Is it my Alternator, or Battery

Discussion in 'Engine & Drivetrain' started by Donnyballgame, Jul 12, 2011.

  1. Donnyballgame

    Donnyballgame TYF Newbie

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    So after 97k miles, I have a problem that is not longer covered under warrenty. I was driving along and I noticed that my voltage was quite low. I looked at my volt meter on my radar detector and it was reading 12.5 volts. I get to where I was going and shut the truck off. It sat an hour or so and started just fine. I look at the volt meter and it reads 14.8 to 15 volts. Great. So I start my 2.5 hour trek home. 10 miles down the road the volt meter reads 13.5 volts. and it kept dropping the whole way home never getting lower than 12.3 volts. I stopped by Advanced Auto and they assured me that it was the alternator after they brought out their little machine and tested it. Not that I don't trust those guys, I just wanted to know if there was anything else that I can do with a multimeter to double check their work? Any ideas? Thanks

    Donny
     
  2. BabyHuey

    BabyHuey Full Access Member

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    The signs definentaly point to alternator, no questions asked. The alternator is responsible for charging your battery when you drive, so effectively your driving off your alternator power.
     
  3. bauer4567

    bauer4567 Full Access Member

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    If it does not drop below 12 volts, it is probably working just fine. The BCM will adjust the amount of voltage the alternator puts out, and therefore the reading on your volt meter, based on the electrical demand for the current driving / operating conditions. It is completely normal from my experience and research from others experiences to go between 12 and somewhere right under 15 in normal operating conditions.

    The theory behind this is that the BCM changes the amount of output from the alternator to gain MPGs during low electrical demand times, through less friction on the alternator pully... Practically, I find it hard to believe the difference is significant, but I can all but guarantee there was a nice government grant or incentive for the development of such a stupid MPG saving strategy.
     
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  4. Donnyballgame

    Donnyballgame TYF Newbie

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    Thanks for the replys guys. Iam thinking Bauer. After reading your post I went out and did another test. I started the truck and let the volts start to drop. When it got down to 13.5, I turned everyhing on. Both blower motors on high, AC on, High Beams, hazards flashing, and brake lights on. THe volts then started to climb back up to 14.2. I guess I will hold on a little longer before swapping out the alternater.
     
  5. bauer4567

    bauer4567 Full Access Member

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    This is what GM says about it all...

    During normal driving conditions, when engine speed is above 1000 RPM, the generator is designed to do two things:
    • Supply the current necessary to operate the vehicle's originally equipped electrical devices (loads).
    • Recharge/maintain the battery's state of charge.
    The following factors may affect generator and battery performance:
    • Non-usage of the vehicle for extended periods of time. The vehicle's computers, clocks and the like will cause the battery state of charge to drop (For example; 30 days in a parking lot and the vehicle may not start because of a dead battery or a vehicle which is driven only a short distance once a week may end up with a discharged battery to the point where the vehicle may not start). This would be considered abnormal usage of the vehicle and the normally expected result for the vehicle battery, generator and electrical systems.
    • At idle, vehicle electrical loads may exceed the low speed current (amperage) output of the generator and when this happens the shortfall comes from the battery. This will result in a drop in the electrical system voltage as the battery delivers the additional electrical current to meet the demand. This is equivalent to the brown outs experienced by homes and businesses when the electrical demand is more than the supply. See Figure 1.
    • Extended periods of engine idling, with high electrical loads, may result in a discharged battery. Attempting to recharge a battery by letting the engine run at idle may not be beneficial unless all electrical loads are turned "OFF".
    • Increased internal generator temperatures from extended idling can also contribute to lower electrical system voltage. As the generator's internal temperature rises, the generator's output capability is reduced due to increased electrical resistance.
    The following are some typical examples of electrical loads:
    System Amperage Load
    Rear Window Defogger 25
    Electric AIR Pump 25
    Heated Seats 5 Amps per seat
    Headlamps (high) 20
    Blower Motor (High) 20
    Headlamps (Low) 15
    Brake Lights 6
    Windshield Wipers 6
    Ignition 6

    Figure 1



    Depending on the vehicle application, generator current (amperage) output at engine idle speeds of 600-700 RPM can be as low as 35 percent of the full rated output. With enough electrical loads "ON", it is easy to exceed the generator current (amperage) output when the engine is at an idle of 600-700 RPM. This is a normal condition. The battery supplements for short periods of time. Items that affect the vehicle's electrical system current and voltage at idle are the number of electrical loads being used, including add-on accessories, and extended idle times. When the vehicle speed is above approximately 24 km/h (15 mph), the engine/generator RPM is high enough and the generator current (amperage) output is sufficient to supply the current (amperage) requirements of the vehicle as originally equipped and recharge the battery.
    Dimming lights at idle may be considered normal for two reasons:
    1. As the engine/generator speed changes, so will the current (amperage) output of the generator. As a vehicle slows, engine/generator RPM slows and the current (amperage) output of the generator may not be sufficient to supply the loads, the vehicle system voltage will drop and the lights will dim. Dimming of the lights is an indication that current is being pulled from the battery. If the battery is in a low state-of-charge (discharged condition), the driver will notice a more pronounced dimming than a vehicle with a fully charged battery.
    2. When high current loads (blower, rear defogger, headlamps, cooling fan, heated seats, power seats, electric "AIR" pump, or power windows) are operating or cycled "ON", the generator's voltage regulator can delay the rise in output. This effect, usually at lower engine speeds, can take up to ten seconds to ramp up the generator output. This is done to avoid loading the engine severely. To increase current (amperage) output, additional torque is consumed by the generator. The engine computer (ECM/PCM) will ramp up engine/generator speed in small steps so engine speed variations are not noticeable to the driver.
    For diagnosis of the battery and/or the generator, refer to the appropriate Service Information (SI) or Corporate Bulletin Number 05-06-03-002C.
     
  6. Warriorpluto

    Warriorpluto Member

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    So just to clarify this is normal behavior when driving long distances? I aborted a trip today thinking it was the battery or alternator. Got thirty miles out and 12.8 volts. Turned truck off then back on and back at 14 voltd
     
  7. NathanJax

    NathanJax Vacation Nathan Staff Member Supporting Member

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  8. iamdub

    iamdub Too cold, too cold

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    As Dan and Nathan said, this is normal operation of a factory feature. It can be nerve-racking and make you wanna watch the gauge more than the road, but just drive it. It's been doing it for the past 97,000 miles, you just never noticed it.
     
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  9. Andre Downer

    Andre Downer TYF Newbie

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    +1. my Tahoe does this, and everything is working just fine.. alternator is newer and I have a dual battery setup..
     
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  10. Warriorpluto

    Warriorpluto Member

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    Oh ok thanks allot guys. Pretty cool feature.
     

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