Technical ALT indicator light wont turn off

hi guys, i jump started my friend with my stock 65 ford f250, 352FE external voltage regulated alternator, and about 2 min later, my ALT light came on, so i took off the alternator & had it tested at napa, & it was bad so i exchanged it for a rebuilt one, & installed that. i hooked up a gauge and when running it reads 14.7 v, so it's working, but the ALT light wont turn off, i dont understand why its still on, the Voltage regulator is brand new also, only 2000 miles. i guess its the sending unit for the indicator light, but i cant imagine that it would go out at the same exact time the alternator did. any help is greatly appreciated thanks

 

While it's running, ck for voltage at stator wire at the regulator (white/black). No voltage, bad wire (white/black) to the alt. or bad stator in the new alt. If you have voltage on the white/black wire then ck the green/red wire at the regulator plug with the key on. Should have voltage. If not trace it back to the idiot light to make sure there's no open wires or bad connections. The alt. light has no connection with the gauges and instrument voltage regulator. The way I recall it the alt. light works on a voltage "balancing act" between the ignition switch and the voltage regulator. When one voltage is different between the 2 it finds it's "ground" and turns on the light. That's basically how it works

 

ok ill give that a try, thanks thats pretty helpful

 

I'm getting 7.3v at the regulator and about 7.25 at the alternator, so the wire is good, but should i be getting more? or is it just supposed to be right above 7?

 

You should get just over 14 volts at the fat terminal. That one usually says "BATT".

There is no sending unit. 57custom300 is correct.

One wire to the alt bulb comes from the battery by way of the ign switch and the other side comes from the regulator. When you turn on the key 12v goes to the bulb and the other lead isn't getting anything (no power being generated until the engine starts.) When you start it the charging voltage starts rising.

When the regulator voltage equals the battery voltage the light goes out, having the same 12 volts to each side now. The voltage will rise farther but not enough to turn the light back on, because the battery voltage rises too, as it is charging.

If the charging system is weak, having less voltage coming from the charging system than the battery, the bulb will glow, & it's just the net difference in voltage which runs the bulb.

If the battery is 12 volts and the charging system makes only 11 volts, the bulb will glow weakly, seeing only one volt. If the battery is 12 volts and the charging system is only putting out 5 volts, the bulb will be very bright, as it now sees a whopping 7 volts.

 

ok thanks, that whole system makes more sense now. Is it the wire from the VR that says "I" that runs to the bulb? and the other bulb wire runs to the ignition?

 

I'm not that familiar with the specific wiring on your car. That's just the typical system used since "idiot lights" were invented. You need a real diagram for this. People can tell you, "It's the pink wire!" but they don't know if your wiring is all original or not. Your wire might be green or red.

 

The "I" terminal stands for ignition feed from the ignition switch. Circuit starts from the ignition switch on a red/yellow wire and runs to the idiot light socket. The wire coming out of the idiot light socket should be a yellow/black wire. It runs to the dash connector and changes to the red/green color that goes to the "I" terminal at the regulator. The white/black wire just runs from the alt. to the regulator. I would check the voltage at the light with the key on then ck it at the I terminal at the regulator to see if the voltage is close to the same. That should eliminate wiring concerns from the ign. switch.

You said it was charging 14.7 so it sounds like the alternator is charging and the voltage regulator is working. That said, just because the alt. is charging doesn't mean it's good. It's must produce the right stator voltage (white/black wire) to turn the light off. Ulu explained the system real good. Assuming the wiring is good, I think you have an alt. with a bad stator in it. You may have let the smoke out of your old alt. jump starting your buddy and when you replaced it you got a bad one off the shelf. You could have them test it and make sure it has the right stator voltage (if they know how) when charging.

 

ok thanks for the info 57Custom. i really do think its the alternator, all the wiring looks good and intact. ill take it back when i get the chance & have it tested. unfortunately my engine also happened to blow a head gasket 2 days ago. & its got 211k so I'm not gonna bother fixing that, I'm just gonna swap out the old 352 for a 390 & call it a day. sometime during the swap i think ill get around to having it tested.

 

Ok, i got the new engine in about a week ago & i'm just now getting to alternator issue again, i had it tested & it passed with flying colors, & the stator is putting out the right voltage too. could it possibly be the voltage regualtor that went bad when jumpstarting?

 

That's what I would try next. Maybe it got dinged when you hooked up the cables. Any of your pals have an older Ford that you could borrow theirs and give it a try? This has turned in to a very informative thread by the way.

 

Silly question I know but is the battery up to snuff? I have seen shorted cells turn a charge light on even though the alternator and regulator check out.

 

yea, the battery is good & holds 12.9 V when the engine is off. but now the light wont turn on, i checked the volts on both the wires going to the light when the ignition is on or at acc, & then engine is off, they both read 12.9,i thought the one to the Voltage Regulator shouldn't have voltage when the engine is off. if i ground one, the light turns on, so the system should work, but when engine is off, should the wire from the voltage regulator have voltage? this is begining to confuse me.

 

Do make sure the bulb is good.

 

Yes, but there's a bit more to it. Ford installs a 15 ohm resistor in parallel with the lamp and if that fails, the regulator doesn't see enough 'load' to put the light out. To check if the resistor is good, disconnect the wire from the 'I' terminal, remove/unplug the 'alt' lamp, then check for ohms between the 'I' wire and the accessory terminal on the ignition switch (switch 'off'). If it reads between about 10 to 20 ohms, the resistor is good. A higher value or 'open' reading means the resistor is bad and this will have to be fixed for the warning light/charging system to work correctly.

It sounds most likely to be a bad regulator. If you have the typical 'electronic' type, those can be killed by jumping sometimes.

 

ok thanks guys, ill have the regulator tested and see about that first then ill get to testing the resistor