Still need help- Voltage / Batt / Alt question

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Aug 26, 2006
High Country CO 2006-2014 -> SW Washington 2014
I've suspected I have electrical issue somewhere for a while. My voltage gauge/needle has not been as high as I recall. I purchased a new interstate battery in October 2015 and the truck did not see much action until October 2017 where I've used it seasonally. If it sits a month, I have to put a charger on it for several hours to get a crank. Once it's starts up, everything works, which makes me think I have a battery issue vs. alternator, but I'm not certain. I read a half dozen posts on the topic this morning, and did some testing:

Cold battery after having been on the charger for 12 hours 12.04 V
80 barely starts
Low idle, the red volt light comes on, and the Volts climb to 13V at the battery
High idle (1600) or so, volts climb to 14.1V at the battery
Turn on AC at 1600, volts fall to 13.2V at the battery
Rev up to 2500, volts climb to 14.2V at the battery
Turn her off, volts fall to 11.9V at the battery

Does this sound like a solid battery problem vs alternator/link/etc?

Called Oreilly's and they have the Optima Red ( on hand. If it's the battery, does it make sense to spend up for the Optima AGM vs. another Interstate 27F at Costco for $90?

I do not know where to test the alternator apart from the battery terminals.

12v on cold battery after being on charger sounds like a connection problem. Have you made sure the battery post is clean and the terminal is tight. It might also be a battery problem so you can stop at an auto parts store and have them test the load on battery. A bad one can show full charge but the min you put a draw on it, it will fall.

All that and you also could have a alternator problem. With a good battery, clean tight connector, and good alt you should be at over 14 at idle.
Then i would start with a draw down test on the battery. Orielly can do this. If you battery was good and at full charge it should start the truck even with no alt power. I am guessing that you may have a bad alt and it has been charging your battery correctly but with the info you posted I would start with battery. If you didn't have the problem with the cold start in the first line of your test then i would say it was your alt but that makes me think battery and possibly alt.
No, INMH. Look ups optima in search. Not what it used to be. Sams carries Duracell agm. Sears used to have a plat agm but i think thats gone also.
12.5V or lower on a battery, particularly after charging for 12 hours means the battery is junk. Have O'Reilly test it. I have an O'Reilly battery in mine and it has worked great for a few years so far.

34RPLT 710 CCA It's about $190 right now but has rebates it says.
Interstate batteries rock, it sounds like you might have a parasitic draw. Get a test light and head the battery, disconnect a cable and clip the test light to the terminal, when you touch the clamp on the battery cable with the test light it will light up if you have a parasitic draw. If you do have a draw then you can proceed to check separate circuits in the fuse block to determine what circuit the parasitic draw is on.
Here is a quick video.
I have an interstate battery store close to me, they will test their batteries for free and offer a warranty if they fail before 5 years also
Test light found, I also have a Klein MM300. Test light results:
Disconnected the red, clamped the light to the post, and touched the light probe to the disconnected cable, it pulsed medium bright on/off. Touched the cable to the post, the light went dead and did not turn on right away, after 10s the flickering returned. Then I started pulling the fuses to the left of the steering wheel, the light flickering remained constant. Then I pulled the fuses in the box next to the battery, when I pulled the "dome" fuse the light quit flickering. I placed the fuse back in the slot and the light flickered. When I open the door, the light goes from flicker to very bright.

I found this post: Dome Light Circuit Parasitic Drain

I attached my MM300 (red) to the red battery post and then black MM300 lead to the free red battery cable, but I don't know what setting is meaningful to look at the mA draw?

I am not much of an electrical guy but it sounds like you have successfully confirmed a parasitic draw and narrowed your parasitic draw down to the dome light circuit. I believe from this point your next step is to check all the wiring, switches, and bulbs on the dome light circuit for a short or something else causing the draw. Sorry I can't be of more help.
Go with the Costco 27F. It is the OEM spec size, made by Interstate and you can't beat the Costco warranty.

Repeated draining of a battery will shorten it's life.

One cause of the drain has been found to be the light ring around the ignition key. There is a small device located in the passenger side kick panel that controls the dimming and timed duration of of the light. Even though the light is off the circuit will drain the battery. Unplugging the device solves that problem. A search should find pictures and more info.
Thanks for all the replies. I've received so much help over the years from members of this forum, I wish I had mechanical knowledge to return. I'm picking up a new battery today and I'll check out that dimming control circuit next weekend. Thanks again.

These two videos may be helpful for someone with the same questions:
80 Series dimmer draw-

Looking for a parasitic draw-
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Update. I replaced the battery with a new costco/interstate. Now battery reads 12.6v when cold. The voltage drops to 12.25 volts when idle, and the dash voltage light comes ( 600-800rpm- Idle wanders +/-100) and will go off when I am around 1500RPM. The output at the battery and the alternator is 14.5 and 14.8v at 1500rmp, and drops to 12.25 back at idle. Under load AC, Lights, ect, I'm at 14.1 at 1500rmp at the battery. When I turn the car off the battery comes back up to 12.6v. I've checked all the connections at the terminal and followed this video with the multimeter tests:

The test where I have the red lead on the battery and the black lead on the B post of the alt reads .37 v which is higher than the video suggests, but I don't know what that means? All the other alternator tests were within the normal range, including testing the ground wire connections. Is this a sign of a failing diode? Failing voltage regulator? Bad connection somewhere I did not check?

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I had very similar symptoms on mine, so I swapped the alternator, only to find the problem did not disappear until the starter died and I swapped the starter.

It does not correlate to me unless I was getting enough contact issues that it was draining the battery.

But I would sit at a stop light and watch my voltage (at idle) start at 13.5V, then continuously drop until it hit about 11.0V, by then the light changed and I was on my way. At 2200 RPM, I was at 14.3.

I had the battery tested and it tested bad, so I got a free replacement because the battery was only 2 months old.

So, I figured it was fair warning the alternator was going out and it was original at 275K miles.

I changed the alternator and 2 weeks later, the starter quit and just gave me clicks. No amount of hammering on the starter made it work, and fortunately, it was still in my driveway.

I swapped in a new starter, and all my electrical issues went away.

I had clean and new battery terminals before ANY of this took place, so it was NOT an issue of dirty terminals that finally got cleaned up.

So, check grounds, main cables as well?

I'm not sure what the readings mean. I don't chase electrons for a living.
I’ve had nothing but good luck with optima batteries before and after production moved to Mexico in 2009. I recently replaced the red top oprimas in my dodge Cummins that were 8.5 years old and still cranking albeit rather slow.

If a fully charged battery shows less than 12.4v it’s time to think about a replacement.
So, the alternator crapped out today, I need to order part# 27060-66070-84 , who is the current IH8MUD preferred vendor?

Baldilocks, I'm in N Clark County too, you doing your own mechanic work? Have a reference for a reputable shop?

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