I have a 2002 Blazer, the two door version. It's occasionally doing something strange. For most of the time, the voltmeter displays a steady 14 volts. But once in a while for no apparent reason, it will drop to 11-12 for a few brief seconds and then goes right back up to 14. Doesn't happen often, the speed on the clower always drops when that happens, runs perfectly the rest of the time. What could cause this? Is the alternator or battery failing?

 

I have very little insight to what this means: "the speed on the clower always drops when that happens," ... in that, I do not have a "clower" in any of my vehicles and have no experience with it/they/them.

If that was a typo, you could've corrected it - still can AFAIK --- but the rest of your post is decipherable.

Batteries do not play games like you mention --- but the electronics can and might.

Are you still needing information on this (late) response to your long-forgotten post?

Lemmee know.

 

If you mean blower, your ac compressor could be be coming on and dropping the voltage. Same thing happens when I turn on rear defroster. Running the heater on defrost will kick on ac compressor to take out moisture. I believe it kicks on when running in other switch positions also, like when switched to the floor

 

In 2002, GM figured out how to suppress the drops in voltages from accessory loads that would be cyclical or switchable from the humans inside the vehicle.

Unless someone put a biga$$ boom box and amp in it, it should run very well with close to zero changes in perceptible RPM changes for causal load variations; ergo: caused-voltage changes

"Cyclical" accessories include:

1. Power steering "calls" for higher pressure at low RPM (it would be noticeable in slow speed turns and turning the steering while also standing on the brake pedal to twist the wheels into the next direction you want to go --- this puts a lot of strain on the PS pump!)
2. AC Compressor cycling at low RPM (it would be palpable during parking lot maneuverings, but not so much above human walking speed).
3. Shifting into Reverse or 2nd/Intermediate gear from a stopped condition. (the pressure-biassing inside the transmission requires maximum pump pressure for those two gear positions)
4. "Forcing" the steering wheel full-lock on either end of the travel and holding it against the pressure relief valve inside the PS pump ... (this is hard on seals and pressure lines too).
5. "Heavy-Amperage" accessories like sunroof motors and high-draw Halogen auxiliary lights (largely phased out by LEDs now).

Other than those --- I draw a blank on "normal load" from normal vehicle accessories - not enough to actually see the voltage drop as much as you state.

Do this:

Run an AC-Output test on the alternator... using a DVOM in AC-Mode and ground either test lead to the engine somewhere and touch the other end to the B+ output lug on the alternator.

There are some old-school views that anything OVER 0.015 VAC is bad, but the current idea is that 0.025 VAC is OK.

I opt for the first value in that I hate to hit electronics with too much AC Voltage at all!

After this test and what you report --- we can go from there.





PS ---> GM uses a Low Speed 800 Hz square wave generator to swamp the alternator with a false-flag higher "perceived voltage" in the Field - causing the alternator to artificially be induced (gaussed) to produce more output current.

You can hear the fields being clipped if you put your ear close enough to the running alternator when this system is activated.

It is a moderately high pitched squeal -- and even at my age, I can still hear it when it's working.

This is supposed to activate at low RPM - and when the battery load is considered "negatively flowing" - IOW: discharging) --- and will be active until the throttle hits "tip-off" and then the alternator does its own thing again, acting more-or-less, normally.

The GM evaluating device for the when-if-how part of this system - is that funny-looking device on the negative battery cable. It is an inductive sensor for ALL the power flow through the negative cable from-to the battery --- from everywhere.