I have an old beater 1990 240 with about 375000 miles on it. It has developed a charging problem. On start up, the battery light stays on, and there is only 11-12v at the battery while running. But, rev it just off idle, and the battery then starts charging at approx 13.90 volts.

But now, this too has become inconsistent. I pulled this alternator off another 240 that was charging, and soldered new brushes into the regulator. The thing is, it did the same thing (batt light on until revved) with the original alternator on it. So now I am starting to wonder about the fat red wire that goes to the alternator. It is pretty corroded at the alternator, and quite a few strands of the inner copper wires that make up the cable are kind of fraying at the ring connector. I am going to crimp on a new ring connector and see if I see any changes in my voltage readings. I had to crimp on a new ring on the red cable that goes from battery to GM fuse on inner drivers fender

I also had a bad battery, which I have since remedied. That was also making diagnosos a little tougher. But now days, I suspect the battery in about 7 out of 10 electrical problems in these old cars, and with the crappy quality of batteries anymore.

Any help is appreciated.