Battery light on all the time but truck seems fine?

[fiddlermd]

I've had the battery light on for at least a month.. probably more. I did have some weird issues with cold starts but they seemed to have gone away now. In any case, truck starts, battery doesn't die. It runs ok but the low battery light is always on.
What else could it be?
Not sure if it's related, but I noticed recently that even on cold start, my engine temp gauge jumps right to the middle immediately even though obd2 reports something like 97 degrees.. guessing one of those sensors went bad?

 

[fiddlermd]

Anyone? @RonD ?

 

[rubydist]

You need to put a voltage tester on the battery with the engine off and again with it running. The light will be on if the alternator is overcharging also, and from your description that may be happening here.

 

[fiddlermd]

When I previously tried it, i'd get normal readings except at idle it would be a bit low (mid-13's).. So like 12.5 engine off, 13.5-13.7 at idle, and 14.5 above 1k rpm or so.. i can check again tho

 

[RonD]

Unplug the 3 wire connector on Alternator, this is the wiring for the internal voltage regulator
Turn on the key, engine OFF
Battery light should stay off, if not then light green/red wire has a partial short to ground

If Battery light stays off then most likely issue is the voltage regulator in the alternator is bad
From your description of "running voltage" this is most likely the issue

But test the Yellow wire on the 3 wire connector while its unplugged, should read Battery voltage(12.5v)
This is the Voltage Regulator's "feedback" voltage

".....13.5-13.7 at idle, and 14.5 above 1k rpm or so." <<< this denotes a probelm
After start up battery voltage should go above 14volts, but only for 5 minutes or so, to recharge battery quickly after it was drained by starter motor
Then voltage should be under 14volts, 13.5v to 13.9v the rest of the time engine is running
It can go up when you raise RPMs but should drop back down under 14v within a few seconds, that's the voltage regulator working, the point of having a voltage regulator, its there to maintain a steady voltage regardless of RPMs or AMP draw
So at idle if voltage is 13.7v and you turn on Headlights and Blower fan to HIGH the voltage will drop for a few second but then come back up to 13.7volts because voltage regulator has increased AMP output to match Demand
Same, as said, when you raise RPMs, voltage will go up but will drop back down if you maintain the higher RPMs

Voltage regulator sends 7 to 9 volts to the "rotor"(part that spins in the alternator)
This 7 to 9 volts generates 13.5v to 14.9volts in the Fields(case of alternator) which goes out on B+, stud/nut terminal on back of alternator
That's how it generates the extra voltage/AMPs to run vehicle systems and keep battery charged

A car battery likes to be trickle charged at about 1.1volt higher than its at rest voltage
2 volts higher is fine for Fast recharge , but will "cook" the battery long term
Which is why a voltage regulator is needed

If you raise and lower RPMs and voltage goes up and down with it then there is no "voltage regulation", its just sending the rotor a fixed voltage, say 8volts, and never adjusts it to match RPMs or Demand
That could cause Battery Light to come on, not always
And it can "cook" the battery, shorten its life, if voltage stays over 14volts while driving around at 2,000rpms

 

[fiddlermd]

@RonD You're a legend. Thanks for all this info. will check!

 

[fiddlermd]

OK so the light did go off when I unplugged the 3-wire connector. I then did a voltage test on the system which you can see in the video below. Basically, when it's idling and accessories (lights/AC) are on, voltage drops to around 12 - 12.5. With what I'm seeing above, I guess bad regulator indeed?

finally, if it is the regulator, could you share a link to the right part number that won't fail in a month? thanks!

 

[fiddlermd]

btw. lights / AC turn on around 1:30

 

[RonD]

Good work

Yes, bad regulator, could possibly be a bad field but doesn't matter which.............bad alternator

 

[fiddlermd]

i ordered a regulator.. we'll see what happens. if that doesn't do it, i'll swap the alternator. thanks!

 

[fiddlermd]

so some updates on this. I installed the regulator, battery light turned off. I had the battery die twice but then took a long ride and everything has been fine for about 2 months. 2 days ago, i was messing around with FORScan on my laptop and had the key in the 'on' position for a while without engine running so battery drained. I boosted it and ran the engine for a while to charge. Then i had to take a 30 mile ride. On the ride I noticed battery gauge was kinda low. the ride was mostly highway. when I arrived at my destination, i noticed the car was kinda going nuts when i'd go to idle. gauges would bounce all over the place, lights flicker, etc.. almost stalling. i managed to keep it going by giving it some gas here and there. while parked, i turned off the lights/ accessories and had the engine running. probably around 20 mins. Then on the ride back, at a stoplight, car totally died/stalled. couldn't restart so had to boost again (i have one of those lithium battery booster packs - amazing). I made it back home since most of the ride was highway but now i'm in this state.. so is this an alternator issue or a battery issue? or something else entirely?

@RonD ?

 

[RonD]

You need to use a Volt Meter to test voltage at battery terminals with key off and then with engine running
12.3v to 12.8v key off
13.5v to 14.5v engine running



Your current battery will be on its last legs after being drained so many times, more than twice
Car batteries are not made for that type of use
This is NOT your current problem, just a heads up that a new battery will be needed sooner than later
But you must fix the current problem or you will ruin the new battery

 

[fiddlermd]

yeah i've done this test in the past (see video above) and it was not meeting these requirements. I'll test again but I know that with key off, it's been less than 12.
If it is a charging/current problem, would might it be?
I had the battery tested at an advance auto parts before i got the new regulator.. they said it checked out

 

[RonD]

Need the actual voltages to determine the problem

YOU can test a battery with a volt meter, auto parts place can not

 

[fiddlermd]

ok will let you know when i test