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Re: Battery will not hold a charge.
I don't know how you figured out that your battery was good, but if it drops to 9 volts I highly suspect that it is bad. You need to have it load tested. This will tell if it is bad under load. Also if it is bad it can cause the alternator not to work even if the alternator is brand new.
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a bad battery would give a wrong judgement of an alternator output. take off your battery and replace with a working one, test your alternator output using a DMM it should read about 12.5 when engine off and 14v when engine start running any figure more or less shows you have a bad alternator or an incorrect input current to the alternator....hope this fixxx it?
The Avalanche are famous for the dead battery of not driving for 30 days, Your ECM and various module has a parastatic draw that is normal for you vehicle.You also have the ignition problem. Even though you turn off the ignition remove the key the ignition draw power and this has been a porblem with your year make and model You will need to replace the ignition switch Here is the TSB Bulletin No.04-06-03-002.
When you say battery voltage how are you testing it? Are you looking at a gauge on the dash? Or are you physically testing it with a multi meter? If the truck is dying and you recently replaced the alternator and battery I would first suggest you to check all them connections again real good. Snug them all up. Check for corrosion and clean as needed. Did you replace the battery and alternator with new/re-manufactured battery and alternator? The alternator needs to put out at least 13.6 volts to charge your 12 volt battery or the battery will not maintain a charge. However the alternator runs the vehicle once it is started. The battery is used only to start the car and then the alternator runs it. So if your vehicle starts and runs and then dies you may have a bad alternator. Check your connections first and then test the alternator with a multi meter. Anything less than 13.6 volt will not suffice. 14+ volts would be the hopeful output of a good alternator.
Sounds lie the battery is not getting charged by the alternator. If you slow charge the battery overnight and it retains a charge for a while then eventually dies, it may be good and the alternator needs to be replaced. If the battery does not hold a charge, you will need to replace the battery and have the alternator tested to see if it is faulty.
Yup, time for a new alternator. Most models have the regulator built into the alternator, so it makes it a little bit more expensive - but it will resolve your problems. If you are driving at night with a load on your charging system (lights and radio on), you will probably notice you have dim headlights. If you don't change it soon, you'll end up having to replace your battery too.
If you're getting no power at all when you turn the key to start, try disconnecting the battery and then hook it back up. Chevy has had some problems with an ignition relay solenoid that's mounted under the driver's side dash. Disconnecting power could reset the switch....If that seems to be it, a replacement is around $250. at a chevy dlrshp.