Alternator and regulator puzzle - not a Z, it's a Ford

[Zed Head]

I have a charge light coming on my other car, but only at RPM above idle.  I checked the system with a meter, and at idle speed voltage is 14.3, but anything above that and it drops to no charge.  The reverse of the typical bad regulator or alternator problem.  Kind of weird to see.

Anybody have any guesses on what might be bad?  It's a Ford truck, with the replaceable regulator attached to the back of the generator.  There are quite a few videos pit there about replacing the regulator and/or brushes but I'm new to Ford land so don't know what works well.  Still leary of remans and Ford truck parts are expensive.

[SteveJ]

What do you mean by "no charge"? Is the voltage dropping to battery voltage? Are you measuring current?

Is this a really old truck with a generator, or are you using alternator and generator to mean the same thing?

What are the chances that you can find a wiring diagram to post here?

[Zed Head]

It's a 2003 Ford F150 with 4.2 liter V6.  Service documents are hard to find, I haven't found a wiring diagram yet.  Yes, voltage drops to battery voltage.  14.3 at idle...add RPM...12.3...dash light comes on.  I'd noticed some odd behavior from the radio noise that reminded me of when my old Nissan alternator crapped out.  A day later the battery light started lighting, then going off at stop lights and from standing starts.  

I'm leaning toward the higher current of higher RPM causing a regulator fault, or bad brushes losing contact with RPM increase.

I have a manual with a troubleshooting flow chart to go through that will show something, hopefully.  Just happened last night.  The truck came from a super dusty environment, so worn internals wouldn't be a surprise.  I'll know more when I take it apart.  Hoping somebody's seen similar.  Even the internet only has two correct examples, with no resolution.  Google keeps coming up with the opposite problem, no charge at low RPM, and I don't know how to make Google understand.

[Zed Head]

Here's a shot of a typical alternator and regulator.  There's a three pin plug with an A, S, and I.  A jumper wire from A runs over to the small one pin plug.  The back of the regulator has an A and an F terminal with F exposed for testing.  Instructions that say "ground here to test" molded right in to it.  The manual has a test procedure that illustrates checking the slip ring for grounding, a "hung brush' test using A and F resistance, some loose fuse and connection tests.  All pretty interesting but it might just be more direct for me to remove it and take the regulator and brushes off and inspect.  Pretty usre it has 75,000 miles on it.  Debating...

[Zed Head]

Here's a shot of the test instruction overview.  Of course, "S" does not mean Sense.  Gotta learn a new language now.

[SteveJ]

I would suspect that it's an issue with the stator and/or brushes. They are more likely to be affected by the increase in speed. I wouldn't see the regulator caring.

Does the alternator go back to 14.3 when the engine goes back to idle?

[Zed Head]

It goes back to 14.3 when the RPM get low enough.  Sometimes even idle wasn't low enough, the light only went off when i was lugging the engine at take-off.

But, with Google as my guide, and my $6 eBay service manual, I was able to get it pretty well figured out.  Attached some pictures.  The only test it failed was resistance across A to F.  3.3 ohms instead of the 5 limit, the "hung brush" test.  Wasn't really hung, just gone. 

 

[Zed Head]

Alright, that was something different.  Back to our regular schedule...

Thanks for the help.  Sunny and 65 out here, so I'll probably go test wrecking yard alternators now that I know what to look for.  The good remans cost over $150 at the parts stores.

[SteveJ]

You only needed reinforcement for what you already suspected. 

Rockauto has a decent price on a Bosch brand listed as new.

[Zed Head]

Describing the problem helps understand it, is what I tell people often.  Had to follow my own advice.

I found a decent Remy "Gold" unit with about 3/4 left in the brushes, good resistance A to F, and smooth quiet bearings.  Had a coupon, only $22 outlaid, with a $7 year-long return period extension.  Got some sun, saw a bald eagle harassing the flocks of geese at the wildlife refuge next door...overall, a nice day in the wrecking yard.  Got it home and installed and all is back to normal, had 14.5 volts at idle.

Then found a couple of places on eBay that sell rebuild kits.  $30 for bearings, slip rings, regulator and brushes.  I might swap the Ford regulator for the kit's and maybe keep the Ford bearings, depending on quality.  But I should be ready for the next go-round, whenever it happens.  A good rainy weather project.

Edited October 26, 2017 by Zed Head
conflated sentence fix - word!