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I chased the same problem for a year on my 81 242. Just now got it fixed.
Check the voltage across the battery at idle unloaded. It should be about 14 volts. Mine was 12.5. Check for voltage drop along both battery cables and the cable to the alternator. From the fat red cable at the alternator to ground should be close to the same as across the battery. Check the ground wire from the alternator to the block. There are guys on this board who know all about this stuff. Unfortunately, I'm not one of them. I just know what happened to me.
On my car, I checked all this stuff, then took the alternator in for a check. The guy put new brushes and bearings in it and said the diodes and everything else was fine, and that if I still had the problem to replace the regulator. I still had problems, so I replaced the external regulator with a $50 Bosch. No help at all. So I took the alternator off again and had it checked, this time by two different shops. Both said it was fine. After driving about six more months, with the alt light getting brighter and brighter, I took off the alternator a third time and took it for another check. The shop said it was fine. Finally I called my friend who has an ace mechanic for a son. His son told me, "You're taking it to the wrong shops. Take it to 'Warehouse Rebuilders' over on Gladiola. He'll fix it and your problem will be gone." That's what I did, and that's exactly what happened. He saw a problem with the rectifier that, the other guys just didn't have the experience or equipment to see. Problem gone.
I hope your problem is a lot simpler and quicker to solve. They usually are. Mine was a weird case, but worth bearing in mind if you have trouble finding a solution.
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Thanks everyone for all the information and advice, Doug C. 81 242 Brick Off Blocks, stock, B21F (non-Turbo), M46; 86 244, B230, 140k , auto.
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