Bad Alternator ?

My 2000 Blazer LT 4.3.....noticed tonight that my dash lights were dim and I could hear a whining from my speakers, only had my headlights on....tested it with a Voltmeter and it read roughly 12.23 volts at idle BUT.....when I rev the motor it goes to 14 volts....could this be the alternator ? Battery is only 6 months old, AC Delco....

Thanks !

Yip, the alternator is not producing the right voltage.

This could be caused by a bad battery with an internal short or the alternator has bit the bisket early.

I would get a warranty replacement. It is covered by a years warranty.

Yep, alternator was the problem....got a Bosch rebuilt at NAPA (it's nice to have a buddy there !) and now it reads 14.7 volts at idle with EVERY accessory I could think of on plus the headlights.

Thanks BlazerLT !!!

You are welcome!

Glad to see you happy.

Well I THOUGHT all was well.....now all of a sudden when I am idling the Vots drop all the way down to 12 and then back up to 14......if you sit and idle about every 30 seconds or so this happens, the lights dim, and then it goes back up to 14.

New alternator gone bad ??? I am getting so frustrated with this truck now...

I would get your battery tested.

If it will not hold a charge, the alternator will be constantly working to keep the battery up to snuff.

This will burn it out in short order.

Battery tested 100 % OK....

Check all of your wires and connections, make sure they are tight and in good condition. Also make sure the mounting bolts are tight. Look and see if the belt shows signs of slipping.

Also make sure your battery is fully charged before you connect the new alterator, or your alternator will be DOA.

Battery tested 100 % OK....

Who tested it?

Did you hook up an voltmeter to it?

I took it to Autozone and they checked it...

Teheheh, don't trust them.

Take a voltmeter out and connect it to the battery when the engine is off, and again when it is running.

Teheheh, don't trust them.

Take a voltmeter out and connect it to the battery when the engine is off, and again when it is running.

OK now that we finally had a nice weekend I decided to dig into this again....Voltage at the battery with the engine/key OFF - 12.76 volts, Voltage at the battery with the engine RUNNING - 14.58 volts.....

I went ahead and got a warranty replacement alternator and installed it. Voltage reading with the new alternator and engine running is 14.61 volts.

STILL having the same problem, many times after coming to a stop and mostly at night the volts will drop down to 12, then rise back up to a little past 14. I am also noticing that when idling with the turn signals on the volts needle will bounce back and forth slightly...

Is any of this normal ? I NEVER noticed this before with my original stock alternator....

Any help is appreciated !

The needle bouncing with the signal is normal, the dropping to 12v is not.

It ONLY happens at night?

Yep...I've only noticed it in the evenings when my headlights/dash lights are on.......BUT doesn't do it everytime I stop either....

Odd, sounds like a short of somethign somewhere.

OK I'm opening this up again....I am on my second Bosch reman alternator and still having the same problem. It seems as though it's because these motors idle soo lom (600 RPM) If you lightly tap the accelerator the volts go normal. I am going to try a different brand of alternator now before I bite the bullet and pay for a GM alternator. Lucky for me to have a friend at NAPA who will take these back....

NOTE this NEVER happened with the original AC DELCO alternator....

Cmon now with all of the GM techs on this board no one has any insight to whether this is normal or not ?? I know I'm not going crazy.....

i have something of the same problem. at night if i have my headlights on and come to a stop it goes slightly below the 14 mark but then recovers. it could just be a lazy regulator. and theres not much you can do about it other then changing it out. If i have the headlights on and the heat and the rear defroster and i'm sitting at a light it will be about 12-13 but as soon as i turn the rear defrost off or take off it recovers and is back up the the right voltage. you could always take it to a shop and have the alt tested to make sure its putting out the right amount of current and voltage.

Cmon now with all of the GM techs on this board no one has any insight to whether this is normal or not ?? I know I'm not going crazy.....

Check to make sure that the reman Alt is the same amperage rating as the Delco. You might be putting an undersized Alt in place of a higher rated one. :iceslolan

pulley size?? a larger pulley on rebuild will cause the shaft to turn slower a smaller pulley would have the same effect as raising the idle speed or accelerating. recheck all electrical connections

OK guys I'll double check all of that......thanks !

Sounds like you might want to check the current draw on the alternator under the conditions when this happens. You'll have to hook an ammeter in series out of the alternator to do this, but if there's a short somewhere that's drawing a lot of current, better to find it sooner than later.

OK so the new rebuilt alternator (not a Bosch this time) is 100 AMPS....all of the ones I've replaced so far have had the same sized pulley as well....does anyone know what AMP alternator comes stock on a 2000 4.3 Blazer LT ?

Albert this happens anytime day or night, not everytime but mostly when I come to a stop and idle.....usually I only have the radio on...on occasion I did have the heated seat on but turned it and the radio off and no improvement.

hmmm ... assuming battery and alt still good, prob only when headlights on ? ... excessive current moving thru light circuits ... proper power draw lights ? ground wires and connections clean and tight ? bulb sockets in good condition ? i'd likely remove 1 bulb at a time to see if voltmeter improves, to locate actual light circuit prob if that cause. is headlight switch getting hot ? (short) also, isn't there a headlight power solenoid or something ?

do you have trailer hitch wiring?? wonder if you have asalt build up between lighting circuit and ground!!!! only when light s are on ??

Mine dips to about 12V when my rear defroster is on, it's just a heavy load when idle.

Mine dips to about 12V when my rear defroster is on, it's just a heavy load when idle.

Something is wrong there.

Shouldn't dip like that. The engine rpms should raise a bit to counteract the load.

At night, headlights on (bright), defroster on, radio loud, amp and subs on, foot on the brakes (brake lights) mine dips just barely below 14....bounces slightly when using turn signal.

i'll have to look into it then. it came from a gunk yard for $30 and i think it might be from a 4.3 car motor or older blazer. it might not have the same amp pull.

A normal reading for a healthy alternator should be 14.0 - 14.5 volts at idle (with a good battery). If you have a gauge in your truck, it will tell you. If it indicates less than 14 volts, have your battery, alternator, or both checked and replaced accordingly. In most cases, if you have been driving on a bad alternator your battery may need to be replaced. I paid approximately $300 for both (a lifetime warranty came with the alternator). Hope this helps!

OK I'm opening this up again....I am on my second Bosch reman alternator and still having the same problem. It seems as though it's because these motors idle soo lom (600 RPM) If you lightly tap the accelerator the volts go normal. I am going to try a different brand of alternator now before I bite the bullet and pay for a GM alternator. Lucky for me to have a friend at NAPA who will take these back....

NOTE this NEVER happened with the original AC DELCO alternator....

Have you had your voltage regulator checked? Funny as it sounds, your truck is operating normal. From your description, something is pulling more current (intermittenly). Do you have any aftermarket accessories installed (stereo sys., fog lights, etc), anything running that is NOT factory installed? Depending on the type of equipment, an upgrade to your charging system may be neccesary to compensate for the drain. Or, something electrical installed on your truck COULD be going bad. This could be an early warning sign. Just a thought.

I would just go buy a battery. i mean thats the onlything you havent done yet. yea it was tested, but by autozone, just buy it and throw it in, atleast, if thats not it, your narrow it down

I am having the same problem but with a new twist. My truck wouldn't start the other day and when I jumped it, it read normal. I took it to Auto Zone and they tested it saying it was putting out 3.8 amps. Being the only vehicle I had and needing to go to work I carried my trusty Jump Pack with me just in case. I had to jump it 3 times that night and the next morning, it started fine and is now reading 16 on the dash gauge. Is the alt bad or did I just have an electrical gremlin f'in with me.