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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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If the OP had a real DVM with a small labscope capability (Sears sells them pretty cheap - maybe RadioSchlack does too IDK) and he could actually see the bad diode in the A/C side of the screen.

ALSO - in the DC-Voltage mode - you should/might have a frequency reading - and frankly, there shouldn't be a frequency on the DC reading.

You will be seeing AC-ripple on the DC side and there's a maximum level for the sake of the battery (it'll kill a DC battery to have AC shoved into it) .... and the bad tempered 'electronical' devices that do NOT like AC on the DC side either!

Honestly - I can see no real rational for NOT having a good digital meter for all the things that go bad in your house, car, computers and anything else that cannot hack a big current pull of an analog meter.

Do NOT use an analog meter nor a test light in a circuit that you may or may not know is a computer low-current/low voltage circuit! You NEED a Hi-Z type meter that does not put a big current pull or injection into a circuit that is designed to have little to very little current or voltage above 5VDC.

Don't go sticking a probe into any circuit that you do not know what it is and what it is for. Period!

Never use an Ohm Meter on any vehicle circuit that is not isolated - they do not like the internal battery of the meter's 9VDC slug of current into their system.

Here's a meter description from Matco --- [LINK HERE]
  • Features include frequency, duty cycle, diode and continuity tests in addition to all the standard meter features
  • Auto range function automatically selects the correct range for testing, eliminating damage to the meter caused by incorrect range settings
  • Data hold feature allows you to examine leads and test points carefully while the meter holds the highest reading
  • Provides accurate readings for volts, amps and ohms
  • Frequency: 1Hz ~10MHz±0.1%
 

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I'm going to bed now ------ nite!
 

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Nite, and just in case the news did not make it to Montana, Radio Shaft, err, Radio Shack went out of business a few years back, so no more running there to purchase their test instruments of various qualities.
 

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Aha, yes.... but we still have a Radio Shack here ... and a Sears, a JC Penny and a K-Mart.
 

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2004 GMC Envoy SLT 4.2
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Aha, yes.... but we still have a Radio Shack here ... and a Sears, a JC Penny and a K-Mart.
Sounds a lot like the little town I grew up in . Always figured it was at least 10 years behind the rest of the world . LOL
 

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They just haven't gotten the memos yet...
 
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