This is weird. At least I think it is. When I hit try to turn on the high beams, my headlights shut off entirely. Turn them off, and they flash on and switch back to the lo's. As you can imagine, this is basically opposite the effect I'm going for. I'd like to get this fixed today, because I'll be driving winding dirt mountain roads tonight after sunset for a backpacking trip. I checked the fuses, which are fine. My next guess is to go after the switch and see what's up with that. If you have any ideas on what else to look at, I'd greatly appreciate it.

Before you go replacing the hi/lo relay, remove each of the headlamp connectors from the headlamps, check for corrosion, add some silicone grease, and re-seat them several times. Then repeat for the two-pin connectors with the red and blue wires, located just aft of each headlight pair. Then, similarly, clean and tighten the grounds, one at the washer bottle bracket, and the other at the end of the hi/lo relay mounting rail. Also, do the same for the hi/lo relay sockets. The relay itself is amazingly robust so long as the aluminum cover hasn't disintegrated.
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Art Benstein near Baltimore

Bakers trade bread recipes on a knead to know basis.

This may not be of much help, but i recently had my turn signal switch replaced cause it broke-duh- :oÞ~~~-!-:o/Þ----(what was i saying?-oh)-> and as the guy who was fixing it was putting in the "new" one-the high beams flickered. He said "dont worry 'bout it, it's fine". The turn signal switch when pulled back makes high beams on or off-SO-with that said-maybe could be something in your turn signal switch that is loose and shorting? (I wonder if ones/my- brain can short out?¿) Like i said-donno if that's any help. (BTW I didnt 'worry 'bout it and it's fine")! Ha. :o)Þ

Weird hi/lo beam behaviour does suggest the beam switching relay and/or maybe its connections, but this is a good time to check your headlight switch and connector. Overheating of the connector and partial melting of its plastic body are quite common in these pre-86 cars. This happened on my 83 and resulted in occasional intermittent headlights, especially on high beam.

After (for? during?) the 85 model year, Volvo incorporated a relay to take the heavy electrical load off the headlight switch and several of those cars' owners report that the overheating has now moved to the relay and its connector.
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Bob: son's 81-GL, dtr's '94-940, my 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.

Thanks gents, I was starting to think it's the relay too.

Sounds like a relay tripping out. But I haven't heard of that in your (and my, since I also have an '84) model year -- only in the newer cars.

Are you using higher wattage headlights, by any chance? Alternatively, if you have a bad ground, that could also be causing the problem.

Good luck on the drive tonight.

i bet it's either hi-lo beam relay next to your battery, or the plug that plugs onto it.