I have a 99 Ranger and it has been a great little truck. At the dealer last week for an oil change -- said the battery was "marginal". I checked it later - about 12.2 V, was 5 years old, so I replaced it to be safe. Driving it at night a couple of days later I noticed the voltage gauge drop down. Got it home and checked - it was only charging about 11.6, not good. The next morning it was charging 14 V, then later dropped again. I checked the wires, figured it was the alternator which had a lifetime warranty, and got a new one free. It was charging 14. I drove it later, and it did the same thing again after a few miles -- the gauge dropped down, the lights dimmed, and before I got home it even started running rough. I checked the voltage at the battery - under 11V -- so I shut it off. After a while I started it back up -- battery was fine -- and it was charging 14 again. I've checked the fuses, all good, which they should be with the problem coming and going. It does it everytime I drive it now. Charges fine for a while, drops down like shutting off a switch, then it might kick back up. The light only comes on with the ignition on without it running, which is normal I guess. The voltages at the alternator check good according to things I've read, but they all drop to 11V or less when it acts up. I'm assuming the voltage regulator, being new, is OK but it acts like an old-time regulator -- off/on, off/on. I'm also figuring it was probably doing this before the battery was deemed "marginal" and before the new alternator. I might also mention that while doing all this other stuff I replaced the plugs, wires, and coil pack for a rough idle and missing at times -- it's done that for a while. It runs great now but if the charging voltage drops below 11 or less it begins to run a little rough. Sorry for the length of this but I wanted to try to paint a complete picture. If you have any fresh insight let me know. I can give you the voltage numbers.

 

Welcome to the forum

You should have a Battery Light on the dash and it should come on when key is turned on.

Does that happen?

And it should have come on when voltage at alternator dropped below battery voltage.

1999 Ranger has a pretty simple charging system, no "smart charging" yet, lol.

Alternator 101
If you apply voltage to a spinning rotor's windings it will create a magnet field around the rotor.
If you place other windings in a fixed case around the rotor, the spinning rotors magnet fields will create an AC voltage in the case windings, these are called the Fields, alternators have 3 of these.
Each Field will have 2 diodes to change AC volts to DC Volts

Voltage regulator
Because rotor is powered by engine RPMs the voltage coming out of the fields would fluctuate wildly with engine RPMs
So the voltage regulator is used to stabilize this
The voltage regulator supplies the power to the rotor
If voltage regulator gives rotor 8volts at idle then fields will output 13volts, 9volts then 14volts, and so on
As RPMs increase voltage regulator will decrease voltage to rotor
As electrical load increases, i.e. lights and heater fan, voltage regulator will increase rotor voltage.

Car batteries
Called 12volt batteries but are pretty much dead if they actually have 12.0 volts, lol
New batteries should be 12.8 volts
3 years old about 12.5 volts
5 years old 12.3volts
At 12.2 volts they are almost done, under that and they are done

The above are "at rest" voltages, battery needs to sit for a few hours after charging or draining, and then be tested.

Testing a charging system
Key off
Test battery voltage, lets say it is 12.6volts
Back of alternator has the larger wire(s), that is B+ terminal, output of the 3 Fields, test voltage from alternator's metal case to B+ should show Battery Voltage, 12.6v
If not then wire or Mega Fuse is blown
BUT.....test again from Battery negative to B+, could just be a bad alternator ground issue, Alternator needs a GOOD ground

3 wire connector, the voltage regulator
Yellow wire, this is the Reference voltage used by the voltage regulator to monitor system voltage
Right now system voltage should be Battery voltage, 12.6v
Test it, alternator case to Yellow wire, 12.6v, if not then blown ALT Fuse in engine fuse box

Green wire, should be 0 volts with key off
Turn key on
Should now show 12.6v, battery voltage
Green wire is the Battery Light circuit AND the on and OFF switch for the voltage regulator, so very important.
It gets power via ignition switch and Fuse 15(7.5amp) in Cab fuse box

Green wire Powers the rotor so no power here then no output at B+

White jumper wire connects voltage regulator to Fields, as longs as it is not frayed it will be OK.


Voltage regulator monitors battery's charge, just after startup battery AMPs have been drained, so voltage regulator should increase rotor voltage +2 volts, so you should see Battery voltage(12.6) + 2 volts, so 14.6volts, can go as high as 14.9volts but stay below 15volts.
Charging a battery at +2 volts will cause it damage over time, so voltage regulator will lower this +2 volts to +1 volts after a few minutes, so after driving vehicle but before shutting it off, if you test battery voltage it should show battery voltage +1 volt, 13.6volts, this is maintenance charge, keeps battery charged but won't damage it.

Load
have engine idling and test battery voltage, say 13.6v
Turn on the lights and fan to high
Voltage should drop and then come back up to 13.6v
If voltage drops to say 13.2v then increase engine RPMs, if it comes back up to 13.6v then you have 1 damaged Field, so only 2 working fields.
This causes the common "dimming head lights at idle", no it is NOT NORMAL, lol, no auto maker specs in an alternator that can not power all the electrics at idle.

Intermittent charging can be a few things, test wire voltages as described and move the wiring harness around while testing, you could have a damaged wire.
Bad voltage regulator or alternator.
new used to mean "tested and it works"
new now means you test it and we will replace it if bad
Labor to test each part is expensive, warranties put labor cost on end users so FREE labor, you do the math, lol.

"Its a new part" means NEVER tested so grain of salt

New Batteries are the same, "you test them we warranty them"

 

Thank you for the reply, RonD! Your explanation fills in a couple of gaps for me. Sadly it is snowing, windy, quite cold, and almost dark here so I won't be checking it right away.

