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My wife's 2005 TL has been having intermittent issues with both low beams not working for a few months. They either both work or both don't work. High beams and running lights always work.
Did a bunch of googling, stumbled across threads suggesting the headlight lowbeam relay may be failing. This diagram seems to support this as a possible culprit:
I opened up the exterior fuse box and saw the fuses mentioned in this diagram and some relays, but not the headlight relays. Where are they? One thing I read said they're not serviceable which sounds odd. Is this true?
Can't get this to fail when I want it to so can't off more info. It's working fine right now.
You wrote that low beams sometimes don't work, but high beams always work.
The problem with that is that both high and low beams use same ballast, same bulb, same relay, basically same circuit. The only difference is that when high beams are ON small metal "cover" inside the projector opens, allowing more light to pass through. ("High/low beam switching solenoid" on your diagram)
So if low beams don't work, but if you pull left stock toward yourself, and headlights turn on, then I would test the headlight switch.
Problem here is that the "underlined red" wires going from switch (from switch control unit) to relay control module, and between RCM and MICU are 1s and 0s (digital communication) so there is no way to check that.
Anyways here's the switch test:
If you suspect the switch, you might decide to short the "low beam" wires together for now (pin #2, #7, and #12), and check if problem still occurs. Of course assuming the lights will turn off after the car is turned off - my '08 does that but no idea about yours.
And you are right about headlight relays - they are build-in into the fuse box, meaning not really replaceable. But if high beams work, those relays must be good.
Thanks for the very clear explanation. It makes sense that the switch is the most likely culprit. That also makes more sense to be intermittently working (corroded terminals etc). Now to figure out how to get that apart...
