'99 B3000 102k mi.
Truck died today while driving. "Battery/Charge" idiot light NEVER came on? Got it home by jumper-cable jump starting it every 5 miles, in between it dying. (didn't have a tow vehicle, only "battery' vehicle following me)

Truck not running: I checked for voltage on alternator post (+) with big cable that leads to battery & I have 10v., because of the obviously low battery. (Have a 12v. charger on it)
Truck running: I started truck & voltage at big post (+) on alternator lowered because of ignition drain & did NOT increase to 13.9v-14.2 volts .
So my alternator is bad.
I've ordered a new alternator & waiting for it to come in.

When ever I've had alternators die on me, the "Battery'' or "Charging" light always came "ON" to let me know the alternator or charging system had died. Not this time!
Why did my "battery/charge" idiot light on instrument cluster NEVER come on?

Thanks in advance,
jonny

 

Does/did the battery light come on with Key ON, it should, if not then Fuse #15 in Cab fuse box may be blown, and that also turns alternate OFF, even when engine is running

Testing the B+ terminal on back of alternator is good, yes, should read battery voltage all the time key off or on doesn't matter

But you also need to unplug the 3 wire connector on alternator and test those wires
No test for White wire
Yellow wire is same as B+, should show battery volts all the time, if not then blown ALT SYS fuse in engine fuse box
Light green wire should show 0 volts
Turn key on
Light green wire should now show Battery volts<<<<<this is the ON/OFF switch for alternator, also the Battery light wire, if no volts then blow fuse in cab fuse box, and no battery light either



FYI
An alternator is similar to an electric motor in many ways
If you were to leave it "powered on" with engine off it would try to turn the engine using the fan belt, it can't of course but it would drain the battery trying, lol
So it needs an ON/OFF switch, thats the battery light circuit, it also provides Startup Voltage for the alternator

A 12volt light bulb needs 12volts AND a ground(0 volts)
If you hook up two 12volt wires to a bulb what happens..................nothing because no voltage is FLOWING thru the filament so it doesn't heat up(light up)

Battery light
With key on battery light gets 12volts from Fuse #15(in 1999 Ranger)
Its other wire is the light green wire on the alternator
When engine is off alternator is producing 0 volts.....................so battery light has 12v and 0v which makes filament heat up and light is on
When engine is started then alternator is at 14volts, and fuse #15 now has same 14volts, so...............battery light has same voltage on both wires, no heat and no light

An alternator has to have startup voltage to start generating voltage, this 12v from fuse #15 passes thru the battery light bulb to the voltage regulator on light green wire, which was at 0volts, its still at 0volts but HAS 12volts to use, that's the above test, light green wire has 12v key on
So when alternator/engine starts to spin this 12v is used to "prime the pump" rotor is sent 12v so it can generate an electrical/magnetic field that will start to generate voltage
Once voltage reaches battery voltage, or higher, Battery light goes out because it has equal voltage on both wires
If alternator voltage drops BELOW battery voltage then battery light will start to flicker or glow, the lower the alternator voltage drops the brighter the battery light gets

In your case my guess is that fuse #15 was blown, so one wire to battery light is broken(blown fuse)

 

That's exactly what I forgot to check before I left the truck today... "does the idiot light illuminate with key ON?" (17 degrees outside working on truck is no fun)
Yes, Fuse #15 :7.5amp "Alternator Indicator Lamp"
Will check first thing tomorrow AM along with the 3- wire connector you suggest. I did check all fuses in engine bay before I left.
That's the direction my mind has been going & everything you state,is crystal clear now & I know, or thought I "knew"?
I remember working on German BMW cars where the "Idiot" light bulb completes the Charge Circuit & if that bulb filament in the instrument cluster goes bad... the charging system won't work.So your ''#15 fuse'', albeit a slightly different wiring scenario, makes perfect sense!
Thank you so much.... !

 

In the Fords there is a by-pass resistor on the battery light, its a higher resistance than the bulb so bulb is the power pathway, but if bulb burns out the 12volt would still go to alternator thru resistor, so it could start up and power all the systems and keep battery charged

 

My alternator went out several years ago and the battery light didn't come on until the engine was starting to run crappy. The battery light seems to be pretty useless if it does not warn of a failing alternator. Fortunately, I always keep one of those small booster jump starters in my Ranger. I hooked it up to the battery cables. It started the engine and got me home.

