My battery light came on today. The dash battery gauge hadn't moved (It shuts down when off. But it seems to never move from a certain reading. This may need to be look at closer later!). The truck (1998 F150 4.6L, not bad for 220K miles) still ran fine. I opened Torque on my phone and it showed it hovering around 11.8 volts. Occasionally, while driving it home, the voltage would rise to ~ 12.5 volts. When I got home, I measured it with a multi-meter and it was confirmed to be hovering around the same 11.8 volts. The battery is new (~ 1 month, replaced because of a small crack that was leaking. The running voltage at that time was ~ 13.5 volts). When the truck is shut off, the voltage rose a little above the 11.8, but not by much. Maybe 0.5 volts. I suspect the regulator in the alternator to be the issue.

My questions are as follows:
With symptoms like this, are the windings, or other internal items, ever found to be a more likely suspect than the regulator?

My thinking is that it seems the bulk of the internal structure is working. It is just a regulation issue. It seems like it is getting enough current to maintain the battery. It just may not be enough for additional load, including ample charging. The regulator seems to be weakened, but not dead. I can just swap the whole thing. But, I can't lie. I am also attracted to a cheaper fix!

So, is there any experienced alternator techs willing to chime in on my tale of woe?

BTW, I do believe it is the 3G 95 Amp variety. Correct me if you disagree. (The pic was taken when I did some other work years ago. It is added for reference.)

 

Your alternator is not charging at all if you have 11.8 volts while running. The common wear point on alternators are the brushes. After that, it could be any of a number of different failures. On the old alternators with the separately mounted regulator, there is a test you can do to test the alternator and see if the problem is the alternator or something with the regulator. I bet there is a way to do this also on this alternator, but I am not familiar enough with it to give any advice on how to do it.

P.S. You have some fuses related to the alternator also, I think I would take the alternator off and take it to the store and have them check it to make sure it is bad.

 

Thanks Dave. I agree with your assessment. Fuses all look good. Wiring load tests all rule out wiring issues (Whatever ~12 volts, and not 13-14 volts, can test. I think it is close enough.). I ordered a regulator and a new belt (The belt had 40K on it. I felt it was time to change it.). I am theorizing that one of the 2 brushes is not working. I will know more when the part arrives and I remove the old part and can then view any damage. If the regulator makes no improvement, I'll get a replacement alternator assembly. I can't complain after getting 220K out of it.

Separately... I have a panel mount digital voltmeter that I am going to wire up until I can determine why the regular dash voltmeter does not fluctuate from the static reading. I now worry about the oil pressure gauge as I don't think I've every seen it change either. (Though, I have never been low on oil.) They are always in the same reading on the meter. Temp and Fuel move as their changes are easily seen.

 

I am not sure about your model, but I know the year my model came out (1989) they had a "fake" oil pressure gauge system. My truck has the full gauge set with the oil pressure gauge, but if you look out on the engine, it has the small oil pressure switch style sending unit. The real oil pressure sending unit is a very large device screwed into the engine oil pressure port, while the switch is very small.

The switch has continuity to ground any time the oil pressure is below 5lbs. What they did was add a resistor to the gauge cluster circuit board, that when the oil pressure gauge sending unit wire is above ground, the resistor makes the gauge read in the middle. When your oil pressure goes below 5lbs, the oil pressure switch grounds out and the oil pressure gauge reads zero. So my oil pressure gauge reads in the middle or zero, there is no inbetween.

I m not sure if they continued to do this in 98 or not, but if they did it could explain why your gauge never gave any meaningful readings. If you can spot your oil pressure sending unit, you should be able to tell what you have.

Here's the real oil pressure sending unit.



Here's the switch type sending unit.

 

Thanks again! I'll have to look. That is both clever and disheartening at the same time. If it is so, they could have at least explained the concept.

I am just curious. If the pump function were to somehow fail, I know that damage would likely be certain. However, if the pressure were to drop, and not trigger the reading, how do we know that damage is not already caused by the issue? At what pressure would damage be considered most likely? Could it be 6 PSI? I don't expect you to know or say the answer. But dang! (Is my frustration starting to show?)

As a question, are the sending units interchangeable, to get the functionality to work as expected? Or, is the more required to get it to work?

 

Some people do convert theirs back. They have to get in the cluster and get around the resistor, and then go buy a real pressure sender for the gauge, and then it works. Myself, I just go buy a aftermarket gauge with numbers on it.

You can tell when your pressure is really low, the engine lifters will usually start pecking, they are the first to start losing function when the pressure is really low.

 

So, I replaced the regulator only, earlier this week. It works just fine. My charge voltage is back to normal. The load test passed with little voltage drop.

Attached is a picture of the original regulator. One of the brushes shows that it was making very little contact. You can see it has just a sliver of fresh wear compared to the other. Incidentally, it was also a lot shorter than the other (1/4" or so). I suspect it wore down faster and just reached it's limit of making contact.

I did not take a picture of the rotor where the brushes make contact. Though, I meant to. It was worn down a little. The wear was even and smooth. While I am not too worried, this wear does make an argument for a rebuilt alternator (that is, as long as the rotor contact is also replaced at rebuild time!). Getting back up and running cheaply is good. It also allows me to plan for an alternator purchase (read as: save up for a good one!).

Info for future reference: The alternator must come out. You cannot replace the regulator while it is still mounted. I do wonder if there is a spec as to the minimum diameter of the rotor contact. This would let me know if I have to be more aggressive in my complete alternator replacement, due to this wear.

Again, Thanks to Dave for his contribution.