Battery light blinking - voltage checks done
 

First up - not a Ranger guy. But I've scoured the Interwebs and many of you (especially @RonD ) really seem to understand the circuity behind this light and the alternator.
I have a 1994 Ford Thunderbird SC. Battery light comes and goes. Circuit is exactly what you guys have. I've read through most of the troubleshooting here, but still at a loss. Here's what I have for voltage. Hoping you guys are willing to help a fellow Ford guy!

Key off, engine off -
Battery - 12.97v
Back of alt - 12.97v
Yellow/white - 12.97v
Lt green/red - 0v

Key on, engine off -
Battery - 12.62v
Back of alt - 12.56v
Yellow/white - 12.60v
Lt green/red - 12.28v

Now here is where it gets interesting. If I put a test light on the lt green/red, it's dim. Really dim. So with the slight load of the test light, I put my meter on and get 8.13v.

Key on, engine on, alt harness unplugged still -
Battery - 12.41v
Back of alt - 12.40v
Yellow/white - 12.39v
Lt green/red - 11.75v

And if I again put a load on the lt green/red, it drops to 7.76v.

Once I plug the harness back in, battery and back of alt come up to 14.45.

Alternate is the original one that came on the car, but I had it rebuilt by a local shop probably about 10 years ago.

Thoughts? Light have been flickering for some time, but have never been able to come with a resolution. I DID have the cluster out quite some time ago, but I can't recall if the timing coincides. It's been a few years for both of these events.

These are the only tests that mattered
Key off, engine off -
Battery - 12.97v
Back of alt - 12.97v
Yellow/white - 12.97v
Lt green/red - 0v
^^^^ GOOD

Key on, engine off -
Battery - 12.62v
Back of alt - 12.56v
Yellow/white - 12.60v
Lt green/red - 12.28v
^^^ just OK,
green wire should be 12.6 as well

Battery light circuit is pretty simple

12v(key on)---------Battery Light Bulb---------------------------(green/red wire)--------------------------Alternator/voltage regulator(Ground OR 12v)

12volt bulb lights up when it has BOTH 12v and a Ground
If both terminals are 12v then no light, if both terminals are grounds then no light

But not quite that simple, if one terminal is 12v and the other 5volt electricity will FLOW thru the bulb making it glow or flicker

With key on/engine off alternator's voltage regulator is 0 volts, so electricity flows thru the green wire(0 volts) and Battery Light is ON

When you start the engine, alternator puts out say 14.5volts
So battery light now has 14.5v from fuse box(whole vehicle is at 14.5volts)
And Voltage regulator should also now have 14.5volts, so Battery Light is OFF, no electricity is passing thru it, both bulb terminals are 14.5v
IF............voltage regulator's green wire circuit was falling below 14.5volts then Battery light would glow/flicker
So could be voltage regulator issue


The green wire is also Start UP Voltage for the alternator
Alternators can not "generate" voltage just by spinning them
Generator can, but not an alternator
An alternator needs startup voltage
That's where the Battery light and green wire come in
With key on the battery light gets 12v and that 12v passes thru the bulb and to the voltage regulator, 12.3v on your test, should be same as battery volts, 12.6v
But alternator only needs 7 volts to Start UP, so it does start up because it is generating 14.5v

But the voltage loss on that circuit could be causing the flickering Battery Light
Could be corroded connector on the "thru firewall" connector, the big round connector, or at the back of the cluster

RonD - thanks for the info. Used several of your previous threads to get this far, so really appreciate it. I'll pull the cluster (Ugh, surround is LONG out of production and so easy to break.) Has to be something back there, at least that's my guess. I'll update once I get some time to mess with it.

Once you have access to the cluster end if the green wire try grounding it to dash metal then use OHM Meter at alternator end to see if it has 0 OHMs, which means wire and connections are OK, continuity is not the same test