Chevy & GMC Vans

When the engine isn't running, the battery gauge seems to stay about where it should. As soon as I start it, the gauge jumps up to just below the yellow mark. Sometimes it goes up into the yellow. If I press the brakes, it typically fluctuates up and down somewhat. Is this something I should worry about?

Possibly.
If the regulator is fried, you could overcharge the battery which could be very dangerous. Hopefully, your gauge is simply not showing the true voltage. To be sure, hook up a battery tester to it (it should also be able to diagnose your charging system), or even a regular voltmeter.
Voltage should be:
14V while running
12V when off.

14.2-14.8volt idling
13 to 13.8volt off

Hook up a volt meter to it.

JaxSPL wrote:13 to 13.8volt off

This is incorrect.
A 6-cell lead acid battery will be no more than 12.8 V at full charge in an open circuit.

My 99 shows 18 volts on the gauge when the engine is running. I hooked a multi meter to the battery on several different times and it always shows normal at the battery. I was thinking of opening the cluster and pulling the needle off and moving it back so it Shows the proper voltage.

JaxSPL wrote:14.2-14.8volt idling
13 to 13.8volt off

The idling voltage is correct but check the voltage with the engine off overnight.

I bet I can get a volt-meter from O'Reilly's. I think they loan those out, if I'm not mistaken.

I highly recommend spending the $10. These things are indispensable for any kind of electrical work.
http://www.princessauto.com/workshop/ha ... multimeter

Spend a little more and get a digital unit they are way better to read.
They are about 15-20 bucks at radio shack.

Kidhauler is right. The digital ones are better because you can read values with more precision. I provided the cheapest example I could find because I understand the OP is on a tight budget. But an extra $5 for the digital is good value. The current measuring function in my multimeter is good up to 20A which is good for measuring loads.

The other thing you can do is take the alt out and take it to Canadian tire, or pepboys in the states and have it tested. I know Canadian tire up here will check it for free on their machine to test the output. That was how I found I burnt a wire and my alt was still good a few years back.

timelessbeing wrote: This is incorrect.
A 6-cell lead acid battery will be no more than 12.8 V at full charge in an open circuit.

Incorrect,
JaxSPL is correct

reaper wrote:

timelessbeing wrote: This is incorrect.
A 6-cell lead acid battery will be no more than 12.8 V at full charge in an open circuit.

Incorrect,
JaxSPL is correct

I think you are mistaken.
With the alternator going, yes the voltage has to be higher to charge the battery. But a battery on its own will never be at 13V. Not that I've seen in my lifetime.

Here are just a few web pages that support this:
http://en.wikipedia.org/wiki/Lead-acid_ ... mon_usages
http://www.scubaengineer.com/documents/ ... graphs.pdf (page 3)
http://www.pbase.com/mainecruising/batt ... _of_charge
http://www.batterystuff.com/tutorial_battery.html#6
http://www.solarnavigator.net/battery_charging.htm
http://wiki.xtronics.com/index.php/Seal ... plications

Are they all wrong?

I am going by standing voltage IN the vehicle.
12.7 is standard thru QC with manufacturers
although there has been 13+ sent thru
& landed at retailers.

So what you're saying is, it's an exception rather than a rule.

I'm probably buying a new battery this week. I'll start taking bets on what people think the voltage will be.