Alternator or Battery?

[BG1988]

Wondering if anyone has insights here on this problem.....2004 Yukon, start it up, battery light on and gauge reads about 11 volts. Rev engine and it goes up to 14.5v or so and stays there even after the engine comes back down to idle. I can turn the engine off and on again with the gauge going right up to 14.5v at idle. BUT, let it sit overnight and when I fire it up in the morning the battery light comes on and the problem repeats. I put a new battery in it since the Advance Auto guy's tester said the battery was bad, as well as cleaned and tightened the connections. But it didn't fix the problem. Before I put in a new alternator, is there a chance it's something else like a bad belt tensioner or a bad ground somewhere that I should check?

check with a cheap mutimeter they have it at harbor freight for like 10$ also try unpluging the battery current sensor (should be on the ground wire if equipped)

tow mode also will trigger a higher voltage

 

[SnowDrifter]

"I put a new battery in it since the Advance Auto guy's tester said the battery was bad". OP replaced the battery.

New doesn't mean good, for what it's worth.
but testing should rule in, or out, regardless.
Just.... Something to keep in mind when chasing diags

 

[9C1C]

Well, new battery, new idler pulley, and new alternator and I'm back in business. The alternator turned out to be the problem after all.

 

[MassHoe04]

Good for another 20 years or 200k miles!

 

[rockola1971]

simple way to test is to put a volt meter on the battery terminals right when you start it up, by default the alternator should be putting out at least 13.4v for the first minute or so, if you do not have 13.4v right after you start it then it's possible the alternator is no good. The reason the system does this is to make up for the power that it just used to start the engine.
There is always a possibility it could be something else but that's a pretty typical test

Another test while you are measuring the output of the alternator is put your voltmeter on AC voltage and on this alternator, a shorted diode would have yielded a substantial AC output which you should never see going to a battery. A little leakage well under 200mv is acceptable.

 

[LaxCoachT]

Shorted diodes in the alternator. They are what is killing your battery overnight.
Your rotor (spinning center of alternator connected to pulley and your main serpentine belt) spins inside a stator which is the coils of wire that are stationary inside your alternator. An alternator is wired so there are actual 3 voltage/current producing systems in the 1 alternator. This is the same principle as 3 phase industrial power. An alternator internally puts out 3 phases of AC voltage that are 120 degrees apart when looking at their phase. Well our vehicles need 12vdc+ so that is where the diodes (also called rectifier or full wave bridge) come into play. In an alternator electrical circuit the diodes convert AC to DC and since there are 3 phases to be converted its convenient because we need some fairly high amp output alternators for vehicles to run everything and charge your battery efficiently for its next cranking the starter cycle. After the AC is converted to DC in the diodes it then moves on to the voltage regulator (havent seen an external voltage regulator on a GM since the 80's). The regulator sends the DC out on that red/pink wire on the rear of the alternator to your battery and maybe even the fuse box too. That 12v+ wire connected on the rear of the alternator is actually connected to a stud which is the output of the voltage regulator. The regulator of course also regulates the DC produced by varying voltage to the stator and/or rotor coils of wire (coils). The regulator actually varies the AC of the 3 phases but if those go down then your available voltage at the diodes goes down so the DC output of the diodes goes down and vice versa. This is a continuously self adjusting cycle.

The reason an alternator can ruin a battery is because a start battery that we use to start our vehicle engines are designed to produce alot of current for a very short period of time. Long enough to crank your engine over and it starts. After that, your engine ignition, headlights, stereo, brake/park/turn signal lights, heater and ac blower, etc are all being powered by your alternator after engine is started, not the battery. If you are running off your battery then you have a problem and its likely the alternator. A bunch of cycles of shorted diodes which electrically are acting like electric heating elements loading your battery once you turn your engine off. Remember the battery is designed to produce alot of current for a short period of time. The diodes are making your battery do the exact opposite. The battery begins to chemically change within during all this funny business and starts to produce electrical blocking chemical compounds (sulfates) which kill a batteries ability to charge up and produce power (high current).

Most auto parts chains like autozone, advance auto parts and the like will pull your alternator in their parking lot free of charge and put it on a dedicated alternator tester and it will spin it up and check your diodes, regulator and output. They will even install the replacement for free. They just want to sell you an alternator if you need one.

**NOTE** when the battery is attached to the battery cables, the red/pink wire on the back of the alternator is hot all the time. You touch that to anything metal under the hood and you will just made your own 12vdc arc welder and the flash will be brilliant and people in the vicinity WILL make fun of you. So disconnect the negative battery cable or the positive if it is more convenient.

Fantastic expanation...this should be a sticky at the top of every automotive electrial forum.

My brand new battery died last night after sitting for 6 hours. I think I'm disconnecting the alternator, looking for parasitic leakeage, then reconnecting it and checking again.

 

[OR VietVet]

Just attach the DVOM between the negative cable end and the battery and see the parasitic draw and unplug the alternator and likely, that draw goes away, if in fact the alternator is the problem. I think.....?

But you need to know that some minimal draw is expected for the Keep Alive Memory. I remembered, when I first started, that a 50 ma, or less, draw was acceptable but I think now days, up to 100 ma draw is allowed on some vehicles. Been long enough out of the shops that I am not 100% sure.

 

[MassHoe04]

Running a little slow on the uptake today... It took a minute for the old brain cells to figure out what DVOM was. After the smoke cleared, I finally got it!! LOL!

 

[LaxCoachT]

FYI mine turned out to be the rear defroster. Pulled the fuse and my voltmeter went from 10A (yikes!) to almost zero. Now I need to chase down the relay location...my rig is a 2009. Guess I'll slink off to the correct forum and see if there's anything there.