Ford pickup low voltage at idle

Shorted diodes would drain the battery when the engine is shut off. He made no mention of that. An open diode might be possible, but my bet is on worn-out alternator brushes that aren't making good contact anymore. It's the most common alternator problem. Brushes should be checked every so often, like maybe 50,000 miles or so. We do them at 500 flying hours in our aircraft, and they are Ford alternators. They don't go beyond about 1,000 hours without being short enough to need replacement. 1,000 hours at an average speed of

40 MPH would be 40,000 miles.

Dan

Is this an alternator where there's a bearing and brush kit readily available at my FLAPS or do I need to actually send it off to be professionally rebuilt? I have perfect confidence in my ability to do the work, but in some cases (e.g. Bosch) I have been unable to source the parts through consumer-level channels.

nate

Forgot to mention, if they fail for you at appx. 1K hours, they probably are toast as vehicle has about 140K miles and no record of alternator having been touched (PO kept good records, I have a surprising amount of documentation)

nate

I'd get the part number off that alternator and go see if your parts supplier has some brushes. Bearings, too, are a good idea. If the slip rings in the alternator are chewed up they'll need turning on a lathe. Otherwise, they should be cleaned up with a bit of fine sandpaper and polished with Scotchbrite. They get oxidized, which represents resistance, so that the field current can't reach maximum and the alternator's output falls off.

Dan

You called it, I called my FLAPS at lunch today, they said they needed to know which alternator I had to say whether or not they had parts for it, so I pulled it when I got home from work (good thing I'd done it before, because it's about 95 degrees out, had to wrap a bandanna around my head to keep my glasses from getting sweated on, and I was getting eaten by skeeters) took it into the garage, started to take it apart, brushes are nubbins. Unfortunately FLAPS does not carry brushes for the one I have (75A) I'm on hold with Crap Boys right now...

I'd rather throw new brushes into the original than pay $80 for a reman of dubious quality. the other option is to pay the $$$$$ to have it gone through by the local rebuilder, but they are in Annapolis which is about 50 miles away, not open on Saturday, and pricey...

nate

Spoke too soon. (don't know why my previous message is timestamped

9:15; I actually composed that maybe 2 hours ago.) I found a local-ish parts store with brushes, bought 'em, installed 'em, they work and I certainly feel better about the alternator now that there's some actual spring tension holding the brushes against the slip rings, but I still have the same problem exactly as described above, alternator is still very hot too. Any ideas?

I did not completely disassemble alternator, just removed the black plastic piece that holds the brushes (is that the regulator?) and cleaned the slip rings. The rear slip ring was a little worn looking but I was in a hurry and don't have air tools so I didn't completely disassemble to clean it up. I did wipe them clean with solvent so it should be mostly OK.

I too don't think it's diodes; truck regularly sits 2-3 days at a time with no apparent discharge. Didn't have to jump it when I went to test drive it either, and who knows how long it was sitting then. I don't recall if I tested for key off current draw but I don't feel like trying it now, because it's freaking hot outside and I'm sitting in my nice (relatively) cool basement typing this. And it's late and I haven't had dinner yet, which makes me cranky :)

thanks,

nate

"Nate Nagel" wrote

You do realize that just as engines don't put out maximum HP at idle, alternator's don't put out max amps/volts at idle either. If you're not spinning the alternator fast enough to keep up with whatever load you put on it, no regulator or brush set in the world is going to help.

Have you tried doing a LOAD test with a load tester? A battery could have 12V and just enough AMPs to start the engine yet is unable to keep up with the amp draw being uses Vs., what the alternater can recharge. I have seen may batterys be so dead it will not even light up a marker light and yet have 12V.

Nate, I've worked on vehicles for 55 years (of course somebody's gonna say that may be just 1 year experience, 55 times!!!) and if I needed an alternator that yours would fit, I'd buy & enjoy it! And if your battery will withstand a load-test with a proper tester, I'd buy a good used battery also! Luck, & don't worry--unless battery is going dead or such. HTH, s

Hey Nate, we're not too po-dunky a town, and yet there are 2 shops here who reman just about any alter. for $30, sometimes $35. Just did one off a

2002 Z28 for me for $30. Check around--but see my post *up* one or 2 first. Luck, s

I understand that, but what I'm seeing appears to be drawing power off the battery at idle with only headlights and A/C running. I certainly don't want to be stranded somewhere with a dead battery simply because I got stuck in traffic after dark. (A/C is mandatory here in Our Nation's First Swamp.) Certainly it wasn't like this when new otherwise nobody would have ever bought a Ford.

nate

my 71 comet had low voltage at idle.. coil resistance checked good but it was 36 years old so replaced it and solved the problem

There's one other possibility: the battery has sulfated and its internal resistance has dropped too far so that it can't keep up with the battery's demand plus everything else. I'd try another battery (borrow one) and see if it helps. The alternator's heat might say it's working too hard somehow. Modern alternators are capable of producing plenty of power at idle. It's the reason we switched from generators back in the '60s.

Dan

but I still

I can't say for sure, but you symptoms sure seem 'normal' to me. On all the recent AC Delco alternators I have been using, none will start a good charge according to the dash meter until I get to 800 rpm and above. Sitting at a 650 Idle with everything on causes the volt gauge to drop. Sounds just like your Ford....

I am on my second alternator in this vehicle and the current one was an expensive remanufactured unit, not rebuilt. My last GM pickup was the same and even my last Volvo 240's with Bosch alternators were the same, no charge on the volt gauge at idle.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

Hmm, my 92 Explorer with 135,000 miles on it still has the original Alternator in it. Original Starter too. No sign yet that the brushes are worn out.....

You may want to pay attention. Every Ford I,ve owned since

1986 has routinely had alternator failure between 140-160k miles. All except one have been brush wear. Repair has been varied depending on how much time I had to personally deal with the problem. If you have time, they are usually easily rebuildable. It is nice to have access to a lathe to clean up the rings.

Lugnut

Brushes wear faster when alternator loads are higher and the field current is therefore higher. Driving a lot at night will do it. A daytime commuter vehicle will get more time out of its brushes, but sooner or later they'll quit, and probably in some inconvenient spot.

Dan