Hi all. Our '86 240 wagon that we just got had a vexing charging problem. When first started, the battery light did NOT come on when cranking, and the output would be just above the red zone on the gauge in the dash. The previous owner had told us about this, but said that if you revved the engine to approx 3500 rpm or so, the gauge would suddenly go up to normal level and stay there,even when the rpm's came down to normal. Sure enough, he was right; the meter would stay up there until the engine was shut down.
Well, after perusing this wonderful forum, I got enough info to give me a clue what the trouble was. Thank you bubba240, for your sage advice I saw on more than one post regarding alternator issues.
I went to the car this afternoon after work and located that " little red wire going to the back of the alternator". As I reached down to pull it off the terminal to clean it off, it literally fell off in my hand, being completely disconnected from it's terminal at the end of the wire. I pulled the terminal off its spade post, cleaned it up and crimped a new female spade connector to the freshened end of that wire.
I reattached that little red wire to it's terminal and replaced the battery cable. Fired up the car, noticing that I had a battery warning light for the first time. The car started and the gauge went immediatly to normal and stayed there.
Thanks so much bubba240 and the rest of the members here for being here. I hope this post may help someone else avoid an expensive alternator/voltage regulator replacement.
I am not clear how the "pre-excitement" system works or even why it is there, but I sure am glad mine was an easy fix.

 

The problem is, why did the wire go bad? Usually it is caused by overheating due to a partial short circuit in the wiring harness or the alternator. If it were my car, I would run a couple of checks.

With the ignition off, connect an ammeter in series with a battery lead. How much is the current draw? It shouldn't be more than 50 or 60 milliamperes; i.e., .05 or .06 amps. If it is more than that, disconnect the large ("B+") alternator wire, wrap it in tape so it can't contact anything, and check the current draw again. If it is now significantly less than it was before, your alternator is probably shorted. But if it's still the same, you have a short in your wiring harness. The insulation probably disintegrated. Fix it now. It only gets worse, and when it does, it takes out your alternator.

Then reconnect the battery terminal and measure the voltage across the battery with the engine off. It should be about 13 or 13.5 volts. Now start the engine. The voltage should increase to about 14 to 14.5 volts. This shows that your alternator is charging.