Hello fellas! I have an 85 f250 6.9 and the batterys aren’t charging. My original alternator was bad so I bought a reman Motorcraft one from NAPA and it wasn’t working. So I bought another one. Same thing. I took one I know works off another motor and tested it at the battery’s and at the alternator itself. NOTHING. Does anyone know why this is? Is there some switch that tells the alternator not to charge that may have gone bad? Please help. Thank you!!!

 

likely the voltage regulator has given up the ghost. On my 89 it was a square box behind the passenger side battery, not sure if it was there on the 7th gen trucks.

 

So, no voltage at all coming off of a running alternator ???

 

Field Terminal needs a bit of a voltage to "boot" alternator into producing output. Until there is magnetism in the alternator, it can't produce.
In the case of your model, there'll be a regulator under the hood, near the solenoid above the RF wheel. New alternator is probably good, and regulator or connector / wiring is faulty. There should be around +12v at the field terminal when running, as regulator s/b trying to excite Field. Terminal is marked "F" on back of alternator.
I've seen many bad connections at the F Terminal and also at the regulator plug. The plastic on these connectors is getting very brittle and weak, allowing individual connectors to slide out of header and lose contact with regulator. Also, you say your alternator was "so bad" , maybe it shorted internally and blew the regulator. I recall some Fords, years ago, needed current through the "charge" indicator bulb on the dash to "boot" the system and if bulb was burnt out, vehicle wouldn't charge. Many years ago now, so I'm not sure which ones.

On the bench, you can jumper the big + terminal to the F Terminal, spin it clockwise with a drill, quickly apply 12v to the terminals and alternator will start putting out until you stop spinning it. You can connect light bulbs, etc for a load and you'll feed the increase in torque required by the drill. If you short the alternator, it'll grab the drill right out of your hands and jump off the bench.....watch your toes....

 

This truck is a WIRING MESS. I checked the regulator and there was a little ground wire with a weird connector that was unplugged. My battery’s went from the mid 11s to 12.08 but when I turn my lights and other accessories on it drops down into the 11s again. Which is obviously not enough. I replaced the regulator just for the heck of it and nothing changed. I also checked all of the wiring on the RF and it’s most certainly molested and Jerry rigged but the diagrams say everything is wired correctly. So I cleaned things up and took the little regulator ground plug out and turned it into one wire. Still just 12.08 and 11s when accessories are on.

 

it doesn’t seem like it. When I increase the rpm the voltage doesn’t change. I have 12.08 and 11s when accessories are on.

 

I would recommend upgrading to the 3g alternator, and getting a genuine motorcraft alternator from vehicles,

1995FordWindstar
1995FordTaurus
1995MercurySable
1994FordTaurus
1994MercurySable

3.8 litre, large case 148mm with 8.25 180o mounting ..

These are very easy to wire up, and are a direct fit in the stock alternator location..

But if you want to continue using the stock externally regulated alternator, this is how it works..

(A) wire, yellow, goes to positive 12v always hot source, suggests starter solonoid hot post, this is the sense wire, it will sense your 11.5 volts and instruct the regulator to pulse the field to generate a desired seek voltage of 14 ish..

Make sure this wire is hot all the time..

(S) wire, green, goes to key on ignition hot, this will excite the stator and give it the initial juice to excite and produce electricity.

Make sure the green wire gets hot keyed on ignition...

(F) red wire, this is the wire going to pulse a hot signal to the communicator to acheive the desirable 14 total volts, it will shoot electricity to generate a magnetic field and charge through the main power outlet.

The only real difference in how the internally regulated 3g works, is that there is no red wire, because the sense and regulatory managment is internal, it gets a yellow hot sense wire, a green excite hot on run wire, that jumpers from the plug, from the middle position s on the plug, to the space s for excitement, and the lamp ignition is just for the dash light, because it backfeeds 12v when the exciter is lihht and that bulb gets hot hot on both sides so it look 'off' no ground..

But on the stock alternator those are your wires, make sure b+, hot on key, field wire is good, and then it's either a bad regulator or alternator if all the wiring is ok.

 

Ok… guess I figured out the problem. I’m missing a wire. So I put an alligator clip on the white and read terminals on the alternator and it charges great. So I think I’ll run it to the + on the glow plug relay and call it good. My temporary permanent fix.

 

thank you for the info! I’ll have to double check all that. I’ll also keep a lookout for a 3g alternator to do the swap.