99 percent. Its targeted generator output voltage is 12.9 volts. The BCM will exit this mode
once the criteria are met for Charge Mode.

12 Posts
Discussion Starter · #6 ·
Thanks so much for the reply, that is the information and explanation I was looking for. My understanding of alternator charging system designs is very outdated, so I guess I can say I just got "schooled!" I have been working and traveling and have not been able to respond.

Now that it is cold out, the system is always in charge mode, above 13 volts. Before, it was in economy mode even when I had a full electrical load.....except I never tried turning on my wipers, but everything else was on including the stereo! When I start the vehicle, it does charge for a while before entering the economy mode.

I had an interesting situation happen with a brand new battery. I started to notice that a few days after the installation, my battery voltage was between 13.5 to 14.5, but dropped under 12.4 volts when I shut it off. The battery went dead while I was running around town with multiple starts/stops. Long story short, I wiggled the battery cables and they were both loose. The battery cable connectors didn't stay firmly attached because the new battery terminals were slightly undersized compared to the old battery(it wasn't obvious at first). I replaced the connectors with the standard large lead battery connectors and immediately noticed a decrease in charging voltage. Somehow the system can sense the resistance or electrical load of the battery and it tried to compensate for it. What I found is that a weak battery at warm outside temperatures yields higher system voltages verses a new battery. Yeah, its complicated, but seems to work well so far(250,000 miles).

My saturn operates a little different then the above mentioned service manual, but it is close. The manual I was using was not this specific. If my 2007 has a different charge state, I will post what I observe in the spring/summer when the outside temperatures are back up. I was not aware of the new designed alternator charging system and was seriously confused when I tried to check applied stator voltage. I could not get voltage readings that made sense so I blindly bought and replaced the bearings, brushes, and voltage regulator. It took me a while to figure out that the drive pulley with the one way bearing is right hand thread, not pressed on! Luckily I zipped it off with an electric impact driver. $70 parts vs $200 to $300 for a new one! I really like this vehicle platform but it seems that the price and complexity of being green comes at a huge expense.
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