07 Milan overcharging battery. Replaced alternator. Still overcharging battery. Original alternator bench tested good. What would cause this.

Change the alternator

charging voltage should be 14.5-16..not 12.5.. thats too low.. get the alternator replaced and your charging issues should disapear..

I'm going to assume your meter is accurate and you have verified the 18.4 reading.
Something has to be bypassing the regulator to make the alternator charge at full power. The regulator breaks the circuit dozens of times per minute to maintain 14 volts. Does the alternator charge with the regulator unplugged ?

check voltage from battery car running. if alternator good it will produce 13.6-14.2 volts. if lower replace alternator. good-day ! make sure battery posts and wires are clean and tight.

Your car's symptoms (voltage fluctuations under load that shouldn't happen) seem to
strongly indicate that your voltage regulator has failed. The voltage regulator is the component
that regulates voltage coming from the alternator, and which is supplied to the rest of the car.
Your new belt tensioner indicates to me (and should have to your mechanic) that the
alternator and/or voltage regulator weren't able to operate properly, and a faulty belt tensioner
has killed more than one car's alternator/voltage regulator.
Your do not need new headlights. Do fix the turn signals.

In the 2005 Sunfire, the voltage regulator is an integral component of the alternator, and is
not separately serviceable. So your 2005 Sunfire needs a new alternator.

2005 Pontiac Sunfire Alternator - Duralast Part Number: DL2305-15-4 $199.99 available at:
http://www.autozone.com/autozone/parts/2005-Pontiac-Sunfire/Alternator/_/N-jmqtpZ93xme

The national parts chains (Advance Auto Parts, AutoZone, Pep Boys) will (for free) administer
a "Charging System Test." Which will telly you for sure that your alternator/voltage regulator
is bad. Do this before replacing alternator, since its a pricy part in the 05 Sunfire.

Just in case, Here's the 2005 Sunfire manual entry on (a) Diagnosis & testing the alternator, and
(b) replacement of the alternator

Alternator replacement is a do-it-yourself project, though you may need to get a
"Serpentine Belt Kit" on your favorite parts store's loan-a-tool program to remove/re-install the serpentine (drive) belt.
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Diagnosis & Testing - 2005 Pontiac Sunfire Alternator


Alternator Load Test

1. With the engine running, turn on the blower motor and the high beams ( or other electrical accessories to place a load on the charging system).

2. Increase and hold engine speed to 2000 rpm.

3. Measure the voltage reading at the battery.

4. The voltage should increase at least 0.5 volts from the voltage test. If the voltage does not meet specifications, the charging system is malfunctioning.
NOTE
Usually under and overcharging is caused by a defective alternator, or its related parts (voltage regulator), and replacement will fix the problem; however, faulty wiring and other problems can cause the charging system to malfunction. Many automotive parts stores have alternator bench testers available for use by customers. An alternator bench test is the most definitive way to determine the condition of your alternator.




Alternator No-Load Test

1. Connect a tachometer to the engine.
CAUTION
Ensure that the transmission is in Park and the emergency brake is set. Blocking a wheel is optional and an added safety measure.


2. Turn off all electrical loads (radio, blower motor, wipers, etc.)

3. Start the engine and increase engine speed to approximately 1500 rpm.

4. Measure the voltage reading at the battery with the engine holding a steady 1500 rpm. Voltage should have raised at least 0.5 volts, but no more than 2.5 volts.

5. If the voltage does not go up more than 0.5 volts, the alternator is not charging. If the voltage goes up more than 2.5 volts, the alternator is overcharging.
NOTE
Usually under and overcharging is caused by a defective alternator, or its related parts (regulator), and replacement will fix the problem; however, faulty wiring and other problems can cause the charging system to malfunction. Many automotive parts stores have alternator bench testers available for use by customers. An alternator bench test is the most definitive way to determine the condition of your alternator.


6. If the voltage is within specifications, proceed to the next test.



Voltage Test

1. Make sure the engine is OFF , and turn the headlights on for 15-20 seconds to remove any surface charge from the battery.

