Hi folks,
First post here. I'm getting close to finishing up my project Ranger (an 03 FX4), and I'm running into a strange electrical issue. The truck starts fine, but immediately displays the battery light on the dash. Over the next 3-4 minutes, the voltage gauge will slowly rise from close-to-nothing, up to normal operating voltage. On top of this, the headlights and all dash lights are flickering rapidly.
A little bit of background, the truck has 240k miles, a new battery, and I was able to drive it over 100 miles home when I bought it. I don't recall any flickering during that trip. Currently the taillights and charcoal canister are disconnected due to some frame repair I had to do.
My first thought was alternator, but my last truck (an older GMC) had similar issues due to a loose connection elsewhere in the engine bay. So I figured I'd ask before shelling out the cash. Any ideas?
Thanks!

 

well if its that rusted it needs frame repair id clean every ground i can find. particuarly by the fusebox, main frame grounds, engine to frame grounds, taillight grounds, etc.

 

I'll check those tomorrow. In particular, I'm interested in the "taillight grounds" since the taillight harness is disconnected at the moment. I'll try reconnecting that and see if it makes a difference.
I'm also going to swap the battery, and if nothing comes of that I'll swap in my spare alternator (I've got a 97 parts truck)

 

Shooing from the hip here... poor ground or voltage regulator.

 

After a little more research, I pulled the alternator and swapped out the brushes. Also cleaned off all grounds I could find. Issue is still present.
One thing I am finding a bit odd is that the voltage drops mostly when the engine is accelerating. If I'm coasting or braking, no issues whatsoever. However when accelerating I'll get battery light, flickering headlights/dash lights, and the radio will actually power off. As soon as I let off the gas, everything goes back to normal.
In addition, when idling, the battery light flashes once every 4-5 seconds consistently.
Any ideas? I'm going to try belt, alternator pulley, and regulator next, unless someone has a better idea.

 

You need to use a Volt meter

New battery has 12.8volts
3 year old battery 12.5volts
5/6 year old battery 12.3volts, and time to shop for new battery, cold weather will kill it

Battery is ONLY used to start the engine
After engine starts system wide voltage is 13.5volts to 14.8volts from the Alternator, so even a new battery at 12.8volts is too low to add voltage to the system, so it PULLS IN VOLTAGE, to keep itself charged up for the next "Startup".

Battery Light is a simple circuit
The green wire on alternator runs to the dash and one side of Battery Light bulb
Other side on the bulb is connected to a fuse in the cab fuse box.

How does a 12volt light bulb work?
12volts on one side and Ground on the other side makes it light up, I know duh
But what if is has 12v on one side and 12v on the other side?
Or 14v on one side and 14v on the other side?
NOTHING happens, light bulb is off
For a light bulb to work it must have two different voltages

Back to battery light bulb
When you turn on the key the bulb gets 12v on the fuse from the Battery
And alternator is off, so 0 volts, Green wire is a ground
Battery light bulb is on, two different voltages

You start the engine, alternator makes 14volts(as an example), so FUSE now has 14volts, and Green wire on alternator has 14volts so..................Battery light is OFF

If alternator voltage drops below 12.8v(battery voltage), the FUSE has 12.8v and "failing" alternator has say 9volts, so Battery Light bulb has two different voltages so comes on or even flickers if alternators voltage is not steady 9v.
As you accelerate, you raise alternator RPMs so it might get up to 12.8v or higher, so battery light is off again.


So get a voltage meter
Test battery, key off
Start engine
Battery voltage should now be 13.5v to 14.8v
Higher voltage just after startup, above 14volts
After 5 minutes of running it should drop down to 13.5-13.8volts, over 14volts for too long will ruin the battery, "cooks it"
Turn on ALL the electrics in the vehicle, fan in HIGH, voltage needs to stay above 13.5volts

An alternator generates AC Volts, Diodes in the alternator convert the AC to DC, it diodes sets fail then AC can leak into the system, cause flickering and all sorts of odd issues.

 

Okay, I did some of these earlier.
Engine running, battery light off: ~15.5v
Engine running, battery light on: ~13v
These were taken at idle, after driving around the block. When it idles, the light comes on once every 5 seconds or so (which corresponds to the voltage drop)
From your description it sounds like the alternator is definitely the issue. Does the alternator ever really "break", or is it just the regulator and brushes? Can anything else in it go bad? This is my first go at alternator repair as opposed to replacement.
Thanks!

