Headlight Wiring

[sleep_deprived129]

I have an '88 camry and I recently put a set of HID's in it, the stock one just weren't cutting it around where I live. However, the new bulbs do not have a 'high beam' part to it. What I wanted to do was to install a set of driving/fog lights and hook them up to the high beam circuit. So esentially when I hit the high beams, I want to have the low beams and the driving lights be on. What would be the best way to go about wiring this up? I know currently when the high beams are 'on' the regular headlights do not get power. Any help is appreciated!

[nopain00]

I assume that this may not be a problem, but you may want to make sure with your local regulations/state inspections that this is allowed. I don't imagine it to be a problem, but there are some restrictions on the intensity of driving lights.

That point aside, it would be a fairly easy fix in theory, but the logistics of where to mount the wiring and components may take some work. I'm not at my car at the moment, so I may be mixing up some of the color codes, but the principle should hold.

The basic headlights operation is as follows:
The main 40A fuse provides constant +12 to both the coil and the switch of the headlight relay. When the light switch is turned to "on" (vs. off or parking lights only), the coil is connected to ground, closing the headlight relay.

Once this happens, +12 is supplied to the LH and RH fuses, and then to the + terminal of the headlights. Each headlight has 3 terminals: 1 positive terminal, and 2 grounds (one for low beam and one for high beam). Depending on whether the combination switch is at the "low" or "high" setting, the current goes through the positive terminal and down the red-white or red-blue wire to ground.

If you were to connect these two wires together--the two grounds--both the low beam and the high beams would go on at the same time. This is almost what you want, since you want both only if the switch is at the high position, not at the low position.

To accomplish this, you'd need to install a diode between these two wires. As you may know, a diode allows the flow of electricity in one direction, but not the other. It's represented by a triangle followed by a line in schematics. The triangle "points" in the direction of the current from + to -. If you installed a set of diodes for each bulb pointing from the low beam to the high beam terminal, when you hit the high beams, the diodes would allow a secondary path to ground for the low beams, but not vice versa.

A few considerations:
-The headlight fuses are rated for 15A each, so make sure that your HIDs+driving lights don't exceed that.
-To do this wiring, I suggest you use a small hobby box to include the diode and have some sort of quick-connect terminals pigtailed from it. That way you can mount the box in a relatively dry location and easily swap back to the factory wiring if you need it.
-You'll need to choose a diode that will handle the current of both sets of lights together.
-You should do this separately for LH and RH bulbs.
-This will likely also cause both sets of lights to turn on when you flash-to-pass and some HIDs don't like this. Wiring this to flash only the high beams (now the driving lights) is a bit more complicated.

Hope this helps.

[white90dx]

Originally Posted by nopain00

A few considerations:
-The headlight fuses are rated for 15A each, so make sure that your HIDs+driving lights don't exceed that.
-To do this wiring, I suggest you use a small hobby box to include the diode and have some sort of quick-connect terminals pigtailed from it. That way you can mount the box in a relatively dry location and easily swap back to the factory wiring if you need it.
-You'll need to choose a diode that will handle the current of both sets of lights together.
-You should do this separately for LH and RH bulbs.
-This will likely also cause both sets of lights to turn on when you flash-to-pass and some HIDs don't like this. Wiring this to flash only the high beams (now the driving lights) is a bit more complicated.

Hope this helps.

I will second the possible/likely illegal nature of this modification...

That said, I do want to correct one technical detail - you will not be able to power the headlights or HIDs directly through any diodes. You will need to trigger a separate relay using diodes to power the HIDs. You can power high beams directly from the high beam wiring though. I'd have to pull up the diagrams for more specific details than that though...

-Charlie

[nopain00]

I didn't have the wiring diagrams in front of me. Charlie, why are you saying that you can't power the headlights directly through diodes?

[white90dx]

Originally Posted by nopain00

I didn't have the wiring diagrams in front of me. Charlie, why are you saying that you can't power the headlights directly through diodes?

You loose 0.5-0.7V through the diode, and it has to handle ~5A continuous current... It is technically possible, but it is a waste of power (and you have to buy fairly high current diodes) and is bad for performance of the lights.

By using the diodes to activate a relay, you can get brighter headlights - since you can use much better wiring directly off the battery for the lights (through the relay).

-Charlie

[nopain00]

Originally Posted by white90dx

You loose 0.5-0.7V through the diode, and it has to handle ~5A continuous current... It is technically possible, but it is a waste of power (and you have to buy fairly high current diodes) and is bad for performance of the lights.

By using the diodes to activate a relay, you can get brighter headlights - since you can use much better wiring directly off the battery for the lights (through the relay).

-Charlie

Yeah, I see your point. I've used Schottky diodes for other projects, which have lower voltage drops, but as you said, it is still a waste of power.

Your idea of the diodes' activating a relay is good. Actually if he does that, he may be able to wire a second relay in series so that his flash-to-pass setting on the combo switch will only flash the fog/driving lights.

I'm imagining a normally closed relay in series with the other one, which gets energized when terminal 14 of the combo switch connects to ground (during the pull flash-to-pass feature) and subsequently opens the circuit. That way the HIDs go on with the high beams (pushing the combo) and not when he pulls it.

[sleep_deprived129]

Yeah I have decided to nix the HID idea, enough research of people saying they don't work well in our cars and the legal issues as well. Gonna go with some sylvania silverstar ultras or ge nighthawk platinums, anyone have an opinion on those? Also, I would still like to put in some fog lamps/driving lights. Anyone have some suggestions for good ones of those? What is the difference, if any, between fog lights and driving lights? I'm just lookin for a little extra light on dark streets and on the highway.

[white90dx]

Generally, fog lights project a wide short beam of light to allow for driving in poor visibility conditions (supplementing low-beam lights or alone for reduced glare). Driving lights project a long, thin beam of light to aid in high speed clear weather driving (supplementing high-beam lights).

-Charlie