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Hey all,

I have an '02 rocking the original alternator and a 2.5 year old Super Start battery. Shortly after fixing my A/C I noticed that the battery gauge while running was not near 14 but rather looked like 12-13v. I hooked up a meter to the cigarette outlet and it says the battery resting was 12.3V and when running it was exactly 13V. Revving it did not change it at all. Turning the A/C off makes it go up to 13.4V on the dot. No battery lights on and I haven't had to jump it.

I should mention it's gotten quite hot outside rather quickly as only a month ago we were in high temps of 80-85 and now we're going to be at 106 tomorrow.

I rushed to Autozone and had them load test the battery and alternator. Both reportedly passed but the man mentioned the machine told him that the battery was only at about 60% charge but tested healthy. The alternator also passed the load test and the slip it printed showed 13.4V charge. I asked if that was normal and the guy wasn't much help. I've looked on the forums a bit and can't quite find a post showing what the normal alternator outputs are so I post here.

Am I worrying over nothing or will the truck eventually strand me somewhere? I'm suprised the guy didn't try to sell me a new battery so that's why I posted here. Is there something else it could be or am I stuck paying $129 for a new battery or $100 for a new alternator :(
 

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2004 chevy trailblazer_lt
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Have you measured the voltages at the battery using a digital multi meter (DMM or DVM)? You really cannot get an accurate reading from the volt meter gauge on the instrument panel - they are notorious for going all over the place and not being very accurate. So, before you do anything else, measure the Key Off Engine Off battery voltage at the battery and the Key On Engine On battery voltage at the battery, and compare those values to the information in the next paragraph. Also, make sure that where the battery cables connect to the starter and to the block as well as to the battery are clean and tight. Be careful with the battery end as you do not want to over tighten the bolts and strip the threads in the battery.

I have always heard/read/been told that 14.2 VDC is the "ideal" output voltage for an alternator that is putting out current to charge a battery/provide the juice for headlights, or HVAC fan, etc. In reality, I think the voltage range for being considered acceptable is more on the order of 13.8 to 14.2 VDC. Anything above 14.2 VDC indicates a bad voltage regulator, and anything below ~13.8 indicates something funny is going on inside the alternator such as a bad diode in the diode trio, a bad regulator, a mechanical problem.

Now, here is the kicker - the alternator's output is controlled by the PCM, so the low end of the voltage range for a good alternator could drop as low as 13.0 VDC on some PCM controlled vehicles. I do not know what the lower voltage specification is for a 2002 TrailBlazer with the 4.2 L I6 engine.

If the voltages you measured using a DMM/DVM are not within the 13.8 to 14.2 VDC range, I would take the alternator off the TrailBlazer and take it to another auto parts store (such as NAPA, O'Reilly, or Advance Auto Parts) and have them test it. Or, if you know of an automotive electrical shop that rebuilds alternators and starters, take it there and have them test it using their methods (which will likely include looking at the output voltage with an oscilloscope). If they find something wrong, let them repair it and they will return to you an alternator that will likely be superior to anything you can buy at a local parts house.

Good Luck!
 
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