 

Also need battery voltage Key OFF, its the reference voltage for your volt meter
15.5v is too high, should never go over 14.9v, but could be your meter reads high
New battery should be 12.7-12.9v, Key OFF
And thats what you use to determine alternators condition

Just after startup battery will be drained, so alternator's voltage regulator will cause alternator to put out 2v higher than battery voltage
So 12.8v + 2v = 14.8v
12.5v + 2v = 14.5v
These are approx., not exact

Yes, alternators can fail out right, usually bearings go, but brushes can stick.
Voltage regulator can be replaced, as a separate part.

But test alternator wiring first
Key OFF
Use volt meter black probe on alternators metal case, the Ground
Red probe on B+, terminal on the back of the alternator, should read exactly the same as battery voltage

Unplug the 3 wire connector
Red probe on Yellow wire, should read battery voltage
Red probe on Green wire, should read 0volt, under 1v

Turn key on, engine off
Check green wire again, should read battery voltage, may be .1 volt lower, that OK, and thats the battery light wire, battery light would be off with green wire unplugged, just FYI

If all the wires check out then Truck wiring is OK


How alternator works
The case of the alternator has 3 Fields(each is a small wire wrap around a metal core)
Alternator's rotor has 2 bearings and 2 brushes, and windings around metal cores

An alternator can NOT generate voltage by just spinning it......................
It must first HAVE VOLTAGE to start generating voltage
That's where the Battery Light circuit comes into play, when you turn on the key the Green wire has 12v and that's the Startup voltage the alternator needs to start generating voltage by spinning it.
Once alternator is generating voltage it will be self powered

That 12v goes into Voltage Regulator, voltage regulator controls the voltage at the Brushes.
Voltage regulator sends 7 to 9 volts to the brushes, 9 volts for startup
This voltage creates a magnetic field around the ROTOR'S windings
When the alternator starts to spin this magnetic field cause AC electric current to build up in the 3 Fields in the alternators case
Each Field has a pair of Diodes that convert the AC to DC current.
If a Field or Diode set fails then alternators output drops by 1/3, dimming head lights at idle is a sign of a failed field, just FYI, no its NOT normal, lol.
Higher voltage at the rotor's brushes = higher AC voltage in the Fields, so higher DC voltage out of B+ terminal

The voltage regulator gets "battery voltage" from the Yellow wire, but it is current flow, not actual "voltage" number.
As the battery gets reCharged after starting the engine, it draws less current so voltage regulator lowers Brush voltage down to 7v
As you turn on headlights, heater fan or ???, current flow changes and voltage regulator may change to 8v to brushes or 9v, to maintain a stable voltage, like 13.5v.

IF your voltage changes, goes up as you increase RPMs and hold it steady at say 1,500 then voltage regulator is stuck on one voltage to brushes.
i.e. if voltage at idle is 13.5v then it should also be 13.5v at 2,000rpm, voltage regulator lowers voltage to brushes as RPMs increase to maintain the 13.5v or whatever it was at idle.
You could also have a failed field
Raise RPMs to 3,000 briefly, if voltage goes up to 15v then voltage regulator is bad, stuck on 9v, and it will "cook" your battery

 

It's been a few days, sorry about that.
Anyway, I replaced the serpentine belt (turns out the one in it was two inches shorter than spec) and the alternator voltage regulator. The battery light is no longer coming on, ever, and the voltages have adjusted:
Engine running, battery light off: ~14.4-8 volts, fluctuating every couple of seconds.
However, the dash lights, radio, and headlights are still flickering just as much as before. The radio no longer seems to care about the engine RPMs, it is completely random. Just to make sure, I unplugged the radio harness but the dash/headlights are still pulsating.
The battery voltage is at 12.5, so it's possible I got a dud. Going to try out another one, hopefully tomorrow.

 

Something is wrong with alternator, voltage should be stable, no jumping around.
Not a vehicle issue, strictly an alternator/voltage regulator issue

 

Thanks for the advice. I did notice one more thing that I want to check out before replacing the alternator altogether: when idling, the belt tensioner is doing a little dance, hopping up and down at the same rate as the light flicker. My guess is that having a too-tight belt on it for however long wore down the tensioner spring. Gonna do that later this week, and check the bearings on the idler pulley and alternator while I'm in there.

 

alternator could be pulsing so it makes power one sec causing drag then freewheels like a idler pulley the next preloading the belt.

Any auto parts store will test the alternator for free. Heck they will do a starting charging system test for free as well. Drive the truck there and see what their magic computer says.

