Easy 5 min anytime fog light relay mod DIY

Hey Jeff, I'm willing to give it a go on my 2020. I'm not positive, but fog wiring should be the same as 2018, right? I've got the 2018-21 wiring diagrams, although a noob at deciphering such. Not pink and gray, but red and gray?

 

If you just do the under dash mod on a 2018+ truck the fog lights will stay on all the time if switch is on. Even when key is off

 

2018 - 2021 Tundras (at least my 2020); Factory defaults

Fog Light Switch On

Headlight Switch Status:

• Off: Fog Lights are off
• DRL/Parking/Running: Fog Lights are on; DRL LEDs around headlight dim
• Low Beam: Fog Lights are on
• High Beam: Fog Lights are off
• Auto: Fog Lights are off until low ambient light sensor automatically turns on the headlights, then Fog Lights are on unless High Beam on
• DRL Off: Fog Lights are off

Ignition Off: Fog Lights are off​

Fog Light Switch Off

Yep, Fog Lights are off​

So, the only times that I cannot operate the fog lights, with ignition on, is High Beam on, or DRL off, or Auto. I don't mind having DRL on with Fog Lights, indeed, I prefer that. While it would be nice to be able to turn on the Fog Lights, independently of the headlights, when in Auto mode, I think I can live with this limitation, as there is a work-around: switch from Auto to DRL. Thus, the question, for me, becomes do I have a need to turn Fog Lights on when high beam is on? This I cannot answer; without experiencing high beam and Fog Lights at the same time, and on my Tundra, I don't know if such would be desirable, or not.

From what I can tell from the 2018 - 2021 wiring diagrams, the two wires under our dash that need to be de-pinned (cut?) and jumpered together, are Red (not pink) and Gray. The Red wire runs from our factory Fog Light switch to pin 26 of the J165 connector of our Main Body ECU. The Gray wire runs from pin 25 of the 3H connector (which is internally connected to the ECU) to the fog light relay in the engine compartment. These two connectors are in close proximity, under our dash, with the J165 located near the in-cab fuse box, and the 3H located a little to the right, closer to the steering column. It should be relatively easy to de-pin these wires from their connectors and jumper them together. Doing such would effectively bypass the Main body ECU, and enable the factory switch, itself, to directly ground the coil of our fog light relay, thus turning on, or off, Fog Lights, via their switch, regardless of the headlight switch status.

The issue with doing the above, however, is that starting in 2018 Toyota connected the fog light relay, both coil and switch sides, to a 15 amp fuse that is powered directly from the battery. Thus, if we jumper the red and gray wires under the dash, and forget and leave our Fog Light Switch on, we could drain the battery, as the fog Lights will remain on even with the ignition off. So, we need to change the direct-to-battery source of the power to the Fog Light Relay coil to an ignition controlled source, and this is not simply done. The pin on the relay that needs to be bent to connect to an ignition power source, cannot be bent and still plug the relay back into its position in the relay/fuse box. If that pin is bent, the relay/fuse box will need to be modified (slot cut to accommodate the bent pin and attached wire) for the Fog light Relay to fit back into its place. Alternatively, as Jeff suggested, the engine compartment relay/fuse box could be taken apart and internally rewired so that an ignition powered source is connected to the relay. Perhaps the easiest alternative is to leave the relay dangling in the relay/fuse box, connecting all its four pins via wires, as opposed to plugging it back in to where it should go.

This has all become too much for me to mess with, given that I do not know if the benefit, simultaneous high beam and fog lights, will justify the effort.

Someone convince me. Describe a scenario where I will absolutely wish I had fog lights when I feel the need to run high beams.

 

Not sure if relevant to anyone, as my setup is likely different than most on here, but just in case anyone browsing finds it helpful....

I have a 2021 that did NOT come with factory fogs, and therefore no switch. Rather than adding a switch I have it wired up as shown in the picture below. Fogs do not come on with DRL, but come on when it gets dark and the low beams come on. They remain on with the high beams. I had to add the relay as that space was empty.

 

Jumping the fog relay coil ground to the headlight H-LP Lo relay ground is a great solution when you don't have a switch. Did you have to add the 15A fog light fuse also?

And, do you find it useful to have fogs with high beam?

 

Fancy!

 

This is the same procedure I did with my IS300 back in the day ( although the relay was in the passenger footwell), except I was able to control fogs with the stalk still when ignition was on. I wish the tundra was the same way. But will be doing this as soon it warms up past 0 degrees

 

Just did this mod on my 2015 and it works great. My question is about adding an illuminated dash push button switch. I watched the video but I’m unsure how to wire up a switch.

 

I'd like that wiring diagram.

 

I would just like to see how to tapped to those relay terminals... It would also be nice for others if you just gave the details of how this was all wired up.