Wiring diagrams can be an interesting mental exercise. They are just so logical - like looking at a road map - but keeping track of what is flowing where and which polarity is which can make your brain hurt.
I had some pretty specific ways I wanted the aux lights to function.
The lights
Fog lights: As you may have read in an earlier post, I added some cheap 100w fog lights to the car some time ago.
Spot lights: I also built a removable light bar with three 100w spot lights
Magnetic spot light: Long ago, I built a magnetic mount for a remote-controlled 100w spot light so it can be easily attached and removed. It’s fun to aim from inside the car, and attaching it up high on the roof really lights up the road ahead. This light has its own control box including an on/off switch, so no extra switch would be needed in the cabin.
The behaviour
I wanted these lights to work in a specific way. TSD rally regulations insist that auxiliary lights must turn on/off with the high beams. This makes sense, and it also makes the lights more useable day-to-day if you can turn off all the brights with only one button.
I wanted to make use of relays to handle the high current. The alternative is to run big wires into the cabin to big switches, which is not only bulky and potentially more prone to short-circuits, the extra resistance of the wires can actually make the lights less effective. Relays are essentially remote-controlled switches - you can use a small, low-current wire to switch the relay on and off, in turn turning the high current on and off.
Also, I desired to have the lights all operate independently. Everything would be independently switched, including the high beams, so that I could tailor my lighting to the conditions. Essentially this would mean the old high beam switch on the steering column would become a toggle between low beams and whatever combination of high beams and/or auxiliary lighting I had selected on the switches.
The plan
I’ve had lots of time to think over how I want to wire things since I first added the lights. What I came up with is this.
I would cut the existing high beam wire just after the stalk switch and wire that to the new high beam switch, mounted with the fog and spot light switches. I assumed the existing high beam wire to be a relay-activating wire and therefore low current. This wire would also feed the other two switches and the three relays with positive power. Everything would be individually fused and mounted in a nice package.
When it came time to implement this plan, I found out that Subaru used a switched negative system instead of a more conventional switched positive. This means that the lights are always fed with 12 volts, but that power has nowhere to go without a connected ground. Only when the switch is activated does that 12 volts have a ground to flow to, traveling through the lights and lighting them up. This means that my plan went out the window.
I scratched my head for quite a while trying to figure out just how I could use the switched negative system to work with my lights in the way that I wanted. In a grand and sudden logical shift, I realized that I would have to make my new wiring a switched negative system too. It would also mean adding in one more relay to the existing high beam wiring to be able to turn them off.
This plan was pretty close to the old plan, but the relays are now constantly fed 12 volts of power, and that power has nowhere to go until both the stock high beam switch and the new switches in the cabin are turned on.
Thinking about things a little more, I wondered why Subaru switched the ground circuit and not the +12 volt side. I realized that it makes more sense to have as many of the resistance-inducing parts of the wiring on the negative side of the system, downstream of the lights (ie. the relays). This should make the lights more efficient and brighter. I slightly redrew the fuse and relay box to make everything fully negative-switched. This will require that the lights are grounded only by a wire and not also through the bolts holding the lights to the car.
And there you have it. I hope that wasn’t too dry, although I can see some eyes have glazed over! It should help make sense of the actual wiring work coming up.
UPDATE: I don’t know if I’ll be done playing with the wiring schematic until it’s finally done and in the car! I’ve made one (final?) tweak to the plan, that is to ground the new high beam relay at the high beam switch. Grounding it there means that there won’t be power constantly running through the switch and the relay, only when the ground is activated with the stock high beam switch. This is slightly more efficient and should help the switch and relay last a little longer.
I realized that the power would be constantly running through that relay after playing around with this handy java circuit simulator I found.
UPDATE 2: Newer wiring diagram posted in Part 10
I had some pretty specific ways I wanted the aux lights to function.
The lights
Fog lights: As you may have read in an earlier post, I added some cheap 100w fog lights to the car some time ago.
