- Edited 3 times, last by BoKu on .
TL;DR: I removed the Sylvania LED tail/brake light bulbs I'd just installed, replaced them with a new set of 7443 incandescent bulbs, and the problem immediately went away.
Yesterday I noticed that one of my brake lights was burned out, so I broke down and bought LED replacements for the 7443 bulbs for the combined brake/tail lights. I installed them, checked that the tail lights came on, and went to bed. Overnight, it rained, and the car was wet when I started it in the morning. I got just a little ways down the block when I noticed that the ABS light was on solid and the TPS (tire pressure) light was flashing. Uh-oh.
My first thought was that an ABS wheel speed sensor wire had chafed through, and rainwater was shorting it out. I don't want to be that guy who swaps out all four sensors in turn to find the bad one, so I tried scanning for a code with my ELM237 and Torque Pro. No joy, Torque doesn't do ABS codes. Or at least, I couldn't figure out how to do it.
Anyhow, I eventually went to the local car fixing shop, and the guy there plugged in his sooper-dooper scanner and found that there weren't any ABS codes. He did, however, find that the car had logged a brake light switch fault. And when he stepped on the brakes, sure enough, the brake lights (with the new LED bulbs) didn't come on.
That was the aha moment. Apparently the ABS system uses the signal from the brake light switch for something, maybe to verify that the brake pedal is actually being pressed when the brake system pressure increases. And with the LED tail lights, maybe the polarity for the brake portion is different from tail light portion--with incandescent bulbs, it doesn't matter, but for LEDs it does. Or maybe the current draw through the LEDs is wrong. Regardless, with the LEDs in there the ABS computer couldn't see the signal from the brake light switch, so it threw up a bunch of lights and went offline.
So I removed the Sylvania LED tail/brake light bulbs I'd just installed, replaced them with a new set of 7443 incandescent bulbs, and the problem immediately went away.
Edit add: I just went out to the car with an ohmmeter, and confirmed that the tail/brake light sockets on my 2006 tC are wired in "CK" manner as described on this page:
That means that if you plug in a standard 7443 LED bulb, in one way the tail light illuminates but the brake light doesn't, and when you turn the bulb 180 degrees, the brake light illuminates but the tail light doesn't. So you either have to buy CK bulbs, or replace the sockets so they are wired conventional, not CK. What remains to be seen is if doing either of those things still causes the ABS/TPS error mentioned above.