Today when i was driving to work i had my headlights, radio, and heater on. I noticed that the battery voltage was dropping and got down low enough that the shift indicator, and odometer were flickering. I turned off my heater and it went back up a little, then i turn off my headlights and it went back to normal. Is this the alternator or just a weak battery. I'm guessing its the battery because there were a couple times previously that i had drained the battery all the way down, and it looks like an off brand cheapy. Also i dont think its the alternator because it has been doing this since around june, or maybe earlier. if it was the alternator i would imagine it would be shot by now. Of course the alternator will probably go soon because its been working harder trying to keep the battery charged. Just looking for any opinions. thanks for the input.
By the way its a 99 ZR2

 

first thing you need to do is give the battery a real good charge. Then you should have the battery load tested and check the charging system voltage. The alternater won't put out properly if you have a bad battery.

 

Start your blazer and let it idle. Pop the hood and while the motor is running, remove your positive battery cable. If the truck continues to run your alternator is fine and the problem is your battery, If the truck dies, the problem is with your alternator. I have found this a pretty effective way to test the alternator without using a voltmeter. If you have a voltmemter that will plug into your cigarette lighter, start the truck and plug it in. It will show you what kind of voltage your alternator is pushing. If it is very low voltage, your alternator is bad. If you have alot of aftermarket accessories (stereo, auxillary lighting, etc..) this can cause your alternator to wear out quicker. To run alot of electrical accessories on blazers you need to upgrade the alternator. If you notice, your headlights will dim anytime and auxillary electrical accessory kicks on. Your interior lights will do this too... Its an easy way to fry an alternator, so if it needs replaced, I would reccommend a higher capacity alternator.

 

I would say that the altenator is bad and the battery could not keep up with your electrical needs, but like Chevylover said put a good charge on it to see if it will recover. The electrical systems on the newer trucks will no longer recharge a battery. Engineers have sacrificed function for weight savings and fuel mileage.

I had the same problem with my '99 G/A the alternator went dead and drained the battery. I recharged it and it was fine until it happened again. Altenator was whining a lot so I kinda figured that it was time to replace. No problems ever since.