ALT light staying on, new alternator

[Nate's69F100]

Hey guys,

So I have a '69 f100 with a 300 l6 that i just put a new alternator on and the ALT light stays on. I think I may have it wired up wrong but im not sure. It seems to be charging fine as I have been driving it at night and running my off-road lights with no problem. Do any of you have any suggestions as to why the light might be on?

Thanks, Nate

[fordman]

the wiring is one thing to check. but also a loose belt too. i would also check the regulator and see how its doing or if the plug got pulled loose.

[Thunderfoot]

The "ALT" light is controlled by the Voltage Regulator (it is what is sensing if the alternator is working) the circuitry in the Voltage Regulator that does this could have gone bad and will cause the light to come on...
There have been some bad "New" voltage regulators that the light came on even though they were controlling the alternator fine and charging the battery fine. Try a new Voltage Regulator and cross your fingers you get a good one.

[Nate's69F100]

Alright, so I got a new regulator and put it on and the light is still staying on. I'm thinking maybe a loose wire or bad ground somewhere? I guess i'll just have to keep messing with it. Thanks for the quick replies.
-Nate

[Thunderfoot]

The way the Alt light works it wont be a bad ground... the light gets power from the same place as the instrument cluster, then the Voltage Regulator makes a Ground Connection to make the light turn on. When the Alternator is not charging or engine is off this circuit in the Voltage Regulator is like a switch and it is turned to ground, after the engine starts and the alternator starts charging this switch opens and the light will go out.
Also this same wire going to the Voltage Regulator for the light is tied into the Exciter Wire, this is the wire that tells the Alternator to start charging, so if this wire was shorted to ground your alternator wound never charge (same thing if it was a bad connection- no charge). This wire is a Green/red striped wire at the VR and inside the cab it is tied into the Alt Light wire which is the same color and also a Black wire which goes to the ignition switch which is the Exciter/Activation wire to tell the alternator to start charging (this black wire is a resistive wire as well and is Purple on some rigs) wire diagram for reference. http://www.fordification.com/tech/wirin ... rging2.jpg

If your alternator is working the only way it could be the wire is if the wire was broken from the wire going to the voltage regulator and it grounded where it broke for the wire to the light but not to the VR...

You can check to see if this is broken by unplugging the VR connector and turn the key to Run, if the light is out then the wire is fine, if the light is still on then you have a wire problem somewhere.

Been more then couple of members here get bad "new" Voltage Regulators (more the one) before getting a good one. New Voltage Regulators and Starter Solenoids are junk these days...