Anyone familiar with an '04 Trailblazer, here's one for your consideration...
My wife just called me on her way to work.
During deceleration (with or without the brakes on) the volts gauge will periodically swing left of center and the lights will dim.
This happens for a period of a couple seconds, and then it swings back to the charge side (normal) again for about 10 seconds, then repeats. The odd thing is that it does not do it at idle, steady cruise, or accelerating. Only decelerating.
My first thought was that the A/C compressor was kicking on, but she said neither the A/C or defroster was on.
No check engine light.
It's fairly cool outside (just above freezing). The only other time she noticed this was the other night, while it was raining.
I haven't driven it yet, but thought I would ask. The collective knowledge here is amazing.

Thanks.

Maybe they snuck in a hybrid engine, and that's the gasoline engine starting up...? j/k... no idea... Probably a loose wire somewhere... check your battery cables

Check the wiring for your taillights. Rain and moisture can creap into the coupler in the rear of the truck. Should be just in front of the trailer hitch. It will cause a "short" short. Had it happen a few times. The dealer couldnt find a problem but after I cleaned and sealed the connector it hasnt happened. Good luck

Well... She called Hardy Chevy. They told her it was "normal". (Ever notice that anything they can't explain is "normal"?)
The Trailblazer Forum http://forums.trailvoy.com/ seem to think it's an air pump.

I'll check the battery cables and all the usual stuff when she gets home.

quote Originally posted by Tommys86: Check the wiring for your taillights. Rain and moisture can creap into the coupler in the rear of the truck. Should be just in front of the trailer hitch. It will cause a "short" short. Had it happen a few times. The dealer couldnt find a problem but after I cleaned and sealed the connector it hasnt happened. Good luck

Good thought. And one that nobody else has mentioned.
If I find the cause I'll post back. Whatever it is.

Thanks, guys!

Just a thought--

Check engine grounding to frame wire.

wait...your wife...looks at the gauges??

mine only pays attention to the fuel (sometimes) and mph!

i wonder, since its decel and its enough to dim the lights, that maybe a wire, maybe off the batt or alt is loose and grounding out/loosing connection?

at a stand still and accel its ok but under decel it moves enough forward to cause the drain?

Alternater belt tight?

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Ol' Paint, 88 Base coupe auto.
Turning white on top, like owner.
Leaks a little, like owner.
Doesn't smoke, unlike owner

quote Originally posted by sostock: wait...your wife...looks at the gauges?? mine only pays attention to the fuel (sometimes) and mph! i wonder, since its decel and its enough to dim the lights, that maybe a wire, maybe off the batt or alt is loose and grounding out/loosing connection? at a stand still and accel its ok but under decel it moves enough forward to cause the drain?

After 9 years of being married to a car geek? She couldn't help but be aware of what her car is doing.
Actually, she's very good about it. Any time her vehicle does something strange, she lets me know.

I still haven't had a chance to look at it. She said that it behaved on her way home from work, tonight.
I'm going to have a look in the morning, when it's cold again. (Seems to be related to cold or wet, so far.)

All good ideas. Thanks!