This is an interesting problem for a guy who really doesn't have much electrical experience. For some time my left front headlamp's lowbeam has not been functioning while the high beams are fine. It is not the bulb as I have tested that on the RH side and it works fine there.

In the process of trying to trace this down I was using my circuit tester on the RH side to determine which wires powered the low vs. high beams. As I did this (obviously not properly...) there was a small arc and now the lowbeam on the right side does not function either.

I have checked all fuses and bulbs. The high beams on both sides work fine as do all turn signals and brake/reverse. Pilot lights in the headlamps work as well.

So, I guess I have 2 questions. First, can anyone enlighten me (no pun intended) with a quick fix to this problem? Second, as I trace the wiring diagram (no idea what I am looking at really) there are Blue/White and Blue/Red wires that I assume power the different beams. How can I determine which is which before I blow out all my lights with inept testing?

Any help is approeciated, even pointing me at previous posts (I have not seen any specific to this issue but many about Darkness in general).

Thanks!
Rob & BamBam

 

HAYNES MANUAL

 

Blue/Red is the low beam, Blue/White is the high beam. Since your hi beams work the problem is not the fuse, as both lo and hi are fed by the same (blue) supply. Initially, only your left lo was NFG, and since the hi and lo share the ground, I suspect a bad connection behind the grill under the slam panel. If your '66 is like my '80 there is a bundle of connections feeding the left side of the car about mid grill under the bonnet latch. Check and clean all of them - they are subject to corrosion because they are open to the weather. None of this addresses what you did to the right lo beam though. I'm not sure what you could have done that caused an arc but did not blow the fuse, unless you blew a line fuse (one NOT locate in the fuse block). According to Haynes, they're located on the firewall, instrument panel, and under the bonnet latch (slam panel). I can't determine the correct diagram for your car. Do you have a 2 fuse block or 4?

 

Before you get too far into it check the obvious, the bulb. You may have zapped the filament with the arc. I learned this the hard way.

 

Check the grounds (most likely)
Check the fuse block wire attachment points for corrosion - the blades corrode and lose contact
Check the switch

 

Thanks all. I've got the Haynes but as stated previously do not have the electrical experience reading the diagrams. I'll start tracing the wires back focusing on the grounds and connectors along the path as bulbs, fuses, and the switch have all checked out.

 

I second checking the bulb first. Anything that arced bad enough to do damage should have popped the fuse WAY before anyhting got damaged.

The bulb is the most likely candidate for failure in this scenario - check that, then move backwards from there if you're SURE it's OK. The fact that you can even USE a circuit tester puts you ahead of the curve - just work slow and check every connection logically, and I'm sure you'll find it. While you're in there, use the time to your advantage, and check and replace all corroded contacts (it will spare you the same diagnostic work later on) and tidy up your wiring as you go - you might as well multi-task and use the time to your advantage. Look at it this way - the next time something happens to fail, you'll already know your way around.

PS - don't feel bad - the Haynes book wiring diagrams are a BEAR to read. Half the time, I wonder if I even have the right diagram (there are so many different ones).

 

Well I have narrowed it down a bit. The connectors up front were corroded for the LH low (blue/red wire). Works fine once cleaned and reseated. For the RH Low there does not seem to be power to that connector. Tracing back it looks like that runs to the switch but I'll have to make sure. The diagram shows both blue/red wires joining and heading to the switch but there are 2 blue/reds running from the front into the main harness in my setup so I think I have to find a different diagram than the one I have been using.

Since there is a spare connection that ties to the LH blue/red do you think tying both RH and LH lows to that one wire into the harness would overload it in the short run? I have tested it briefly and it does correct the issue I have been having.

OK... I know what the answer is so nevermind... I was just looking to validate taking a shortcut against my own better judgement. Thanks for all the input as each little project builds my confidence with working on this beast.

 

Your temporary/shortcut test - loading both LH and RH on the LH feed - should tell you that the problem is with the NON-ground feed to the RH bulb. That's the wire you should trace. If you previously shorted that wire to ground at the bulb, the bulb would not be at fault, as the current bypassed the bulb altogether... directly to ground... hence the arc. Look for a burnt wire going to that bulb. If there was already a flaky connection somewhere in that path, excessive current (i.e. the arc) could have opened it up, thus "protecting" the fuse.
You're getting closer.

 

looks like the wire from the switch to the connector. it runs in the harness so I'll have to run another alongside if I can't find the burnt spot... Thanks everyone