Register now to get rid of these ads!

Technical 3G alternator no charge light wiring

Discussion in 'The Hokey Ass Message Board' started by 51504bat, Oct 10, 2021.

  1. 51504bat
    Joined: May 22, 2010
    Posts: 3,033


    3G alt.JPG charge-warning-light-wiring-14.jpg
    I'm installing a 3G alternator on the 302 I'm putting in my '54 Ranch Wagon. I wired it like the 2 attached diagrams show and it charges fine but I can't figure out the idiot light or no charge light as Ford refers to it as. My guess is that the light assembly isn't just a typical light socket with a hot and a ground. Can anyone shed some light on this? (no pun intended) Thanks.
  2. BJR
    Joined: Mar 11, 2005
    Posts: 7,152


    I believe a regular dash bulb and socket will work. That's all the GM alts use. Not sure, but I don't think a LED bulb will work.
    TrailerTrashToo likes this.
  3. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,092


    Correct on the LED bulb not working - I learned that the hard way on an off-topic field repair... The Autozone indicator light was a LED (not labeled as a LED on the package) - Ended up using a side marker lamp socket and bulb from a wrecked Chevy Citation. Zip-tied the mess to wiring harness under the hood.


    The socket for the regular dash bulb must be a 2 wire socket - If you use a 1 wire socket, the case of the socket acts as a ground and the smoke will escape your electrical system...

    The 560 ohm resistor is a great idea, that way the alternator will continue to work if the indicator bulb burns out. That said, I've never used that resistor (maybe because all the Radio Shack stores disappeared :().

    Bob Lowry likes this.
  4. The lower diagram is closer to being 'correct' but still doesn't quite reproduce the intended circuits. The 'A' wire at the regulator should be connected at the input terminal on your fuse panel. This allows the regulator to boost output voltage to compensate for voltage drop between the alternator and fuse panel under high current to maintain full voltage to the harness. The closer to the alternator this connection is, the less well this feature works. This wire only has voltage on it when the alternator is working and only has low current in it; it's only purpose is to 'sense' voltage output and adjust alternator output to keep output within it's settings.

    The connection for the idiot light is correct. How that works is 12V+ is fed in from the ignition switch, the connection at the regulator is the 'ground'. With no or low output from the alternator, there's a 'difference of potential' between these terminals and the light comes on, with brightness telling you how well the alternator is working or not. If it's working right, the light is being fed with 12V+ from both ends and doesn't light up. This is typical on Ford alternators, so if the OEM 2-wire warning light socket is still in place you're good to go with the right connections to it The one thing I'd doublecheck is the resistor value. I don't know about the 'modern' Ford alternators, but the older Ford systems required a specific-value resistor (usually 15 ohms) connected as shown to 'trigger' the voltage regulator so the alternator would start output. Unlike the GM systems, this resistor had to be in place or you'd have to rev the snot out of the motor on first start-up to put the light out and start charging. Learned that one the hard way...

    And as everyone has noted, a LED won't work.
    Bob Lowry and MAD MIKE like this.

  5. gimpyshotrods
    Joined: May 20, 2009
    Posts: 18,703


    No resistor required on a modern Ford alternator.

    The indicator light is not required for operation but it sure is a good idea!

    The alternator just needs key-on power on that terminal to function. Putting a light bulb in series on that wire can save your day. The bulb is there for the driver.

    The resistor across the bulb terminals is to provide a load, in addition to the bulb, to make a connection in the event that the bulb burns out. Without that, if you lose the bulb, you lose charging. Don't skip the resistor.
  6. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,049


    Read what Steve wrote about three times and follow his instructions. End of story.
    MAD MIKE likes this.
    Joined: Aug 1, 2009
    Posts: 674

    from 94577

  8. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 9,475

    Johnny Gee
    from Downey, Ca

  9. 51504bat
    Joined: May 22, 2010
    Posts: 3,033


  10. That's good to know, so Ohm value should be relatively unimportant. With that said, I think I'd use a resistor somewhere between 20 and 100 Ohms (rated for at least 5 watts) to insure enough voltage for proper excitation in case the lamp fails.

    If you're running a voltmeter, I'd skip both the resistor and the lamp. I'll also point out that if running an ammeter, that may or may not indicate charging system issues with an alternator so the idiot light becomes useful again.
    Last edited: Oct 10, 2021
  11. 51504bat
    Joined: May 22, 2010
    Posts: 3,033


    At this point just going with the idiot light in the stock location. Down the road maybe a volt meter, oil pressure gauge, and water temp gauge. But with a knee knocker A/C hanging under the dash mounting them might be a challenge.
    Last edited: Oct 10, 2021

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.