 

Can't bring it to an auto parts store without it being registered, and I can't register it until it'll pass inspection.
Anyway, I have another Ranger that uses the exact same alternator and has never experienced any electrical issues, so I pulled that one and threw it in this truck. Still having the same issue. Also bought a brand new alternator and tried that. No dice.
Also, replaced the idler pulley and tensioner. No change to the behavior.
Any other ideas, besides the alternator? I'm going to keep looking for shorts and cleaning grounds, but given that this is a consistent issue (the lights pulse at exactly the same rate as the engine RPMs), I really don't think that is it.

 

For anyone still following this, I've learned to just deal with it for now. The pulsation of the lights is significantly less now.

 

Okay, new development!
Last week, my radiator went on me. I had a slow leak for a while, but it turned into a fast leak, very suddenly. I figured while I was replacing the radiator, I may as well replace the alternator (again). So Wednesday of last week, the radiator and alternator were replaced with brand new units.
For the first day following the alt replacement, nothing changed. Then when I went to start it up Thursday night, the battery light was on solid, the voltage gauge was down real close to the bottom, and no more flickering. At this point I'll just list what I know, rather than continuing this narrative style of writing:
- Battery voltage with truck off - ~12.5 volts (it's been very cold)
- Battery voltage with truck on - ~12.5 volts (measured at battery & alt)
- Disconnect battery, truck dies
- Headlights are dimmer than before, but I think "normal" for a Ranger (they were excessively bright when it was flickering, people were constantly "flashing" me thinking my high beams were on)
- Voltage gauge has not moved since Thursday. Increasing radio volume, using high beams/fogs, even an inverter does not affect truck's voltage gauge
- Truck is idling at the same RPM as before, and running just as well as always
We're going to be seeing snow tomorrow, so I plan to swap the old alternator back in later this week. Might wait until Thursday, as I have to do the fuel pump then as well. If no change I'll try a new battery, probably the same day.
Any suggestions are welcome. I'm sure I'm not the last person who will have this issue, so the more info we collect here the better off the next guy will be.

 

Hey RonD, I saw your post regarding somebody else's issue - https://www.ranger-forums.com/showpo...53&postcount=2
I tried your suggestions regarding the B+, yellow and light green wires, and went out and tested these.
Yellow (key off) - ~6v
Yellow (key on) - ~4v
Light green (key off) - 0v
Light green (key on) - ~12v
B+ - ~12v at all times
Based on all this, I'm pretty confident this alt is junk. Hopefully the old one will get my voltage back up to ~14v operating voltage. I may want to throw the battery on the charger tonight, too. Maybe I'll pick up a Motor City Reman 160 amp unit next paycheck.
This doesn't explain the low voltage on the yellow wire. You mentioned an Autozone wiring diagram, however they don't seem to have one for my 03 and the link you posted is dead now. However if it's the same as the 01, you mentioned a fusible link between the B+ and yellow wire. I'll go digging for that after work (~4am).
EDIT: Okay so I thought and thought and thought again, and I'm thinking one or more of the fusible links probably went, and mayhaps the alt is actually fine. I already ordered the MCR alt, but before I pull this one I'm going to try jumping the B+ to the yellow wire and see if that makes a difference.

 

Yes, Yellow wires fuse or fusible link connection is bad, most likely a connection is corroded since you see some voltage on the yellow wire, blown fuse or fusible link would have 0 volts

Yes, you can cut yellow wire long enough so it can reach B+ terminal when its plugged in to alternator, and connect it there on B+
This will give voltage regulator the 12v on the yellow terminal it needs to operate



Just a heads up, you shouldn't unhook the battery with engine running, it can damage the voltage regulator
In the old days with external regulators that used relays instead of transistors it was a common test to see if alternator was working.

Now-a-days most people have a hand held volt meter so can see battery voltage engine off and on and if its the same they know alternator is not working
Also most vehicles have a voltage gauge on the dash, you just need to look at it with key on engine off and see where it is, then start engine, if it doesn't go up then alternator is not working.

So the disconnecting battery thing is not needed and not good for the system, lol.

 

Haha good to know, I figured it couldn't be "good" for the system but now I know.
I ended up running a new wire direct from battery to the yellow wire due to lack of light and time at 4am, but when I swap alts again I'll be running a new wire from B+ instead.
Another thing to note is that the flickering is gone now that the new wire is in there, so both issues were likely caused by the same corroded link.
Gonna call this fixed. Thanks Ron!

 

Good work