Hey all. Got a charging system light that flickered on and off last night while driving, and when got up this morning it now stays on after starting.

I tested with a voltmeter, battery checks out. Got antsy and took it to Advanced. They tested the battery, alt, and starter--all good (though the battery has "low reserve power," the voltage is good). Went for second opinion at O'Reilly who had the same conclusion.

What is this low reserve power business? Do I need a new battery? I was this close to buying an alt, but I want to try to figure out what's going on before throwing money at it. Ideas?

 

Get an OBD reader so you can see the actual codes that are causing the light.

 

There’s no check engine lite, though I’ve been meaning to get a scanner for years anyway.

 

You said the battery light was coming on?

You can get a basic Bluetooth scanner for less than $25 and a free app to read the codes.

 

If Battery light flickers at warm idle or lower RPMs then you have a bad Field in the alternator, for sure, there is no way to test an alternator at a parts store, just FYI

If you have a Volt Meter then YOU can test the charging system

Test battery voltage key OFF, should be between 12.3volts and 12.8volts, remember it
Key is still OFF
Put the Meter's black probe on alternators metal case, for all the next tests
Put Red probe on the alternator's "B+" terminal, its the bigger Stud and Nut terminal, should see Battery Volts(see above), exactly, if not a fuse/fusible link is bad

Unplug the 3 wire connector on the back of alternator, should be a yellow, white and green wire
Test the yellow wire, should see Battery Volts exactly, if not then fuse/fusible link is bad

Test green wire, should see 0 volts
Turn key ON(engine off)
Retest green wire, should see battery volts can be a bit lower but above 12.2volts<<< this IS The battery light wire, also the ON/OFF switch for alternator

If all the tests are OK then YOUR trucks charging system wiring is OK, 100% no doubt at all

Plug the 3 wire connector back in
Start the engine
Retest Battery voltage, should see 14.2v to 14.8volt
Under that and alternator may have a bad Field, it has 3 so if one fails alternator still works but not as well, Battery Light will flicker at low RPMs because of low voltage
Bad Field will also cause headlights to dim at idle, no that is not normal for ANY vehicle, lol
Alternators minimum voltage is 13.5volts, maximum 14.8volts

After 5 minutes of engine running test battery voltage, should be between 13.5v and 13.9v at idle, raise RPM to about 2,000 and hold, voltage should go up just a bit but then come down again even with engine at 2,000rpms

Replace alternator if its not working right


Batteries are rated by AMPs they can delivery quickly
A Battery is ONLY USED to start the engine and not used again until a restart
12.8v is a newer battery, 100% amps
12.5v is a 3 year old battery, 75% amps
12.3v is a 5/6 year old battery and has 50% amp capacity, so time to shop for battery sales

This is assuming battery hasn't been drained of course, lol

 

Hey Ron, just want to say thank you for this in depth explanation. I'm gonna go test this out in a moment.

I'm attaching a wiring diagram that you actually shared in a different thread on a similar problem -- I'm a novice with wiring diagrams so just to make sure I'm wrapping my head around this proper: the 3 wire connector is my main suspect right now; that is shown on the diagram as C168, correct?

 

Added numbers in blue and red below. My battery light is not showing this morning, and it cranks just like normal. I should note that I never had dim headlights, int. lights, sluggish power windows etc. I did get some weird numbers after going through your checklist though:

 

Odd to get voltage on the green wire when its unplugged and key is off

As per 1998 diagram, Pull out fuse 15 in cab fuse box(powers green wire) and test green wire again, key off, should be 0v
If its still 4-6volt, turn on the key, fuse still out, see if voltage changes
Just wondering if green wire is getting stray voltage from dash or wiring harness

Just as an FYI
The way the Battery Light works is pretty simple
A 12v light bulb lights up when current is passing thru it, current needs a higher and lower voltage to flow thru the filament of the bulb to heat it up, the bigger the difference in voltage the brighter the bulb
When you turn on the key fuse 15 sends 12v to the Battery Light bulb
The green wire, when hooked to the alternator, is like a ground since alternator is not generating voltage, so 0 volts
Current flows thru the bulb and it lights up, 12v to 0v
(a bulb is just a small wire, filament, so if you give it 12v on one terminal you will see 12v on the other terminal, the green wire if its unplugged)

When you start the engine and alternator starts to make voltage the whole vehicle is now 14.7volts, in this case
So fuse 15 is 14.7v and Battery light has 14.7v on that terminal
Alternator is also 14.7v so green wire is not 0v any more, its 14.7v
If a light bulb has 14.7v on one terminal and 14.7v on the other terminal, No Current can flow thru the filament so its off
If alternator was not working the Battery light stays on, green wire still 0v
If alternator is only partially working, say 10v then battery light would glow or flicker, as some current was passing thru it

If you are wondering why there is a bypass resistor(470ohm) on the Battery Light, that's a fail safe in case the bulb burns out, the alternator/voltage regulator needs that Key ON 12v from the green wire to start generating its own voltage

 

New data:

Without 15 fuse I get 0v in both off and on positions.

I checked that fuse this morning with a light checker and it looked fine, but went ahead and replaced it for testing. Same numbers from earlier show up, 5.5v in the off position, and 1.767v in the on.

#15 is the instrument cluster -- do you have diagram of what all is on that circuit?

 

So the voltage dropped with Key on and fuse 15 unplugged?
5.5v down to 1.7v
That's good, it means ignore it, the 4-6volts, it has no AMPs so just residual voltage left in the circuit in the dash, not a short or anything to worry about

Fuse 15 powers the Battery Light and Airbag light
Airbag module would be my guess for residual voltage seen on green wire with key off

 

Seems so. The battery light is now gone when I'm idling, so . I'm just gonna carry on and stay aware of it in case I see any electrical sluggish-ness in the future. Through all of this I've noticed no performance drop of any kind, just the annoying battery light.

Thank you for all the help. I really appreciate it.

 

I'm going to assume that this is either a 2.3 l or 2.5 l . If that is the case then what I am going to suggest us that you check to see that your timing belt is actually correct and not slipping because I had the same problem with my 2.5 later and once I got the timing belt fixed the problem went away and also took care of all of the flashing lights and Turn signals that would act weird going down the road. And come to find out it's because from the factory they used a timing belt that was not necessarily supposed to go into truck.

 

It’s actually a 3.0L (realized I never stated, ‘98 XLT 3.0L). All the same, if it’s timing stuff, that’s a little out of my league… though I’ve always wanted to dig in and learn. I think I have a timing cover gasket leak anyway so might be an opportunity.

 

Could have been something as simple as a loose/dirty connector at the alternator, unplugging and plugging it back in fixed it.

 

Hope that’s the case. I came so close to just buying an alternator—in fact, I did, then got home and had second thoughts about testing myself and went and returned it. For the moment seems I made the right choice.

 

Ahhh - you might have a weak super fuse or a slipping serpentine belt then.

 

The serpentine belt did look a little aged. Not real bad but couldn’t hurt to change it out. I’ve only had the truck about 9 months and I have no idea when it was put in.