I frequently see wiring diagrams for Driving Lights that just don’t work in many cars.
Toyota nearly always and Nissan often use a what is known as “switched earth” wiring for their headlights. They do this so that each headlight can have it’s own 12v supply and fuse, meaning in the event of a problem, you only lose one light.
In a switched earth headlight, assuming you are using a H4 bulb with 3 pins (very common) the power is switched twice. +12V is fed through a relay or switch to the common pin, and then either one of the other pins (one for High, one for Low) is alternately connected to ground through another relay or switch. If you go looking for +12V to power your driving lights or their relay in this system, you wont find it easily.
A far easier method is to always wire the trigger for your relay ACROSS the high beam bulb circuit, instead of from +12v to ground. This means that regardless of the vehicle wiring, positive or negative switched, the driving light wiring is the same.
There are several other benefits to wiring in this manner.
a) It works with either positive of negative switched headlights
b) It avoids problems with the relay not dropping out. The high beam indicator inside the car can trickle enough power through to not let the driving lights drop out. It takes about 8-9V to trigger a 12V relay, but only about 4V to hold it in.
c) It avoids problems with the relay not dropping out due to a poor contact on the headlight connection. This common fault can setup a circuit through the other filaments and cause enough voltage to be present on the high beam filament to hold the relay open. This voltage is far less across the filament than in relation to ground.
So how do you wire it?
It’s easy really. I have gone with text, as many people have trouble with electrical diagrams.
Power CCT (Heavy wire)
Battery – Fuse – Relay (Pin 87) – Relay (Pin 30) – Driving Lights – Chassis (Ground)
Switch CCT (Light wire)
Headlight Common Pin – Switch – Relay (Pin 85) – Relay (Pin 86) – Headlight High Beam Pin
Simply tap into the headlight wires / pins with vampire taps.
Presto – I guarantee it will work.
If you don’t have H4 bulbs, even easier, simply go straight across your high beam bulb wires for the trigger.