 

Yes, depends on how an alternator fails
If it stops working, 0 volts then battery light will come on
But if it shorts inside it will have Battery voltage on the Light Green wire so battery light will have equal voltage on both terminals and not come on

BUT.............you should also have a voltage gauge, and it would show lower than "normal" when running on battery voltage, 12volts, vs 14volts

If its a concern you can put a digital voltage gauge in the cigar lighter spot or AUX power port, so can easily see 12 or 14volts
But these ports often have 24/7 power, and while these gauges don't draw much power it ain't 0, so heads up leaving them on all the time

 

Key ON = Bat/Charge warning light comes ON.
Fuse # 15 7.5amp. checks GOOD "Alternator Indicator Lamp/ Air bag Lamp Indicator Lamp/Instr. Cluster Fuse"
Green wire at alternator 3-wire connector has 0 volt with key OFF & same as Bat. (12.9v.) with key ON (fully charged battery over night)
Yellow wire is same as B+ (12.9v)
Fuse #14 30amp. in engine bay checks GOOD. ''Alternator Voltage Regulator Fuse''

START truck.... Bat./Charge warning light goes OFF. Truck is NOT charging. Reads 12v., as ignition is pulling .9v.
I RAPPED on alternator with a hammer 3 times. The voltage tester meter jumped to 14.2v. & stayed there for 4 seconds... then dropped back to 12v.
I did this twice more & the 3rd time it jumped to 15 volt (Danger Zone) for 3 seconds, then went back to 12v.
So, I guess I'm waiting for the new alternator to arrive.

Engine OFF: 12.9v.

 

New alternator is bad, period

If B+ shows battery volts
If yellow wire shows battery volts
And if Green wire shows battery volts, key on
Then vehicles part of charging system is 100%, simple as that

So if "new" alternator does not show 13.5volts to 14.8volts after start up then its BAD, for sure

 

Well I'll be dipped. I'm not the only one!
My warning light NEVER came on, at all. I noticed the Tach go to 0, then the Speedo would go to 0 & engine would die a minute later. This I noticed all after the fact, when trying to get it home.
Not until I got it home & was able to put a tester on it, was I able to diagnose the NOT charging issue.
I used to carry a booster pack years ago. But I haven't had a vehicle die on me in at least 12 years.

 

Ron D,
I want to THANK YOU for your trouble shooting & ability to convey it so literately, literally & logically.

I did search previously through all the threads on this forum, but found nothing about the BAT/CHARGE Warning Light "NOT coming ON when alternator fails".
I just ordered one of those cigarette lighter voltage testers, too.

Edit:I'm coming to the conclusion when these trucks hit 100k mi. "things start to go".
At 100k mi.:
I had a SHORT in my speedo sensor wire running to pumpkin (ran new wire)
New Coil pack
Threw the serpentine belt & had to replace tensioner pulley
I just installed a new radiator 48 hours ago.
Now a new Alternator

Also, #5 spark plug insulator CRACKS every 6K mi. I was told "This is a common problem & to use Motorcraft plugs only."? <- That's just silly!

 

A bad battery can take a good alternator out
A bad alternator can take a good battery out

Batteries are easy to test for age
After sitting over night, "at rest" battery voltage should read
12.8v, new battery
12.5v, 3 year old battery
12.3v, 5/6 year old battery and time to shop for battery sales

12.2v or less is a done battery change it now or it can damage alternator

Start engine
Battery volts should now read above 14volts
Let engine idle for 5 min
Battery volts should now be 13.5-13.9volts, under 14volts
Turn on head lights and Blower fan to HIGH
Should still read above 13.5v
If its lower then you have 1 bad Field in the alternator, so can only get 2/3rds of the AMP rating of that alternator at idle, which is OK but I would shop for alternator sales
Dimming head lights at idle is the same, 1 field has failed in alternator...................unless you have added a bunch of driving lights and/or 2,000watt sound system, lol


Failing battery, lower than 12.2volts, causes alternator to run at full output voltage/amps constantly which can cause it to fail early
Alternator that outputs above 14volts all the time will overcharge battery and cause cells inside to short out