Electrical Guru needed

[Heron]

At the risk of not being very helpful. Recently, after the car had been sitting a couple of weeks while getting an injection pump and fueling issue sorted out, the charging light wouldn't go off when i revving the the trucks engine.  Nothing had been changed with the electrical system except the ignition switch had been replaced several months before without any new and/or associated problems cropping up.  It took about 3 miles of driving before the light went off, something it had never done before and hasn't done since.  The truck already had the alternator case grounded to the battery directly (like the starter).  i might have missed it but am not sure if you have been able to take the car for a short drive?

These cars do strange and un-explainable things; it can get a bit crazy. May be put the light issue on the back burner, relax and watch a game or two, till you can drive it around the block a couple three times?

The truck is no near roadworthy yet. I would drive it and see if possible. Before I picked up the truck from my father's house he had said there were no lights on in the dash. The truck has not been run for more than 15 min since then. I was not running it until I switched out the timing belt.
I'm trying to get the basics in check and work from there but you advice is appreciated. B

I'm going to start by running a jumper ground as Tyler suggested. I'm also going to check voltage at the radio fuse as I read somewhere that fuse controls the guage cluster.
If that doesn't work I'll snip the exciter wire close to the cluster and run a brand new wire.
I'd feel better if there were like 2 or 3 volts at the exciter wire but the fact there are 11.3 volts and it's not enough to trip the field...argh..although like Tyler said it may be current issue. Although wire looks perfect all the way thru, nothing butchered up under the dash.

[Rabbit79]

This voltage on the blue wire sounds all wrong.  I was under the impression that the exciter wire was only providing voltage when the alternator is running.  Not with the key off. 

Sure sounds like something is fouled up with the blue wire.  Shorted to ground from mouse chewing?   

This makes me want to pull out the voltmeter and go pop the hood on my 81 for you but the sun just set and I would only have 15 mins of daylight left.  Not enough to really get started as I don't have a garage space to work in.  Maybe tomorrow if it is sunny.

OR Coaster:
The excitation voltage is there to get the alternator up and running in the start up stage. You could make an analogy with something like the pilot light on a gas stove. It gets the main burner going, but once the main is up and running, the pilot is no longer needed. Once the alternator output becomes equal to the battery voltage it becomes self exciting and the external excitation voltage is no longer needed.

Heron:
As I read it in the Bentley wiring schematic, current for that circuit comes off the number 15 buss (energized with key in 'on' position), goes through the warning light in the dash, and then out to the alternator, where you're getting 11.3 volts. The only thing in that circuit that would cause any voltage drop is the light bulb, and I have my doubts as to whether that little tiny light bulb would cause that much voltage drop. As you say with a wire jumpered directly from the battery you get the alternator to charge, so I'd say you have your problem isolated to that particular circuit. You did say that in the past you've had trouble with the warning lights in the dash. That might be an area to check. I've never had one of those apart but I suspect it's just a circuit card with a bunch of light bulbs soldered to it (or possibly LEDs, not sure if they had those back then). When those things go cattywhompus they can cause all sorts of electrical havoc. In addition to everything that Tyler has already mentioned that might be something to check into.

[ORCoaster]

Thanks for the analogy.  Makes sense to me.   

What is odd here is that the light comes and goes with the key as it should.  But stays on when running.  Which it should not.  But yet the test of the alternator seems to have the needed output back to the battery which should shut the light off.

So is there something wrong with the voltage regulator in the alternator?  There is nothing on the cluster that I can think of that controls much of anything.  It is a piece of plastic with a film on the back that has thin printed circuit like lines that supply voltage to the sensors and light locations.

Could there be some crossing of voltage going on in that big white plug that inserts into the back of the cluster?  Voltage for something like the temp sender being sent to the light on the dash making one think the alternator is not charging?

Gremlins are dancing on your hood here.

Get out the bells.

[Rabbit79]

In a way that little light on the dash does control the alternator. Since the excitation current runs through it, that circuit has to be intact for the alternator to even get started. As the alternator spins up and it's output level reaches the level of battery voltage, it becomes self exciting and shuts off the external excitation voltage(the blue wire). I can't say that I know exactly how it does that though, but anyway that's why you're warning light goes out when the alternator output comes up. Transversely.... if the alternator were going bad, and it's output level dropped below battery voltage, all that would be reversed and the warning light would come on.  In this case, Heron in his troubleshooting has eliminated that circuit by running a wire directly from the battery to the excitation terminal on the alternator in it's place, and in so doing got the alternator to charge. This tells me the alternator and regulator are good, and somehow he's not getting enough current on that exciter wire to get the alternator up to proper output level.

[TylerDurden]

You pretty much got it...

The light goes on when it has 12V at the key/relay end of the circuit and near zero volts at the alternator... when the alternator is generating 12+ volts, the flow of current through the light (led) stops because both ends of the wire then see 12V. (no difference in potential, no flow)

So, why doesn't the wire excite the alternator, when a direct jumper does? Not enough current, voltage or both through the wire. Often strands are broken inside the insulation of the blue wire and it looks ok and meters ~12V... maybe there is resistance in the ground path and the combination of low voltage in the wire and resistance are keeping the coils from adequately generating magnetic fields. 

It could also be something dragging down the system voltage, like a stuck/sticking GP relay... maybe in combination with the other factors. I know I need to rev my 86 Jetta faster by a couple hundred rpm to start charging during the post-glow period.

[RustyCaddy]

Are the cables to the battery good? May be you posted about that already and i have missed it. i don't know why my charging light acted up awhile back and then fixed itself but...

i disconnected my negative battery cable today to check/replace my glowplugs. When reconnecting the battery cable the clamp broke in half; the red suffered the same fate in 2013. Likely it was already cracked. Since the problem goes away when you run a direct lead from the battery that made me wonder about the soundness of the clamps to your battery posts given that the cables on both vehicles are likely close to the same age.  Easy to double check for cracks anyways. Sorry this whole thing is such a nightmare for you.

[Heron]

SOLVED!
I did try grounding the alternator to the battery negative terminal. Did not help.
Basically I cut the exciter wire back at the battery area and tested it there. I had good voltage. I went forward to the alternator, tested it there and saw the drop.
Even though the wire looked good I cut it back right before it went into the the loom(what Tyler said) that travels under the radiator and spliced in a new wire.
Ran it a bit and now it is charging.
Battery is up as I had it charging the entire time I was messing around with this issue but when the alternator kicks in if I have the lights on it drags the alternator down so much I have to idle it up but when the lights are shut down it idles up too much...probably another thread to start..
Thanks much for all of your help with this...Ask me what I did during my time off over the holidays and I'm still not done. 
Look for some more threads..I need to post some pics and get some advice on what to do with this truck.