It's been very cold lately; it sucks. I was taking the kids to school this morning and I started the '01 blazer and checked the gages; all was good. Then I turned on the rear defrost and started driving. I had the blower on #3 and I came to a red light. When I stopped, the voltage meter dropped to near nine volts. I turned off the rear defrost and it came back up to near 14 volts. My '99 blazer doesn't do this. Is this my alternator or battery going bad? Or something else?

 

Did you also have you headlights on? If you did, then when you hit your brakes at the red light your brake lights would have come on too. That would have been quite a power draw. It might have gone back up if you had waited a little, or it might be a weak battery, or alternator. Actually the battery is mostly just used to start the car and sort of a filter, the alt. supplies most of the power.

 

Yeah it's probably your alternator I would say. My '99 has a sucky rebuilt alternator on it and whenever I turn on the rear defrost, the voltmeter drops to about 13 and stays there. Never buy reman alternators!

 

I have the same problem on my 2000 blazer. But, I had the heat on #2, I never go any higher than #2 on my heater. Then, I turned on my Rear defroster and my voltage dropped to about 9 volts. I have a red top optima battery, about 2 years old. But, I have the stock alternator. Could my alternator be bad?? I did not have the lights on. I used to run a huge car audio system--I had a 1600w Kenwood amp powering (3) JL Audio 12w6v1's and a US AMPS 425 4 channel powering the doors and dash. I used 4 AWG wiring and I had the red top when this was all hooked up. You think I might have drained my alternator when I had all this car audio stuff?? I since parted with the system. It sounded nice I have to admit, But needed the space for my fishing gear and what not. How do you test an Alternator?? I have a battery place that does it for free just a mile away. Maybe I should ask them to check it. thanks.

 

I don't know much about all the stereo stuff. Have no idea how much power it would take...If you can get it checked for free and you can trust them to tell you the truth that would be the best way..But you can do a pretty good test yourself...Start it up, turn your headlights on, rev it up a little and have someone take the neg. battery cable off. It should keep running, if not it is probably bad, or weak..

Turn it off before putting the cable back on.

 

Your alternator could be bad, but you should check your belt before condemning the alt. I've had the same issue and all it turned out to be was a stretched belt that would let the alt. slip under load.