I recently bought a used 1998 F150 4.6l v8, seller said it had electrical issues. Battery is not charging, only showing 12.4v at battery and it goes down as truck runs. Seller had new alternator checked at parts store - said it was good. Checked fuses - field fuse, all fuses in driver side fuse box. Found only the airbag fuse blown... replaced that - no joy. Checked continuity in wires - alt single wire to its pin at firewall connector; wires from connector to mega fuse; checked yellow and orange wires at field fuse - good continuity with fuse in; Nothing seems to be bad. The only thing I notice that is throwing a red flag is a. no charge lamp on the instrument cluster - ever - not even when you first turn the ignition on, and b. there is a beeping noise coming from the passenger side floor area beeps like 4 times, and repeats for @ a minute then goes away. I have run out of ideas on what the issue could be.

 

The No Charge light on the dash is wired directly to the alternator so when it's on, your alternator IS NOT working, either because of a bad alternator or bad wiring going to it.

First check that the white/black wire is connecting the two S terminals on your alternator.

Next There are at least three fuses in the alternator/battery circuit. Check them all.

Next, I would turn the truck off, disconnect the battery and then starting checking the wiring with an ohm meter. Check the wires that connect to the battery closely, It's rather common for battery acid to migrate up inside of the insulation and dissolve the copper wire. Make sure that you have a GOOD solid ground to both the battery and the alternator. Also make sure that you have A GOOD connection from the alternator output back the battery plus and (with the battery connected) power from the ignition switch and back to the I terminal on the alternator when the ignition switch in on. That circuit is probably ok since the NO Charge light is part of it and appears to be working. You should also have battery power back to the A terminal on the alternator at all times. There is a fuse in that line and the mega fuse is also in that line and I'm guessing that one of those two is bad or else your alternator is bad. And BTW when I say to check the ground connections, I don't mean just see if there is 0 volts on them, they could be disconnected and would show 0 volts. Reconnect the battery then connect your voltmeter to the plus side of the battery and then check each ground point (the alternator body, the CLAMP and the wire going to the - battery terminal, etc) with the other probe. You should read 12 volts (or what ever the battery is charged to), if you don't then your ground is open.

Schematic of your system is here <http://ww2.justanswer.com/uploads/fordguy4u/2012-02-08_232944_a1.jpg>

 

The beeping is probably telling you that the passenger airbag off light is burned out.

 

No Charge light is all telling.
When you turn the ignition switch to On, the Ignition switch passes fused battery to the Charge lamp.
The other side of that Lamp goes to the Alternator Regulator where it gets a Ground from, to light up.
When the Alternator is outputting it's voltage, the Regulator removes the Ground, the Charge Lamp goes off.
This circuit is Open if the Charge Lamp never lights.
Either the lead is open, the plug at the connector has bent pins or the Regulator is faulty.
.
As for no charge, there is another issue because the Charge Lamp does not control Charge output of the Alternator. The Lamp is only an indicator of the function.
Some one likely has worked on the problems and added more to it.
You have to work it all out by knowing how the system works in order to sort out more than one issue present at the same time that may be causing confusion of the one issue only, thought pattern...
Bottom line is you bought some one else's little mess.
Good luck.

 

[QUOTE=rjb888;5261830Nothing seems to be bad. The only thing I notice that is throwing a red flag is a. no charge lamp on the instrument cluster - ever - not even when you first turn the ignition on,.[/QUOTE]

I misread your post and I thought you said that the light was on. OK if it NEVER turns on then you either have a burnt out bulb, a bad regulator (part of the alternator), a bad fuse, bad ignition switch or bad wiring. Disconnct the the I connection att the alternator, turn the ignition to the Run position then ground the lead that was connected to the I terminal. the No Charge light should light up. If it does, then you have a bad regulator or else the alternator isn't grounded, If it doesn't then it's one of the other items named. You can make a quick check of the alternator grounding by taking a jumper cable and clamping it to the metalic body of the alternator and then clamping the other end of the cable to the - ( MINUS) post of the battery. Don't touch the + post or you'll see serious fireworks! While you're at it, you might as well disconnect the A terminal of the alternator and check the lead for 12 volts. It should have 12 volts at all times unless your Mega fuse or the small inline mini fuse is burnt out. Don't let the wire touch ground since it is hot at all times.

You can check the ignition switch by monitoring for +12 volts at the junction box fuse shown in the diagram that I linked to. Check and be sure that you have 12 volts on BOTH sides of the fuse when you turn the ignition on.

 

Thanks. I was wondering if you had noticed that my charge light never comes on. Thanks for the breakdown, I'll check some of these after work. I know the field fuse is good, checked that, also checked continuity from all the alternator wires to their endpoints (except the one that - I assume - goes into the instrument cluster? That one I checked from the connector at the alternator to the pin at the firewall.) all continuity is good. Also just replaced the voltage regulator, so thats brand new. Previous owner said something about having the dash apart... so I am worried that there is a wiring mess up in there. Would anything up there be able to keep the alternator from putting out voltage? Seems like that stuff should only affect indicator lights.

 

Which fuse is that? I don't see on the diagram how to identify the fuse.

 

That I can't tell you, my own F-150 is far different. The only clue in the schematic is that it should be a 10 amp fuse. You need to find another schematic of that year truck and see if it says or see if your owner's manual lists some kind of Charging Circuit fuse in the junction box or just check all of the fuses there. Or maybe someone else on the forum can tell you.

 

This video may help.

 

UPDATE: After much trial and error, fuse checking, alternator testing, etc. I have found that the green wire into the alternator harness (from the start switch/battery indicator cicruit) is not supplying 12v to the alternator to "turn it on." When I unplug the connector at the firewall and supply 12v to the alternator from there, the alternator comes on and I have 14.5v at the battery. Yay!! Pending an exhaustive trip into the dashboard wiring I would like to put in a temporary fix so that the truck can be used in the meantime. I would really like some input from experienced mechanics on whether or not its safe to just keep that wire always hot, or link in some kind of switch in the cab so I can "turn on" my alternator when I start the truck. Will leaving the green wire hot cause battery drain? burn out the alternator? TIA.

 

Look further "upstream", guys. You're too focused on the "downstream" symptoms and are overlooking elements that are in common to multiple indicated faults.

Both the CHARGE lamp circuit and the airbag diagnostic monitor (AIRBAG light) rely on power from fuse F19 under the dash in order to function. You have no charging and an airbag audio warning (means the airbag light warning isn't working).

Get under the dash and 1) visually check F19 and, 2) if not visibly blown, put a meter on the fuse's test points and verify it is HOT IN START/RUN. F19 is fed directly off one of the circuits through the ignition switch.

 

The Charge Lamp circuit does NOT turn on the Alternator.

 

Not sure what is actually happening then because when I put 12v to it to "simulate" the ignition being "on" the alternator started putting out 14.5v

 

You can define it as a matter of semantics however you wish, but if there is no current flow in the I circuit, the alternator won't charge.

Suggest you follow the instructions I offered earlier. I would not recommend running it very much like you did for testing purposes as there is on current limiter when you do that.

 

I agree. Hooking up the alternator so that power is applied to it at all times will slowly drain your battery since it draws several amps of power. The power that is connected to the alternator should be run through the ignition switch so that it's only applied to the alternator when the engine is running.