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94' Ranger

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My 1994 Ford Ranger 3.0 has been having an electrical issue that revealed itself the other day by catching fire. I had a major direct short on my blk/org B+ alternator wire in between my inline fusible link and the starter solenoid relay. While the truck was running the short was not complete, but when I connected jumper cables with my truck off and sitting still, it started pluming smoke. The short was obvious, there was exposed wire laying on my truck body past the fusible link.
I had been going through both remanufactured and brand new alternators every month or two and was happy to at least identify the problem. I cut away the exposed wire, replaced the fusible link, and warrantied my 4th alternator. I had them bench test it and saw it produced 15.7V, but when I put it in my truck it did not recharge my battery.

I have gone through all the troubleshooting I can find online and my results are as follows:
*13.08V battery with Key Off (charged overnight)*
B+ Alternator wire is 13.08V
Unplugged 3 wire connector (yel/wht, wht/blk, grn/red). - Yellow wire is 13.08V, White wire is 0V, Green wire is 0V.
KEY ON ENGINE OFF
Now green wire (grn/red) is 12.71, same as battery with Key On.

KEY ON ENGINE ON
B+ stud on Alternator is reading same voltage as battery, 12.71V.

My Battery light indicator on my dash is not coming on at any point.
I checked every fuse under the hood and under the dash twice, all of them are good.

There is a third connection to my alternator, a single white (beige cream color) wire that I have seen people call a "jumper wire". None of the troubleshooting I have found has had me test this wire, just inspect it.

I am not sure where to go from here. I've been checking for continuity along every related wire I can get to, everything I've tested checks out okay.


Any advice is appreciated, thanks.
 

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Rock Auto 5% Discount Code: F9A1A579ACFAD1: October 1st, 2021

ericbphoto

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If @RonD doesn't respond first, I'll check my evtm and try to help later this evening when I get back from trading the dog off to my ex. (Shared custody)
 

RonD

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If B+ shows battery voltage
Yellow shows battery voltage
Green/red wire show battery voltage(key on)

Then your vehicles charging system is OK

After startup battery should show 13.5 to 14.8volts, if not replace alternator, period

No back story needed really, those 3 wires are the charging system, if they check out and battery voltage is under 13.5v with engine running, then either fan belt is off, lol, or alternator is bad, there is no 3rd option
It's a very simple system, don't overthink it

White wire does need to be plugged in, there should be a single wire spade terminal on the back of the alternator, near the 3 wire connector, where it plugs in
Outside of "how it looks" there is no other test for that wire


If your battery light is not coming on with key on but you have 12volts on green/red wire with key on, then bulb is burned out

Parts places can not test alternators, just FYI, they will say that can, but they can't
You can
test the 3 wires, if they are OK and alternator doesn't charge, then alternator has been tested(by you) and found to be bad :)
And alternators should never go above 15volts, just FYI
 
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94' Ranger

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If B+ shows battery voltage
Yellow shows battery voltage
Green/red wire show battery voltage(key on)

Then your vehicles charging system is OK

After startup battery should show 13.5 to 14.8volts, if not replace alternator, period

No back story needed really, those 3 wires are the charging system, if they check out and battery voltage is under 13.5v with engine running, then either fan belt is off, lol, or alternator is bad, there is no 3rd option
It's a very simple system, don't overthink it

White wire does need to be plugged in, there should be a single wire spade terminal on the back of the alternator, near the 3 wire connector, where it plugs in
Outside of "how it looks" there is no other test for that wire



If your battery light is not coming on with key on but you have 12volts on green/red wire with key on, then bulb is burned out
Based on your reply I ordered another brand new alternator from O'Riley's, it will be here in 2-3 hours. I'll be stoked if it works but I'm not super confident, with this being the 5th alternator I've ordered in the last year.

In the meantime I'll get to my battery light to verify it's burnt out and replace it.
 

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RonD

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Just ground the green wire and turn on the key, if battery light does not come on then bulb is burned out

If it does come on then plug 3 wire connector back in and turn on the key, if no battery light then Voltage regulator is bad in alternator
 

94' Ranger

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You're the man @RonD

I grounded the green wire and my battery light came on for the first time since the short circuit happened. When I plugged in the three wire connector, no more battery light.
This indicates that they sold me a brand new alternator with a shitty voltage regulator, correct?
Also, this means putting in a new, properly functioning alternator should do the trick, right?

Thanks man, I did more research than I had to but I feel like I know my recharging system fairly well now.
 

RonD

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Yes, voltage regulator is for sure bad, with results of your test

Not that it matters but the green wire is the on/off switch for alternator AND provides Startup voltage so alternator can "make its own voltage"

If you spin an alternator it will not make any voltage, doesn't matter how fast you spin it, thats not how it works

An alternator can increase(multiply) voltage by using the mechanical force of spinning it
So if alternator has 0volts and you multiple that by any number of spins you get................0volts, lol
0 x (anything) = 0

We will call the part of the alternator that spins the Rotor, and the part that doesn't spin is the field(3 fields in this case)

The voltage regulator uses the 12volts from green wire to power up the Rotor(startup voltage), it actually only use 7 to 9volts
Then when engine spins the Rotor the Fields are energized by the Spinning Rotor voltage and this creates AC voltage in the Fields, which is converted to DC volts
So 8voltsDC sent Rotor = 14voltsDC out of Fields(B+) and whole vehicle, as alternator multiples voltage

Now the whole system has, say 14volts with alternator working, and that includes the battery light's wire in cab, AND the voltage regulator connection for green wire also has the same 14volts, from regulator side
And when you connect both a light bulb's terminals to 14volts(or 12volts) what happens..........nothing, power is equal on both sides so no current flows thru the bulb and its off

The voltage regulator can sense the Amps needed by the vehicle, its called "pushback" a "counter-electromotive force"
When you first start an engine the battery is drained by starter motor
So "pushback" is low so volatge regulator might send 9volts to Rotor to increase Amp output, 14.8volts
After 5 minutes or so the battery should be fully recharged so "pushback" goes up voltage regulator will lower the voltage to Rotor to lower the Amps and voltage will drop below 14volts
13.5-13.9volts is best for battery so it doesn't get "cooked" in long term charging, like using a "trickle charger"
When you turn on headlights or fan to HIGH the voltage regulator "sees" that as "pushback" drops so it adjusts the Rotor voltage a bit higher to compensate for the extra Amps now needed
The Yellow wire is part of this "pushback" circuit
 
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