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2018 factory fog light diagnostics

277 Views 2 Replies 2 Participants Last post by  gwork
I'm adding fog lights to a 2018 LE using the factory wiring, so I've added the relay, verified the fuse (thanks Toyota,) added the lights, and swapped out the turn signal stalk to include the fog light switch. No lights.

I've seen a variety of wiring diagrams and at least one "Toyota diagnotics," but none appear to be exactly aligned with what I'm seeing. For instance, the diagnostics I have suggest power should come to the relay on terminal 5. On this one, Terminal 5 goes to the lights and power comes in on terminal 3 from the fuse, with or without ignition/lights on.

Does anyone have the diagnostics that go with 2018s, or practical experience to help me determine where the oddity lies? TIA!
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What you're seeing on the fog relay is correct. Fog light power comes directly from the battery, through the fog light fuse, to pin 3 of the fog relay. So its always there regardless of ignition switch/light switch position. Power goes out to the fog lights on pin 5 when the relay is closed. The relay coil is energized by power coming in on pin 1 when the taillights are on. The relay coil is ground by the body ECU via pin 2 of the relay. The body ECU connects pin 2 to ground when it detects that the fog light switch is in the ON position. With a DMM you should be able to figure what's happening. My only fear is that the body ECU is ignoring your switch status and not grounding the relay because some option is not configured for your car (that would suck).

Also, I recently learned that my local library has online access to Chilton manuals with wiring diagrams etc. I've heard some libraries even have Alldata subscriptions. I highly recommend checking yours.
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Excellent. Then I can verify power is on pin 1 from the taillights and ground is provided on pin 2 when the switch is on. I’ve had the nagging feeling that something might be a config option in the ecu, but I’ll have to verify the switch is working before I go there.

thanks!


What you're seeing on the fog relay is correct. Fog light power comes directly from the battery, through the fog light fuse, to pin 3 of the fog relay. So its always there regardless of ignition switch/light switch position. Power goes out to the fog lights on pin 5 when the relay is closed. The relay coil is energized by power coming in on pin 1 when the taillights are on. The relay coil is ground by the body ECU via pin 2 of the relay. The body ECU connects pin 2 to ground when it detects that the fog light switch is in the ON position. With a DMM you should be able to figure what's happening. My only fear is that the body ECU is ignoring your switch status and not grounding the relay because some option is not configured for your car (that would suck).
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