OK, I'm totally stumped and ready to go to the dreaded dealer service department. My alternator only charges when the battery is fully charged. The battery is good.

My wife was driving our '02 X (6.8L V10) Friday and got stranded with a dead battery. After charging via jumper cables 30 minutes, I started the truck and found the alternator was not putting anything out (voltage at battery was slightly lower after starting truck and slowly dropping). I bought a remanufactured alt from Advanced Discount and installed it.

12.3 volts before starting, 13.75 after starting. Yay!! Life was good. I drove 60 minutes with several stops and got home. I tried to start again and had a dead battery. I charged the battery, cranked it up, measured the voltage at the battery. Nothing from alternator. Checked both fusible links and wires for continuity. I checked all my fuses. All good. I went back to auto parts store and they checked the new alt and said it was good.

I was stumped so I grabbed the old one and had them check it. They said it was indeed bad.

They offered to switch out the new "good" one so I took it home and put it in. I fully charged the battery first for several hours with a 6-amp charger. No load battery voltage was 13.75 so I know the battery is good (also the current meter on the charger went to zero). I connected the truck battery terminals and measured about 13 volts. Cranked it up and measured just under 13 volts and slowly dropping for maybe 3 minutes. Then alternator apparently kicked on and it went to roughly 14 volts. Again I thought all was right with the world.

Then I shut the truck off and turned on the lights for 5-10 minutes until the battery voltage read 12.3 - 12.5 volts. Cranked up the truck and voltage stayed there and dropped slowly. I let it run 20 minutes or more and the alternator never put out anything. I checked the wires for continuity, checked fusible links and spent 40 minutes searching for damaged/chewed/shorted wires. Nothing!

I disconnected the battery terminals, put the charger back on and let it charge all afternoon until the disconnected battery voltage was again at 13.7. Started the truck and measured just under 13 for a few minutes until alt started putting out 14 volts. I let it run for 10 minutes or so. To make it quit again, I just need to drive it a day or two or discharge the battery some.

I also checked the voltage on the plug. I have 12 volts on both wires when ignition switch is in run position.

I do electronic systems troubleshooting for a living, but not on cars. Somebody save me from the dealer please!!

 

With the engine running, are you putting the leads from the multimeter on the alternator to check for voltage output, or are you just checking at the battery?

Stewart

 

Both places read the same. The wires and fusible links are good. thanks for the reply.

 

Sounds like a bad connection at the alt on the small wires that excite the alternator. Look over that plug closely to see if anything is bent or corroded.

 

Ok, with that confirmed, check the o-ring terminal that connects to the alternator stud.

I had one that was broken, but was still held together because of the way Ford attaches the terminal to the end of the wiring harness. It would show a properly charging alternator sometimes, but then a few minutes later it would show a diminishing voltage charge.

If that's ok, check the body ground and make sure there's good contact there. Remove it, clean the metal on the ground wire and the body, apply Noalox, and reattach the ground.

Stewart

 

Exxwhy: I looked the plug over pretty carefully. I pulled it off and measured the voltage between the negative terminal of the battery and each of the two connections in that plug with the truck off and the ignition switch in run position. I measured the battery voltage in each of them.

Stewart_H: When I measure the voltage at the alternator (between the stud and the body/chassis of the alternator itself) I read the battery voltage. I assume you are talking about the heavy-duty ring lug being broken that attaches the battery cable to the alternator positive stud, but that is eliminated from the equation by measuring at the alternator itself. Perhaps I am misunderstanding you.

Thanks for the replies so far.

 

Bad ground.

 

Well, that depends. Lemme make sure I'm the one who understands.

When you say...

When the volts "dropped slowly" were you measuring the voltage drop from the alternator? I know you checked from various points, but for some reason I didn't think you measured the actual drop from the alternator.

If you measured the actual drop from the alternator, then you have a bad alternator. It doesn't matter how new it is, it's bad. If the drop was measured when you were checking the battery, then check the points I advised above, the positive o-ring terminal and the ground wire.

Stewart

 

I would bet that you have a bad voltage regulator on the new alternator. They are quite cheaply made and are very finicky. I've burned up more than one brand new one connecting to a dead battery. Did the dealership do that?

I have a known good alternator I'll let you borrow if you feel like paying shipping both ways. PM me if that will be helpful to you.

 

Hi Stewart,
Sorry that wasn't clear. When I say the voltage dropped slowly, I was trying to point out that the alternator was putting out nothing whatsoever at that point. The voltage was less than 13 volts and just slightly less than what I measured when the engine was off. The drop was the very slow downward trend that indicates the battery is discharging. It was only visible by keeping the meter on there for several minutes and seeing a slow, downward trend in the second decimal place on the meter.

