Over voltage? in a car? Could the spark from the ignition coil blow a filament?

Don't know if you've solved this or not Tony but an interesting little circuit. The only three points I see that would only effect both low beams are the dimmer switch, the point at which the low beam wire connects to the dimmer switch and what looks like a connector in the low beam wire after the grommet. Have you tried just rapidly toggling the low beam dimmer switch when this happens? I thought it interesting that the right lamps are back fed to provide 6 volts to both headlights for DRL's. Pretty crafty and cheap to do.

This is interesting to know about that circuit. That's the kind of information I'm looking for.

One thing though. This car is not equipped with DRLs. So are we sure we're looking at the right schematic?

If I knew where that "connector at the grommet" was geographically, that'd be real nice. That's something I could easily check if I only knew where the connector was.

http://farm3.static.flickr.com/2404/2093849776_af98968550_b.jpg

here's another diagram.
allegedly for a 1995 Achieva SC w/o DRL.

Interesting. Am I reading it wrong, or does that diagram have no fuses or circuit breakers on it?

What about that (alleged) timed-reset circuit breaker that only exists for the low beams?

It sounds like that schematic is not what you got. I would imagine that your car has a close variant though. The grommet is where the wire runs through the firewall into the engine compartment. The connector can be anywhere from there the where the wire splits off the go to both head lights. If this is indeed indicative of your wiring and you have determined that the lamps are not actually failing, you might try toggling the dimmer switch rapidly when this happen to see if the lights come back on indicating that the problem is in the switch or opening the hood and basically just shaking the wiring looms in the engine compartment to see if the lights come back on pointing to a bad connection there somewhere. I think it is unlikely that you would have simultaneous lamp failure. It is far more likely that you have a bad switch. Again, if your wiring is similar to this.

This sounds like the most logical explanation to me.

I did some more digging, here are schematics for achevia w/o DRL.

http://gkb.servehttp.com/images/achevia/achevia_no-drl-sch-6cyl-a.gif

http://gkb.servehttp.com/images/achevia/achevia_no-drl-sch-6cyl-b.gif

The schematics are the same for 6cyl and 4.

Wow! Another nice schematic! It looks like the switch is still the most likely suspect. This one shows a 20 amp circuit breaker but it supplies power for both the high and low beams. I thing I notice is that the flash to pass switch fires up the high beams while allowing the low beams to stay on too! Now that's got to be bright!

Tony, even though you've got some good replies, why not call the Car Talk guys anyway. Stump The Chumps!!! Hahaha.

I was going to say the same thing last night, but they brought the board down while I was mid-post. :-)

The switch, or there is a dead short on the low beam circuit somewhere between the switch and the bulb.

If it's a short the breaker should reset after having time to cool.

You could unplug a HL and jumper from the battery to each terminal to see that the bulbs light.

Or better yet, with the lights off, unplug the head lamps and check, with an ohm meter, between the terminals of the harness and ground. If you get continuity where there shouldn't be, it's a short, otherwise it's gotta be the switch or a bad breaker. Breakers do fail sometimes.

It could be a short, if it is, then the HB's won't work either until the breaker resets unless you use then passing switch, then they should work.

That's great information. I'm pretty sure the high beams work even when the lows are out. But I'm not certain of this. The trick with the passing switch is a good one... Strange they'd design it so that flash-to-pass would override the breaker?

The flash to pass is on it's own fuse. when used it is the only time that both the low beams and the high beams are on at the same time. At 50w / lamp thats 200watts @ 12 volts. Very close to the rating of the breaker.