Analysis of an Alternator Failure - Ford Powerstroke Diesel Forum
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Old 02-03-2013, 02:50 PM Fanatic

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Analysis of an Alternator Failure

So, it started out of the blue...

Driving to work one morning and suddenly the idiot (battery) light comes on. Look over at the Insight and see the voltage has dropped to 11.2-11.5V (normally >13.4V). Oh crap.

Abort the trip to work and while driving home, idiot light intermittently goes out and voltage comes back up to normal... so the alternator is charging... when it feels like it.

Park the truck and order up a 200A 6G Large Case alternator off Ebay. The one in the truck has already been replaced once by the previous owner... at a cost of ~C$700 (ouch!), and that was back in Feb 2010 (3 years ago).

Charged up the batteries yesterday before firing it up to drive down to the shop to work on it. Stuck the ammeter on the charge wire, and after firing it up, alternator was putting out about 75A (I have since determined that this was a 6G SMALL case alt that was on there... so I guess that makes it the 110A version).

Driving over to the shop, did the same thing... charged fine for a while, then intermittently it would stop charging.

Fought with the damn serpentine belt for an hour, trying to find the right combination of tools to squeeze down in to the 1/2" square drive hole to release the tension... next time I should have Googled that procedure first. <rant on>DAMN FORD and their dumbass engineers... why couldn't you leave just a LITTLE room between the tensioner and the fan shroud to get proper tools in there to release the tension?!<rant off>

For those who don't know, there is (on the factory one) a lock to lock the tensioner in the released position... here is what it looks like when it is engaged:

Ok, got the old one out and the new one in... 3 minute job once you get the belt off.

Load tested and charged the batteries individually while everything was disconnected... they're happy.

Fired it up with the new alternator... 110A coming out of it at idle (glow plugs on). I'm happy.

Now... what happened to the old alternator? Why was it charging intermittently? Bad brushes? Faulty voltage regulator? Lets pull it apart and find out...

Pulled the back of the alternator apart (three bolts and pry the back of the case off the bearing). Removed the voltage regulator (had to fish the brushes around the slip rings and the bearing to get it out)... noted a couple of the Torx screws holding the voltage regulator were quite loose.

Here is what the back looks like with the voltage regulator removed:

You can see from the above the three terminals/screw holes for the voltage regulator. You can also see that this is a 6G Small Case alternator (136mm diameter heat sink) by the rectifier part number (FR6003). How nice of the Ford dealer to put the cheapie alternator on when they replaced it... thanks guys!

The cause of my intermittent charging woes was immediately apparent. The loose screws on the voltage regulator had caused the B+ (and the Stator terminal to a lesser extent) to arc and char... the resulting carbon build-up make a high resistance connection that probably worked fine when things were cold, but as they warmed up and loosened, it would go open.

When the voltage regulator lost its B+ supply, it could no longer supply field voltage to the rotor, and presto, no output. When it cooled down enough, the connection would close, and you get the field voltage restored, and start charging again. Note that this is not a high current path... the field is rated for 8A according to the specs.

Now for some close-ups...

Here is the charred B+ terminal on the alternator:

And the stator terminal on the alternator:

The top side of the voltage regulator:

The voltage regulator with the brushes retracted and pinned:

The bottom side of the voltage regulator (that mates with the alternator):

The charred B+ terminal on the voltage regulator:

The not-so-bad stator terminal on the voltage regulator:

The good news... I am quite confident that after I buff up the connections and re-assemble the unit, it will probably be just fine. Will put it back on the shelf as an emergency spare (since these things seem to have a reputation).

One has to wonder why the screws on the voltage regulator loosened. Were they not torqued properly when it was built? Did they back off on their own over time with vibration and all the heating/cooling cycles? Why didn't they use some blue Loctite or split or star washers to try and keep things tight?

I'll probably try going with some blue Loctite when I put it back together and see how that works out.

Long story short... if you have similar symptoms, and you are on a tight budget... might be worth your while to pull your alternator apart and check the connections between the voltage regulator and the rectifier plate. Might be an easy fix that can save you some $$$

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Old 02-04-2013, 12:20 PM
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I have always wondered why these regulators fail so often.

I can see why now!

Thank you for posting this!

I'm gonna pull my Motorcraft off the shelf and do this.

I'm sure some dielectric grease wouldn't hurt either.
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Old 02-04-2013, 03:03 PM Fanatic

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Originally Posted by dfuzz78 View Post

I have always wondered why these regulators fail so often.

I can see why now!

Thank you for posting this!

I'm gonna pull my Motorcraft off the shelf and do this.

I'm sure some dielectric grease wouldn't hurt either.

Yes, some Penetrox or No-ox-id A would probably be a good idea on the connections.

Just be careful with the brushes when removing the voltage regulator, they are very brittle.

If you have long enough brushes, you might be able to put a pin through on the rotor side of the plastic housing to help keep them back while you pull up on the regulator.

The problem is as you pull up, the brushes slip off the slip ring, in to the groove that is between the slip ring and the end bearing... then it is a pain to get them retracted fully, which they need to do in order to pass over the bearing (unless you remove the bearing).

If you can work a wooden toothpick in to the outside brush and retract it, and put a toothpick in the retaining hole to hold it in place, and then work on the inside brush, you might have better success in getting them both inside the voltage regulator case and out of the way so they don't get damaged.

I preferred not to remove the bearing, and was able to coax the brushes back out of the way until I could get them pinned.

Good luck!

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Old 02-23-2013, 05:48 AM
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if your problem is more than a connection like mine was ( bad diode)

you can get any internal part from

nice trouble shooting and article
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Old 06-29-2013, 06:01 PM
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Just bought a replacement unit from Advanced Autoparts, $175 and change but comes with a life time warranty. I searched for a replacement Voltage Regulator as the lower brush was toast. The commutator was worn too far to justify waiting for a replacement regulator off Ebay. Here is why it failed. The forward brush (lower one, when looking at it from the back) has a shield. This shield has vent holes to allow cooling air to pass, keeping the brush temp lower. Because these vent holes were block the wear rate was nearly 75% more than the upper brush. Spring tensions were the same, no dragging or binding. So that leaves heat as the only reason for accelerated wear.

So the moral of this story, pull yours apart, inspect the (voltage regulator) brush housing and commutator for damage before planning on the cheap fix. I would also recommend locating a VP3C3-10C359-AA voltage regulator and having it at the ready. For me, I'll just locate the nearest Advanced Autoparts and demand my replacement. The bad alternator had less than 60K miles and 3 years on it.
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Old 06-29-2013, 06:09 PM
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Awesome write up! Thanks!!
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