Three times now I have fully charged the battery and put in one of two new alternators and had it charge when the battery is full. The auto parts store gave me a free replacement even though their machine said the first was good.

I can try checking grounds again but I don't see how that can affect it if I measure the same 12.x volts at both battery and alternator. The only way I can think a bad ground could be the culprit is if the bad ground was causing the computer to not put the proper voltage on the small gauge wires on the plug to control the field coil. I may be missing something...

 

Just for the sake of clarity (and my sanity, LOL), I'm gonna break things down individually and ask very specific questions. Please answer answer each question specifically.

When it was putting out nothing, where did you place your multimeter to measure the lack of voltage? The battery, or the alternator positive stud and metal part of the chassis?

Measured from what point?

On where? And was the engine running?

Honestly, this means nothing. All the alternators with the lifetime warrantee from Autozone, Kragens, O'Reilly's, etc are a crap shoot. They may last a few years, a few months, a few days, or be crap right out of the box.

I recently had to have one warranteed that I put in my daughters '02 F250 four months ago because it was putting out 15.8 volts. However, the one in my son's F350 has been going strong for a year now.

The regulators on these things are crap.

As long as you're placing the probes of the multimeter in the correct spot, getting a reading of 12.x from the alternator means it is bad.

Drive it over to the parts store and show them with your multimeter how bad the voltage output is.

It doesn't matter what their machine shows when the alternator is off the truck, what matters is how it functions in a real world test.

Stewart

 

I'm so sorry for the confusion! See below for my answers to each of your questions. Hopefully I did this quote thing right. Most of my readings were done at BOTH the battery and the alternator. It seems cut and dry: bad alternator. But if that's the case, why do both new alternators only put out voltage when the battery is fully charged?

Both at the battery and at the alternator. Same reading both places
Both at the battery and at the alternator. Same reading both places.
Both places. Yes the engine was running.
Normally I would agree. That reading was the battery voltage only, meaning the alternator was not putting out anything. However, if I charge the battery fully, the alternator starts putting out ~14 volts. I verified this three times with two different alternators.
[quote=Stewart_H;13627045]
Drive it over to the parts store and show them with your multimeter how bad the voltage output is.

This is an excellent idea.
Also, where does one buy a truly new alternator? I called the Ford dealer and they said they do not sell them, only reman.

 

So if I'm reading your response above correctly, you're saying that with the engine running, you placed the multimeter probes on the battery and it showed a voltage reading of 12.x, and when you moved the probes to the alternator (with the engine still running), it showed a 00.0 voltage reading, correct?

Also, what do you mean "if I charge the battery fully" the alternator starts working? Anything in the 12 volt range is a fully charged battery. Yes, 13.x is ideal, but 12 is ok too, especially with an engine off, depending on how long it's been since the battery was under a charged state.

Stewart

 

Try power b a s t a r d s . Com

 

Sorry, this is hard to communicate without a verbal conversation but thanks for trying so hard. If the wires/links/connectors are good between the alternator and battery, then you should read same voltage at both places. That was the case. I did not read 0 volts at alternator. I read the same voltage as at the battery.

I'm basing a lot of what I'm saying on my extensive experience with lead-acid batteries (I buy them by the pallet for my alarm business).

If I connect the battery to an external charger plugged into an outlet from my house and let it charge fully, then the alternator works after a couple minutes (~14 volts read at both battery and alternator).

A word of explanation: You are absolutely correct that anything in the 12+ range is generally fine (and is in fact typical due to loads from dome lights, etc). A FULLY charged, disconnected battery in good condition is close to 14 volts with no load. It also pulls essentially no current when connected to an external charger. As soon as you connect it to the car terminals (motor off) it will drop somewhat (in my case about 12.9 - 13.3). Once I use it start the engine, I read upper 12.8-12.9 (at both alternator and battery) for a couple minutes with a very slow downward trend, then I see it suddenly jump up to 14. I interpret this as meaning the alternator did nothing for a couple minutes, then started working.

If I then turn the truck off and use the lights to discharge to around 12 volts, then restart the truck, the voltage stays around 12 volts, indicating that the alternator is not working, and eventually the truck will not run. Same thing if I drive it for a while, shutting off the engine at a couple stops. It never jumps up (at all) unless I disconnect everything and use the external charger to again fully charge the battery.