One low beam comes on a half-second late. (Solved, with pics of a charred H4 connector. Thank you all!)

[werdna]

Car: '03 Jetta wagon, stock headlight wiring and switch, actual headlights are the H4 Euro-style ones that everybody sells.

Sometimes when I turn the lights on, the left headlight comes on a half-second or so after the right one. I figured it was a manky bulb so I replaced it, but a new bulb behaves exactly the same way. Once it's on, it's on, and no amount of wiggling the wiring harness, blinker stalk, or light switch makes it get flaky. (IIRC the headlight wiring in the switch and stalk area isn't split left/right in any case, so that's pretty unlikely.) More likely to happen when cold.

Any ideas? A balky relay would fit the symptoms perfectly except for the part where (I think anyway) there's no relay! Aside from a sticky relay or wobbling filament, most things that I can think of otherwise would either work or not work...

Thanks for any ideas! Next up is checking/cleaning fuses, but given the predictability of the failure that seems a little far-fetched.

 

[KrashDH]

Have you pulled that headlight (or the back cover) and inspected the wiring, connections, and contacts that are internal to the headlight housing? I'd start there and look for a poor connection, maybe a little bit of oxidation is providing enough resistance for it to fire a tad late.

 

[compu_85]

I'd make sure the fuses are properly seated in the fusebox.

Does this happen with the lights in DRL mode only? Or only when turning the headlight switch on?

Both low beams are powered from the same point (Headlight switch --> dimmer / turn signal switch), then go to individual fuses before going to the headlight assembly. There are no relays involved with the dip or main beam headlights.

A relay is used to interrupt the DRL circuit when the handbrake is set.

-J

 

[werdna]

I did pull the headlight cover and go through the wiring; everything within the housing is all in very good shape. If the fuses look OK I'll keep digging at stuff between fuses and headlight, though, as that (or grounding for same) seems like where it has to be.

compu_85, thanks very much for the wiring info. I'm 90% sure I've seen it happen with both turn-on methods but will make sure.

...and people ask why I keep contact cleaner in my car. (Though really it's for the turn/wiper stalks when they start misbehaving.)

 

[werdna]

Have you pulled that headlight (or the back cover) and inspected the wiring, connections, and contacts that are internal to the headlight housing?

Turns out my answer to this question is "yes, but not hard enough", although it's been getting progressively worse for the last month so I suspect it's now a lot more fried. As always everything looked great except for the wire that was hardest to see! Interestingly this didn't change light output at all when the wire was wiggled, since the mechanical crimp over the insulation is still doing its job. The inner electrical crimp is crap, though, and is high-resistance until it gets hot. (Once it heats up, I get headlights!)

 

[KrashDH]

Thanks for posting up a solution glad you found it!