DIY yaris III fog lights installation

[CTScott]

Originally Posted by Eforno

I know this post is rather old, but are 3rd gen Yaris in the US pre-wired like the EU ones?

Yes. There is a DIY thread for 3rd gen US ones.

[Eforno]

Originally Posted by CTScott

Yes. There is a DIY thread for 3rd gen US ones.

Thanks! Are you talking about this one?

[Skriptenfresser]

Hello everyone! Although this topic is rather old, I want to answer, since I came here using Google.

I am also thinking about adding front fog light, so I read this thread and perhaps I know the reason for your car's behaviour, I want to give a deeper insight and there MAY be a solution.

At first, we need some general explanation regarding the wiring.
The front and rear fog lights are both controlled by the Body ECU. This ECU controls two relays, which simply connect these lights to the 12V supply. The ECU switches the lights on, when the user requests this by turning a switch on the light stalk. Usually (if factory fitted!), there is ONE(!) multi-position switch for the front and rear fog lights. This switch has three positions:
- Off
- Front Fog Light
- Rear Fog Light

The "Off" and "Front Fog Light" positions snap in, the switch keeps its position there. (like a typical on/off switch). But the "Rear Fog Light" position is DIFFERENT. The switch does not snap in this position, there is a spring pushing it back (like a push button). So if you turn your light stalk switch to "Rear Fog Light" for the first time, the rear Fog light turns on and the switch returns into the previous position (but still keeping the rear fog light on). If you turn it again, the rear fog lights turn off again. All fog lights turn off, if you turn the switch back to "off".

The key point is: The front fog light switch position connects the wire PERMANENTLY to Ground, as long as the front fog switch is in its position. But the rear fog light switch position outputs only a brief connection to GND (as long as the switch is held in the "rear fog light" position).
Furthermore, it looks like the ECU automatically turns off the rear fog lights, when it sees, that the front fog lights are turned off (=its wire is not longer grounded).

I do not think, that the ECU is intelligent enough to KNOW, that both wires are connected and somehow does something to prevent you from doing this.
What you are seeing is the correct behaviour, as expected from the wiring explained above.

What happens is:
- You connect both green wires together
- You bring the rear fog light switch into the "on" position for a short time (since the switch returns into the original position afterwards)
- This connects both wires to GND for a short time
- In the very first moment, the ECU sees the front fog light wire connected to GND and switches on the front fog light
- The ECU also sees the rear fog light wires connected to GND and switches on the rear fog lights as well
- In the next moment, the switch returns into its original position and the connection to GND breaks
- The ECU sees, that the rear fog light switch returned (no action taken)
- The ECU ALSO sees, that the front fog light switch ALSO returned and switches off the front fog light. The ECU thinks, you turned the light stalk switch to "off". The rear fog light is therefore also switched off, since this is the intended behaviour with the original light stalk (the ECU now thinks, you are using this one!)


There may be some solutions:
- Use the original light stalk with the switch for BOTH fog lights. Should work out of the box now.
- Add an additional switch for the front fog lights (on/off switch with two real positions, not a push button!), but that is perhaps not what you want. Nevertheless, this is the solution I would use, since I need to control the front and rear fog lights independently (legal reasons, some situations allow front fog lights but not rear fog lights on).
- Build a more complex circuit: Connect a toggle relay (I am not sure about the name) to the rear fog light switch, so the relay changes its state each time, the switch is actuated. The relay then switches the front fog light wire of the light stalk to the ECU.

I hope this clarifies the behaviour.

//EDIT:
Today, I successfully verified my post on my car (Toyota Yaris Hybid, latest version, bought in May 2015).

Another very interesting fact is that the front fog fuse as well as the front fog relay were already in place. Maybe the relay is integrated into one of the ECUs, I was not able to locate it. I heard it clicking and then I was able to measure +12V at the front fog light connectors. Since I am the first driver of this car, this must already have been factory fitted.
So the upgrade becomes more simple: I do not have to buy a relay or do much wiring. I will just have to add a switch and mount the light. :-)