Alternator seems to be overcharging

[JTalley]

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?

[MountainManJoe]

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.

[JaxSPL]

14.2-14.8volt idling
13 to 13.8volt off

Hook up a volt meter to it.

[MountainManJoe]

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.

[Kidhauler]

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.

[LiftedAWDAstro]

14.2-14.8volt idling
13 to 13.8volt off

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

[JTalley]

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

[MountainManJoe]

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

[Kidhauler]

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

[MountainManJoe]

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.

[Kidhauler]

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.

[reaper]

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

[MountainManJoe]

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?

[reaper]

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.

[MountainManJoe]

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.