Volt Gauge Fluctuating

I recently had some issues getting my 2000 Blazer LS started. It would act as though the battery was dying. But if I let it set a few moments, it would start right up. Then a couple days later, I needed a jump start to get it going. I took it to my local Auto Zone to have the alternator tested because the battery was only 3 years old. (It was a 5 year battery.) They determined that it was the battery. So I went to Walmart and bought a 700 CCA battery. And today the gauge is fluctuating again… Considering it is cold and snowing here, heat, wipers, and rear defrost tend to be essential. But if I try to run all of them, the gauge dumps. And causes everything to bog down. I do not have any aftermarket electronics installed in the truck. And until last Thursday, this was not an issue. Does anyone have any ideas or suggestions? Thanks for your help.

Low engine speed, lights and blower motor on full, rear defrost on, the alternator cannot keep up. Solutions include driving in a lower gear below 35 MPH, letting the engine idle with everything off for 5 minutes at the end of each trip or getting a float charger (1.5 to 2 amps should work fine) and hooking up the battery to it overnight each night.

Sounds to me, like the alternator is a goner

Sounds like the alternator to me as well. But, before you condemn it, run a voltage drop test between the alternator and battery. GM typically runs a fusible link wire between them for overload protection and they can go bad, mimicking a bad alternator. A voltage drop of .25 volts or less is Ok and the alternator is bad. A voltage drop of .5 volts or more means the wire is bad. I had this problem recently with my '88 Supra and a friend’s '03 Tahoe. With the Tahoe, we were able to get a replacement wire at the dealer parts counter.

Great answer @BustedKnuckles‌. People forget about the fusible links. You can get aftermarket battery cables. http://m.autopartswarehouse.com/shop_parts/battery_cable/chevy/blazer.html

I have had alternators test fine but have some problems, especially with the voltage regulator. I think just swap it out.

I’m pretty sure this model uses a junction terminal on the hot side of the battery. All power except the starter motor windings go through that terminal and if mating surfaces are corroded or scaled over all kinds of oddball electrical things can happen. It can also vary due to heat generated in that terminal due to current draw.

The terminal should be on the car chassis near the battery positive terminal and located under a small plastic cover. Just follow the cable from the hot side of the battery, remove the nut and cable ends, and clean all of the mating surfaces.

Do yourself a favor and take ok’s advice before condemning the alternator. Check and clean all of your battery connections and also inspect closely all those little ground wires that run from the wiring harness to ground bosses on the frame, fenders, etc., including the body to engine ground strap and where the positive battery cable meets the starter, if this is used as a distribution point on your specific model of vehicle.

agree. it could be as simple as a bad connection or corroded wire.

Thanks everyone. I will try the connections first. I did find a tiny bit of corrosion when I disconnected the positive terminal to swap the battery. But I have seen far worse without issue. So I didn’t think much of it. I cleaned that off and reconnected the terminal. But I will have to trace some of the wires to be sure the other connections aren’t rusty since she is 14 years old. And I live in the northeast…

My bet is on a weak belt tensioner or bad corroded connections at the battery posts.


Get two of those felt battery terminal rings, put oil on them then put one on each of your nice clean battery terminals and clamp the cables on tight tight. Those pads stop acid that seep though the tiny gap where the terminals protrude from battery. Your terminals will stay clean and wet weather will not be an issue anymore. Then recheck the charging system. Dirty terminals will greatly hinder charging. Alternator is not designed to charge up a battery but only top it off while headlights etc are on. After a jump start (which you should never need or do) you MUST put that battery on a charger for 12 - 24 hours. A battery that has gone dead twice for any reason is no good. Replace it with the right one, not one you think will work. 700 CCA is too low. Making the alternator charge a battery will kill that alternator quick. It sounds like you have slacked on maintenance of battery and run all kinds of blowers heaters etc while the alternator overdrives unable to charge through poorly connected dirty battery terminals! All the while the battery gets drained and less charged and is finally too weak, needs jumped and then the alternator is tortured again some more! Is the belt good or cracking up? Every pulley spinning freely? Do not run anything off your car battery without the engine running and make sure your alternator is of correct amperage.


How would you use those “felt battery terminal rings” on a side post battery?

If the voltage gauge is fluctuating, but that’s the only symptom, besides checking the battery connections, I wouldn’t do anything. The voltage always fluctuates, goes up and down depending on how much current is being used by the accessories, and the engine rpm, the charge state of the battery, the ambient temperate, all affect the voltage. Fluctuation is normal, it’s like the stock market.

It depends on the amount of fluctuation of course. If the fluctuation takes the voltage to below 10 volts, except during starting, that’s usually an indication of one of the problems posted above. Or the gauge itself is on the fritz.

@cravinbob, I respectfully have to disagree with a few things you mentioned. First. The felt rings will cause the posts to only have contact through the bolt going into the battery terminal because it is a side post battery. Those rings are meant for top post batteries. In my case it would be wiser to use the anticorrosion spray. Secondly. The proper battery suggested by Chevrolet is anywhere from a 540 CCA to a 700 CCA for my year, make, and model. (It is an S10, not a full size Blazer.) Lastly. Your assumptions are all incorrect. It turned out that the issue was merely a bit of corrosion on each of the terminals. I was unable to see it in the poor lighting of the parking lot I changed the battery in. Since cleaning those connections, there has not been an issue. I am fully aware of the amount of amperage I am pulling when running the accessories in my vehicle. If you read my original post, I was only perplexed by the fluctuation in the gauge. And I decided to ask for suggestions in case cleaning the cables and posts did not work. I have indeed had the alternated tested. (Both in and out of the truck.) It is working perfectly. (That is why I changed the battery.)

If you have voltage fluctuations while the engine is running it is an indicator of a problem with the charging circuit, or a major short. The battery condition is not related to the voltage guage unless it is really shot, and we can see this is not the case. Either you have something kicking in and causing a major draw, or a bad voltage regulator. A new alternator would be the first place I throw dollars at, unless there is a separate voltage regulator.

Thanks @Barkydog. I have had the alternator tested in and out of the truck. Both times testing normal. The fluctuation seems random. And generally only lasts for a couple minutes. At this point, I plan to just live with it. Seldom do I run mutiple accessories. And again, it’s not every time. And for a 15 year oldvehicle with roughly 200k miles on it… Not sure it is worth the chase.

Without personally testing the truck it is impossible to know what is wrong with it but I am quite sure that running all the equipment that you mentioned will overload the alternator at idle. But yes, the 700cca battery is more than adequate. And while those red/green battery washers won’t correct a problem they can prevent corrosion and if the inner, smaller diameter ring is pulled out the outer ring will fit the side post GM battery. Have you operated the truck at cruising speed without the rear defroster and found there is no problem?

@“Rod Knox” , I notice the issue most often in town. And even then it is random. (Happens one time. Not the next.) And generally momentary. If I press the accelerator it causes the gauge to go back to normal.

Long time but must say thanks for the slam on me. I am 63 next month and have done my own mechanics since forever. Half-assed info gets bad answers but its the info giver not the helper so bite a big one.