But I can tell you something you mentioned that I have found odd. The battery light comes on with engine off, switch in run position (as it should), but even when the gauge sinks very low and the system voltage drops below 11 volts the light has never come on.

I did not check the cab fuses, only under the hood. Could a blown fuse 15 (7.5 amp fuse) inside cause the voltage regulator to act sporadic? I don't see how it could but I'm grasping at straws at this point.

As I recall, with the engine off/key on I had 0 at the green wire but the light was on on the dash (?) I'll get back with you ASAP. Thanks again!

 

A 12volt light bulb will light up when you give it 12volts on one terminal and 0 volt(ground) on the other terminal.
If you give it 12 volts on BOTH terminals then it won't light up

The Green wire voltage passes thru the Battery Light bulb to the alternator.

When alternator is not spinning it has 0 volts, so Bulb lights up

The Green wires 12volts also supplies voltage regulator with power to send to the rotor, startup voltage

If you just spin an alternator it will not create any voltage, nothing happens at any RPM you spin it at.
This is because you must first supply it with voltage to the rotor so it can START creating more voltage.
Once alternator is making its own voltage the Green wire isn't technically needed, this is how One Wire alternators work.

If there was no voltage at Green wire then alternator could never start making voltage

Once alternator starts then you should have 14 volts(approx.) at battery and so 14 volts at Voltage regulator(green wire), so Battery Light bulb has 14 volts at each terminal and it would be off.

If alternator stopped working altogether, 0 volts, then battery light would come on again

But if alternator voltage were to drop below "at rest" battery voltage, then Battery Light should flicker, not bright but should be visible.
i.e. if battery voltage is 12.8 and alternator is sending out 11 then bulb would light up with the 2.8 volt different, barely on.

If the whole system's voltage were to drop to 12 volts then no battery light because bulb has 12 volts on each terminal

 

I finally got a chance to get back to the Ranger and take some voltage readings...

Key off:
Battery 12.3 V
@ Alternator
Green 0
White 0
Yellow 12.3
B+ 12.2

Key on, not running
Battery 12.1
@ Alternator
Green 1.3 (Batt light on dash ON)
White 0.3
Yellow 0
B+ 12.1

Engine Running
Battery 14.6
@ Alternator
Green 14.2
White 7.3
Yellow 0
B+ 14.6

Load Test -- very near readings to above

About 5 minutes into load test -- gauge needle suddenly dropped down, Battery light still off
Battery 11.1
@ Alternator
Green 11.1
White 9.8
Yellow 0
B+ 11.1
These numbers slowly dropped as I let it run.

After shutting off the truck and let it sit a while, we restarted and had 14.6 volts charging again.

Any ideas??

 

Yellow wire is the problem
It should have battery voltage, whatever it is, 24/7(all the time) key on or off engine running or off

It showed battery voltage one time, then stopped, so probably a lose or half broken wire.

It is connected directly to the battery positive via a fuse, 30amp in the engine fuse box


And with 3 wire connector unplugged, test the green wire again, should have battery voltage with Key on, 1.3v isn't right

 

This intermittent stuff is maddening. I thought I might have it fixed today by unplugging the connectors, cleaning, and plugging back in, but no. I re-ran all the numbers and the yellow wire had all the right stuff today -- battery voltage matched at all times.

It ran and charged with a load for 45 minutes plus. I shut it off a couple of times and re-started and it was perfect. I finally took it for a ride with the lights on bright, blower on high, etc. It was fine for about a mile, then the moment I hit the turn signal the gauge dropped like shutting off a switch (no battery light though). I got back home and sure enough it was only charging about 10.8 volts and less. It acts like a voltage regulator to me, on-off-on, new alternator or right signal or not.

I still had about a volt today on the green wire with the ignition switch in "run" but engine off. You pointed out that it should have battery voltage with key on, but with battery voltage all the time wouldn't that keep the battery dash light off? Doesn't the bulb check in that position require basically 0 volts on that side to turn the light on?

All the connections, grounds, fuses look good. The rebuilt alternator was installed by me last week as a free lifetime warranty replacement. The fact that it continued to discharge only at times has made me hesitant to condemn it, but I'm having second thoughts on that view. I didn't check the former alternator to this degree so I don't know if it was exactly the same problem or not.

Thinking I may remove it, scrap it, pay for an all new alternator, and start from scratch. Input?

 

You check the Green wire with 3 wire connector unplugged and Key on(RUN)

When it is not turning the alternator is 0 or low volts so a Ground, Battery light comes on because voltage is passing thru it to a Ground, so filament glows
Once alternator starts to produce voltage then both wires on the Battery Light are equal voltage so no voltage is "passing thru" the filament and it doesn't heat up/glow

Battery light doesn't Check voltage, so if system voltage dropped to 10volts the Volt Meter would drop, but Battery Light still has equal voltage on both wires, 10volts, so wouldn't light up.

I would try this
Unplug the 3 wire connector on alternator
Test battery voltage
Start engine
Test battery voltage again
Now drive it for a short distance, lights and fan off, but use turn signal and other low power draws
Watch Volt Meter

Just looking for a system issue not related to the charging system

 

Sorry it has taken me so long to post but my wife has been in and out of the hospital. I finally got a chance to get back to the Ranger yesterday. I got a brand new (not rebuilt) alternator from RockAuto.com and installed it.

After a long drive yesterday and today it is working perfectly. Apparently the new-by-way-of-warranty, rebuilt Remy was bad. I think the voltage regulator in it was probably the problem.

So far, so good. Thanks for the help!

 

Good work

Thanks for the update and fix

Yes, "new" used to mean "tested and it works"
"New" now means "you test it, we warranty it"

To get prices down now you have to cut labor cost, and testing requires labor