ok, this is a new one for me. in my '86 240DL when i was driving tonight, suddenly the voltage went sky high and grounded my volmeter, turning all of my interior and exterior lights very bright, which i know isn't good. so if i would take it out of gear (M46 tranny), then it would go back to normal levels of voltage. also, if i would put it into a lower gear and raise the rpm's, then it would go back to normal. but eventually, it would always come back. so would this be a voltage regulator problem? i'm not quite sure what it could be, but i want to fix it soon, because i'm not a fan of my voltage being really high. yes, i know that the voltmeters are notoriously unreliable, but when all the lights suddenly get a LOT brighter, i know it's not right. so thanks yet again anyone with advice... and pardon my long absence this summer while i was in europe and at camp...
nate gundy

The most likely problem is the regulator in the alternator, (easy to replace)
The other possibility is that there is a short in the wiring loom involving the warning light wire to the alternator.
You could try temporarily removing the existing wire, fitting a low wattage (5W or less) 12V bulb between the battery positive and the warning light terminals at the alternator.
If the charging voltage returns to normal you probably have the dreaded wiring loom rot.
Colin

Sounds like a failing voltage regulator inside the alternator.

Could you hear or feel an increased load on the engine when the voltage increased? That is, when the voltage went up, the added load from the alternator on the engine should have slowed the engine slightly.

Swapping the VR should be a straightforward task. Disconnect the battery ground cable first.

Speaking of battery ------- the battery should have stabilized the voltage to within a volt, or so. But it didn't. This raises a question about the battery's quality and/or its connections to the alternator and to the battery.
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Don Foster (near Cape Cod, MA)