Hey all. My '97 F150 2WD 4.6L stopped charging on the way home from work yesterday. First I noticed the power windows moving slower than usual, then the voltage gauge dropped to zero, then all the other gauges and radio died, and the ABS and check engine lights came on. Died about a mile from home. When I got my sister to come jump start it, I found the small wire that connects to the + terminal of the battery was broken (does that wire send the alternator a charge for the coils or something?), so I stripped it and wrapped it around the terminal, jump started it, and it was charging again. Thought I had found the problem.

I didn't make a real repair to the connection last night because right after I got home we had thunderstorms, and I figured it would be ok for a day.

Went out this morning to go to work, started fine, charging gauge was good, then within 3 miles the gauge went slowly down to zero again. Checked that little wire, it was still connected, I tightened it up, still nothing from the gauge. Turned around and made it home.

I'm going to put a real connection on that wire and clean up the battery terminals, but thought I'd ask if it's possible to kill an alternator running it for that short amount of time with that wire disconnected? There's a little blade fuse in that wire that still looks good... Thanks!

 

You need to replace the alternator.
It's not putting a charge back into the battery.
Usually the brushed are worn down or the regulator fails.
With a voltmeter at the battery it's easy to see if the alternator is charging enough by looking at the voltage just after cranking.
It would be about 15 volts to force charging current into the battery.
Then as the battery recovers, the voltage tapers down to about 14.2on a fully recovered battery.
.
I don't know what the wire is you refer to. If it's not factory it may be some addition someone put there as power feed for some other purpose.
A single lead like that should have been fused at the battery point of connection to prevent fire if the lead shorts to ground at some other point.
Check all the cables and clean all connections at time of any replacements.
Good luck.

 

Ok, so I left the charger going all night, and the battery read 12.6v when I unhooked it. Then I cleaned up my battery posts and the cable ends, and added a bit of water to the battery, not that it was low enough to see the plates or anything, I just topped it off to the bottom of the filler tubes.

Started the truck, and the voltage gauge on the dash stays right around 14v, but the battery then read 12.0, and it keeps dropping as long as it's running. After an hour of friggin' around, it got down to about 11.75. Still running off the battery.

Put my meter on the neg. battery terminal and the terminal post on the alternator, it read the same as the battery alone. So the juice isn't getting out of the alternator.

Question: What has to break for the juice not to get to the terminal post on the alternator, but still show 14v on the dash?

I'm pretty broke, so I'd like to avoid replacing the whole alternator if there's something inside I can fix.

Oh, and that little wire on the pos. battery terminal is something added by the previous owner, so it's not part of the problem.

 

Take my word for it, don't put any faith in the dash gauge especially when you proved the alternator is not charging by reading the same voltage both ends of the charge lead..
There is a field fuse to check.
If ok then the alternator needs to be replaced.
The charging system is really a simple system but greatly misunderstood on how it works dynamically, by most people.
Good luck.

 

I understand not to trust that gauge as a precision voltmeter. But it can't just invent voltage that doesn't exist, can it? I mean if the alternator was not generating at all, then the gauge would stay pegged at zero when I started the truck, wouldn't it? To me it seems as though the fault must be in the voltage regulator.

If the field circuit was interrupted, wouldn't that prevent any voltage from being generated at all?

Can one replace just the voltage regulator?

 

You may be surprised to learn that the voltmeter dash
'indicator' does not read battery or alternator voltage directly.
I'm sure you never saw it move at anytime but from zero to it's normal needle position and not follow any voltage changes.
The reason is it is fixed operated by the dash microprocessor.
This is why I said not to put any value in that indicator.
The oil indicator is operated the same way and not a true indicator of varying oil pressure because it is indirectly operated by an on/off switch.
Your manual voltage test at the alternator is the real test of no output. Believe it and find the cause.
If you want to see the dash indication, the dash diagnostics has to be entered.
The system voltage will show as an Octal number that is converted to operate the dash indicator.
You don't need to do that for anything but advancing knowledge of how the dash system works.
Good luck.

 

Thanks for the patient responses, Bluegrass. I'm not trying to be a pain, I just don't get it. I did four years as a jet mechanic, I've done all kinds of auto repairs over the years, and I built a solar charging system for my camper, so I'm not a total noob, it's just that I haven't had to do much with alternators before, so I'm somewhat electrically ignorant (I HATE electrical problems).

Oh, and the field fuse isn't blown.

I do believe my test, I know nothing's going to the battery. I guess I'm stuck buying a new alternator, so I hope I can get to work and back (15 miles one-way) on just the battery until payday.

But in an attempt to understand what's going on, you're saying that the gauge can show normal when the alternator has no output at all? What would be the point of that? Doesn't whatever processor controls the gauge have to have some input from the alternator to register anything? If it just automatically shows what it should read when everything's fine no matter whether it's really fine or not, what's the point in putting a gauge in the car in the first place? I must be missing something here.

And I have seen the gauge in different spots than its normal 14v. It took 3 miles to go from from its normal spot to zero. I wasn't staring at it while I was driving, so I guess I can't say that I saw it move. But I did see it sitting at a much lower voltage than normal before it went completely dead and I turned around and went back home.

Thanks again.

 

Ok, 3 hours and $200 later, new alternator and belt are in, and putting out 14.3v. Yay! Well, yay except for the $200 part. Thanks again.

 

I just knew you could do it.
I don't blame you for trying to use old time logic but the dash has a processor that samples the DC line on a non continuous basis and converts the 'analogue' level to an Octal/Binary number for 'digital' diagnostic readout.
The so called gauge is just a dummy that just offers something to look at and it may not tell you the truth or be very useful as you found out.
The oil gauge works the same way. It is not capable of offering anything but there is or is not pressure above about 7 or 8 psi the switch operates at.
I know it's not good but that is the way it's been done for a long time.
One reason is Ford does not want the driver running for service for every little ' observed' change is real gauging, if it was provided.
My presumed take is it get's away from product perception and excessive service calls to a shop for things that are not a real problem.
Just knowing this is a leg up on the situation.
Actually very few have ever had a real problem with the way the dash works.
As you see when there is an issue you will become awhere of it, dash gage or not.
Good luck.
Glad you gotter fixed.

 

No wonder the auto companies are always in trouble. They make a dash that doesn't provide useful information, but pretend it does. They can't even make a gas gauge that tells you how much gas you have left, even though they've been doing it in aircraft for decades. They only do recalls when the cost of lawsuits surpasses the cost of the recall. They should have let GM and Chrysler go under, and let someone who wants to make GOOD cars take over.

 

I don't know if you will understand this but in order to be compatible with a computer system and PATS the dash had to be digitized.
It has it's own diagnostics called HEC.
If you would have known about that it would have shown the alternator was not outputting.
Fuel tank level works the same way. As you fill the tank the level does not rise and show on the gauge but some time later when the electronics 'updates' the fuel level gauge and it's level back to the dash. This gauge and some others are updated in time increments to reflect the state their system is in.
The engine computer is all digital at it's basic operation and needs to output digital signals to even operate the speedometer and tach.
Any outside analogue signals from sensors and actuators are converted to digital by A to D solid state devices for use by the engine and dash electronics.
Think this is bad on your truck? , it's twice as bad on all the new cars and trucks for all makes.
Yes it gets complicated for the owners who do not know these things and wonder why the expense is so high to service these vehicles..
The old days are gone forever.
Good luck.

 

Yeah, I understood it, thanks for the explanantion. But if the digital system is no better than a cable speedometer, and at times perhaps worse, maybe it's time I bought something from the sixties. I don't mind sanding the points.