The other day the battery light came on. Of course the useless "gauge" still reads normal but that is another rant.

Checked voltages today:

Battery reads 12.0 volts key off, 11.83 key on, engine off, 11.65 engine running.

Obviously discharging and I am guessing it is the alternator. Do these trucks have an external regulator that may have failed or is it in the alternator? I'd hate to spend $150-200 for an alternator only to find out it doesn't solve the problem.

Any suggestions appreciated.

 

The regulators are part of the alternator for the last 20+ years.
Good luck.

 

There is a 3-wire cable (the A, S, & I circuits) that plugs into the regulator, which in turn is fastened to the back of the alternator.

There are 2 test points on the back of the regulator, below the connector area. The top test point is "A", closest to the connector. Straight in line further down the regulator is the "F" (Field) test point. They should be marked in the plastic molding of the regulator.

It won't be easy to get in there, but what you want to do is "Full Field" the alternator. Alternator output current is dependant on field current. Field current is run through the rotor's winding, and is controlled by the regulator. By grounding the "F" test point to the case, or other good ground, the field gets full current irrespective of the regulator's internal circuits.

You probably want to have lights on, or the heater fan running on High for a healthy load when you do this, though with your low level of charge battery, it is unlikely that the output will spike up to danger zone and take out electronics.

If the output increases back into the charge range (remembering that you have a low battery, so it may not be real high, just higher), then the basics of the alternator itself are good.

The "A" test point has 12 volts on it through a fuse, it provides the battery current for the regulator to use to create the Field current. If you accidentally ground the "A" test point instead of "F", you will blow a fuse.

The hardest part will be getting to the back of the regulator to get down to the "F" test point. GM cars of long ago had a metal tab in the alternator that you would just stick a big screwdriver in a slot to ground it. That was great, but the regulator was internal, IIRC, which was not so great.

EDIT: You can always look up the voltage regulator on rockauto.com, and find one with a pic that shows the "F" test point, says something like "ground here for test" with an arrow, all molded in the the plastic of the regulator.

Another way an alternator can quit is worn-out brushes. The brushes conduct field current to/from the field winding on the rotor via the slip rings. The brushes are carbon, and eventually wear out. In most alternators the brushes can be replaced, if they have a screw terminal on the end of the copper braid that goes to the brush, they are easy to do. Some just have a contact at the end of a spring that comes with the brushes, so they are self-contained, no fasteners used.
There is a trick by using a length of bare copper wire poked through holes in the side of the brush holders to hold the new brushes back into position so the alternator case halves can be bolted back together without the brushes falling out. Then the wire is pulled out from outside the case, allowing the brushes to extend under spring pressure to touch the slip rings.

ONE MORE EDIT! Cool! I didn't realize that the brushes can be replaced from the OUTSIDE on the 3G alternators by removing the voltage regulator. Still need to use a wire to hold, but no need to take the case of the alternator apart. Just shows I have never had a failure of a 3G yet, and that is good thing

 

I guess my question was worded kind of vaguely - what I meant to ask was is the voltage regulator integral to the alternator or can it be replaced separately. It appears that is can be replaced separately...

NAPA AUTO PARTS

Torky2 - thank you very much for the response. I appreciate the very detailed answer. Going to trouble shoot the truck tomorrow and see what happens.

 

The slip rings on these alternators rarely make it through 2 sets of
brushes
You can change them(sliprings)but the alternator has to be dismantled
and the slip ring connections are soldered.

 

It's rarely practical to rebuiild part of an alternator these days.
If the regulator is replaced and it does not fix the issue your back at it again.
System overall checkout is the best way to evaluate before taking anything apart.
Get yourself an LED stick tester from the auto parts store that has LEDs for 10v, 11v, 12v, 13.5, 14v, and 15volt indicators.
Put on it a cigar lighter type plug in place of the test prods so it can be plugged in.
Note the leds that are lighted.

At cranking note the lowest voltage LED lighted.
After engine startup, note what highest LED is lighted.
These simple LED actions tells you about all you need to know to see if there is an issue.
At cranking the voltage should go no lower than 11 volts on a fully charged good battery. If yes the battery, cables/connections or the cause of it is suspect.
After cranking, the voltage has to go up in the 15v + range to show the alternator is recharging the battery.
If yes, does it come down after a few minutes charging?
If yes the system is working proper.
If the voltage remains high after that time, either the regulator is not controlling the alternator or the battery has a bad cell.
Once you understand all the 'expected' actions you can see what is or is not working.
No need to get into involved testing.
Problems can be the alternator has a shorted rectifier stack causing battery discharge when setting over night.
A bad cell causing the alternator to excessively charge and run water out to quickly, excessive gassing and corrosion etc.
Think about it before you get to involved so you understand and can recognize troubles.
Good luck.

 

Bluegrass7, I must say, your post's, along with many others, are very informative. My hats off to you, sir for your helping words.
May I ask what part of this world your from, I'm new to this forum and
and haven't or noticed your location.
Rick

 

Lehigh Valley at Easton, Pa.
Extreme eastern border 70 mi north of Phila.
Good luck.

 

Based on the voltage readings I took a couple days ago I went out on a limb and assumed it was the alternator. The battery is less than two years old - I bought it for my F250 shortly before I sold it.

Anyway, alternator swap was probably the easiest I have ever done. It went very smoothly except one bone-headed move of letting the 12v wire from the battery touch the case of the new alternator. Except for needing to change pants, no apparent harm done. I was pleasantly surprised that all three bolts holding the alternator came out easily. I changed the serpentine belt while I was at it....getting the new one on was the trickiest part.

Battery light is now off and all the voltages are back where they should be. Thanks for all the help. I do appreciate it.

 

I know, I know - I should have disconnected the battery. I got away with one.....

 

Hey, no welding!

This is where we're supposed to give you a safety lecture, so here it is: you knew better

IIRC, one of the 175 Amp Maxi-fuses is in the line from Battery to Alternator output, so I guess your welding arc was less than that ... or quick enough...

Glad you got it going again. The quality of rebuilt starters and alternators I think is at an all-time low for all the problems I read and hear about. I suspect that other than shining them up, very few parts are being replaced in them. Like only the exact part that caused the failure, and not anything else that is shakey and about to go. And what parts they do replace are probably low-quality=dirt cheap.

If I take something apart, and decide to only replace the exact part that failed, and not fix anything else that is badly worn, then that would be my (poor) choice. But if I'm going to pay $$$ for something that they advertise as "remanufactured", I expect them to do just that, look at it all the components and replace those that are going or are out of spec. But that takes time, people educated and capable of performing it, and some $ for quality replacement components. I wonder if we will end up getting down to a choice between low-quality rebuilds, and low-quality new imported parts

 

Of course there is always the third choice - waaaaay over-priced OEM parts. I honestly would have got the alternator from Ford but the price for their remanufactured alternator was $265 online.

Thanks for the safety brief, BTW. I really do know better.

 

Just curious, but for future reference what were the three 'after' voltages?

 

I couldn't find my scrap of paper with the numbers on it so I went and did it again.

Key off/engine off was 12.28v
Key on/engine off was 11.84v (I turned on everything just for fun)
Key on/engine running was 14.75v (this was shortly after starting it)

Something I forgot to mention was that the old alternator apparently had bad bearings for a long time - the mystery rotational noise from under the hood is gone now.

 

If a somewhat the same problem returns check thru the cables and connections for internal corrosion/blue-green stuff, under covers, inside the cables/connections/ends. It usuall crops up at the 5 year + level of age in vehicles. A very expensive-drive you nuts complication of ownership.