2. Using a DVOM set to volts DC, probe across the battery terminals.

3. Measure the battery voltage.

4. Write down the voltage reading and proceed to the next test.



Removal & Installation - 2005 Pontiac Sunfire Alternator


2.2L (VIN 4) Engines

1. Before servicing the vehicle, refer to the precautions section.

2. Remove or disconnect the following:

Negative battery cable

Accessory drive belt

Alternator mounting bolts

Alternator electrical connectors

Alternator





To install:

1. Before servicing the vehicle, refer to the precautions section.

Alternator. Torque the upper bolts to 37 ft. lbs. (50 Nm) and the lower bolt to 22 ft. lbs. (30 Nm).

Alternator electrical connectors

Accessory drive belt

Negative battery cable





2.2L (VIN F) Engines

1. Before servicing the vehicle, refer to the precautions section.

2. Remove or disconnect the following:

Negative battery cable

Accessory drive belt

Alternator mounting bolts

Alternator electrical connectors

Alternator





To install:

1. Install or connect the following:

Alternator. Torque the bolts to 16 ft. lbs. (22 Nm).

Alternator electrical connectors

Accessory drive belt

Negative battery cable

Over charging could be the result of several things including:

Faulty Voltage Regulator
1. A car's battery will overcharge if the voltage regulator isn't working correctly. A voltage regulator is usually part of the alternator, and is used to keep a steady flow of voltage to the battery. If the voltage regulator is defective, it will send either too little or too much charge to the battery. If it sends too much, the car battery will overcharge. The voltage regulator is easily replaceable at an easily affordable price, sometimes for as little as twenty dollars, as of 2009.
Faulty Alternator
2. Sometimes the alternator itself can be at fault. The alternator is the device that converts the mechanical power of the engine into electrical power to charge the battery. When an alternator breaks, it usually stops creating electricity for the battery, which will then eventually die. However, if the wrong alternator is placed in the car, or if the alternator is running at the wrong pace, it will create too much energy for the car battery, causing it to overcharge. The alternator is another easily replaced part.
Incorrect Charger Use
3. If a battery charger is used to charge your battery outside of your car, improper use of the charger can result in overcharging. If a battery is placed on the charger too long, it can result in overcharging, and a significant decrease in your battery's lifespan and efficiency. This is why it is important to read about your specific battery and understand how long it needs to charge to be effective. Too much charge will lead to problems.
Faulty Battery Chargers
4. Sometimes chargers can be faulty. Their settings may be wired incorrectly, or the charges labeled incorrectly. As a result, your battery may be getting overcharged, even if you are carefully monitoring your charging. This is a problem that is hard to avoid, as manufacturing mistakes can happen anywhere, anytime without warning. It is a good idea to test your charger regularly to see if it is running correctly.
Heat
5. Extreme heat in the summer can also have an adverse effect on the car battery. If the battery has been previously overcharged, extreme heat can increase the problems caused by overcharging, and exacerbate any other problems with the battery. This problem can be hard to avoid if you live in a warm climate. The only real way to ensure safety against this problem is to avoid overcharge in the first place.

The first thing to do is have the battery tested and then the charging system checked, this will show you if its overcharging,if it is overcharging then take a look at the wiring going to alternator,and also a bad battery can also cause a problem so first find out if message center is displaying the correct message, this will show if center is okay then move on to battery testing then alternator test, there may be a problem with the wiring or a defective alternator.

Have the battery tested and replace as necessary. Once you are sure the battery is good, have the charging system tested. The voltage for this is between 13.5 volts and 14.7 v. Anything greater than that is just too much and can cause the battery to overheat and have to be replaced prematurely. Let me know if you need anything else.

You need to replace the battery to start with. you cannot test the charging system with the battery the way it is or you will get a false reading. Once you replace the battery, you can then test your charging system to see if the alternator is causing the problem. If its charging more than 14.8 -15 volts, its too much and you will need to replace the alternator.