Alternator going out!

[mbagley]

Hi all, I'm in a strange situation with my truck. It is a 2006 Ranger STX with the 3.0l engine. The alternator is going out, just not putting out enough to fully charge the battery, and yes I did have the battery checked. My problem is, for some reason NOBODY around here has a replacement. I've tried the parts stores and salvage yards, nothing. It is listed as a (3g) 115A unit. Does anybody know of another 3g alternator that will fit, perhaps a 130A unit? I can't understand why nobody carries this part.

 

[Tractor Dan]

What is your location? I'm shocked because the 3.0 is a popular engine. Even to get a 115 amp alternator i don't see why that would be any more special then a 95 amp or any other rating found on an RBV. Maybe check www.rockauto.com if you can't find it local. They seem to always have anything i can't get local.

 

[cp2295]

^i second that.. try rockauto.com. their prices will beat anyones as well as having the part.

 

[Earl43P]

Most alternators need a new set of brushes at about 120k miles or so.

It's a cheap part that is relatively easy to replace.

http://www.amazon.com/Voltage-Regulator-Alternator-Brush-Holder/dp/B005LPR1HY

Typical symptom of worn brushes is that the Battery light starts to come on at higher engine rpms, goes off at idle. As the brushes deteriorate further, the light may stay on or flicker at idle, depends how long you let it go.

I'm not going into a long explanation how to replace that part, but it's not dificult at all with standard tools and common sense. You could probably find a Youtube video showing how.

 

[mbagley]

Thanks for the advice. As I said, I also think it's weird that nobody stocks it. I've always driven Fords, because I've always been able to get any part I needed. I have found several online at a reasonable price, but I was trying to get it local, put it in and be done with it. The truck only has ~60K miles. I think it's a diode opening up because if the battery is fully charged I'm getting between 13.9 & 14.3 volts when I first start out. After it gets hot it starts dropping. It will go as low as 12.5v, no matter what the RPM. I'm going to hook the scope up to it this weekend and measure the ripple to verify the diode's. If it looks ok I may try a regulator module. By the way, the dash light never comes on or flickers, even at higher RPM's

 

[Andy D]

Hmmn Sometimes that light is used in the charging circuit. Earlier models of my car wont charge if the bulb burns out. Dunno if Ford did the same.

 

[doorgunner]

Just wondering.......does the warning lite work when the key is turned on (lamp-check.....I think that's the term) and you probably know that there ought to be an in-line resistor that will take over if the warning lite burns out....go with a higher output alternator if'n you get a new one...then you can add stuff without worrying bout low battery all the time.

 

[RonD]

Yes, it seems that your 2006 alternator, Motorcraft MIGL665RM, was only used in '06, '07, and '08 3.0l engine, so a bit of an odd duck, not cheap either.

Rebuild might be a better plan as suggested.

Alternators have 3 "fields" that supply AC voltage to the diodes, diodes change voltage to DC.
Spinning rotor with voltage applied(electro-magnet) induces voltage in the fields built into the case of the alternator.

The rotor voltage is what decides the output voltage, the rotor has 2 brushes, the voltage regulator powers these brushes, it might send 7 volts to the brushes/rotor for a standard 13.6v output, or 9volts for a 14.6v output.
You are not getting voltage for free, i.e. 7v to 13v, the engine spinning the rotor supplies the extra 6volts

Generally brushes do get worn and can be replaced, in your case, with 12.5v that could be the case, low voltage at the rotor lowers output voltage, I would test the alternator output voltage after disconnecting the B+ cable on the alternator(the big wire), you are probably just reading battery voltage not alternator output, alternator output is probably 0v or well below 12v.

It could also be the voltage regulator itself, it is not sending rotor the correct voltage, usually there is a ground bypass on the alternator, you can put in a screwdriver and that will apply full power to the rotor, that would test if brushes and diodes are working.

The key switch provides the voltage that powers the voltage regulator, and that voltage powers the rotor, that's because the alternator would drain the battery if it had rotor voltage all the time, it is much like an electric motor so would suck power trying to turn itself.
The battery light or Amp meter is usually on that power wire that comes from the key switch, so if the light or meter is not working then the power might not be going to the voltage regulator in the alternator, so alternator is never "turned on".


Fields also fail, 1 field failing won't effect you much, head lights dimmer at idle, but battery would still be charged and maintained, and no.......head lights shouldn't dim at idle, lol, that is not "normal" that is either too small of an alternator or 1 failed field.

 

[mbagley]

The dash light is normal. On at key on and off when running. Been checking and It looks like a 130A 3g alternator from an Explorer, ~98-00, will mount up without any problem. Looks like all plugs are in same position, but can be re clocked if needed. For around $25 at a u-pull-it, it will at least confirm that it's bad/not bad. IF IT WILL STOP RAINING HERE!!

 

[mbagley]

Problem solved.. I went U-PULL-IT this morning and pulled a 130A alternator from a 97 Explorer. Fit perfect. Took it out on an extended test run and it settled down at 13.5v with everything turned on max blower, lights and AC. This was at idle and at 55mph. With normal blower speed and AC, it was a steady 13.9v. After I run it a while I will probably replace the 16yr old brushes. They are only ~$7.00. Only cost me $25.00. A lot better than $155.00 at the parts store, if ordered..

 

[RonD]

Nice work

 

[wildbill23c]

At least you finally got it fixed. You should keep your old alternator and see about rebuilding it and toss it on a shelf for later use if this replacement shall need to be rebuilt one day.