Spot lights: I also built a removable light bar with three 100w spot lights
Magnetic spot light: Long ago, I built a magnetic mount for a remote-controlled 100w spot light so it can be easily attached and removed. It’s fun to aim from inside the car, and attaching it up high on the roof really lights up the road ahead. This light has its own control box including an on/off switch, so no extra switch would be needed in the cabin.
The behaviour
I wanted these lights to work in a specific way. TSD rally regulations insist that auxiliary lights must turn on/off with the high beams. This makes sense, and it also makes the lights more useable day-to-day if you can turn off all the brights with only one button.
I wanted to make use of relays to handle the high current. The alternative is to run big wires into the cabin to big switches, which is not only bulky and potentially more prone to short-circuits, the extra resistance of the wires can actually make the lights less effective. Relays are essentially remote-controlled switches - you can use a small, low-current wire to switch the relay on and off, in turn turning the high current on and off.
Also, I desired to have the lights all operate independently. Everything would be independently switched, including the high beams, so that I could tailor my lighting to the conditions. Essentially this would mean the old high beam switch on the steering column would become a toggle between low beams and whatever combination of high beams and/or auxiliary lighting I had selected on the switches.
The plan
I’ve had lots of time to think over how I want to wire things since I first added the lights. What I came up with is this.
I would cut the existing high beam wire just after the stalk switch and wire that to the new high beam switch, mounted with the fog and spot light switches. I assumed the existing high beam wire to be a relay-activating wire and therefore low current. This wire would also feed the other two switches and the three relays with positive power. Everything would be individually fused and mounted in a nice package.
When it came time to implement this plan, I found out that Subaru used a switched negative system instead of a more conventional switched positive. This means that the lights are always fed with 12 volts, but that power has nowhere to go without a connected ground. Only when the switch is activated does that 12 volts have a ground to flow to, traveling through the lights and lighting them up. This means that my plan went out the window.
I scratched my head for quite a while trying to figure out just how I could use the switched negative system to work with my lights in the way that I wanted. In a grand and sudden logical shift, I realized that I would have to make my new wiring a switched negative system too. It would also mean adding in one more relay to the existing high beam wiring to be able to turn them off.
This plan was pretty close to the old plan, but the relays are now constantly fed 12 volts of power, and that power has nowhere to go until both the stock high beam switch and the new switches in the cabin are turned on.
Thinking about things a little more, I wondered why Subaru switched the ground circuit and not the +12 volt side. I realized that it makes more sense to have as many of the resistance-inducing parts of the wiring on the negative side of the system, downstream of the lights (ie. the relays). This should make the lights more efficient and brighter. I slightly redrew the fuse and relay box to make everything fully negative-switched. This will require that the lights are grounded only by a wire and not also through the bolts holding the lights to the car.
And there you have it. I hope that wasn’t too dry, although I can see some eyes have glazed over! It should help make sense of the actual wiring work coming up.
UPDATE: I don’t know if I’ll be done playing with the wiring schematic until it’s finally done and in the car! I’ve made one (final?) tweak to the plan, that is to ground the new high beam relay at the high beam switch. Grounding it there means that there won’t be power constantly running through the switch and the relay, only when the ground is activated with the stock high beam switch. This is slightly more efficient and should help the switch and relay last a little longer.
I realized that the power would be constantly running through that relay after playing around with this handy java circuit simulator I found.
UPDATE 2: Newer wiring diagram posted in Part 10
- mounting the fog lights
- building the removable light bar
- wiring part 1: new bulbs in OEM foglight switches
- wiring part 2: the plan (wiring diagrams)
- wiring part 3: switches (more diagrams)
- wiring part 4: parts and pieces
- wiring part 5: under the dashboard
- wiring part 6: fuse and relay box
- wiring part 7: prepping the lights
- wiring part 8: final connections
- wiring part 9: aiming the lights
- wiring part 10: updates and fixes
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