# Headlights System Diagram

Joined May 19, 2013
214
Consider the following circuit:

I understand all of it except the part with "to existing wiring harness". I would want to know how the rest of it looks.

I suppose that there are two switches: one switch for the high beam and one dip switch for the low beam. And the switches are connected with one end at the "to existing wiring harness" and the other end to the battery.

Is this the case? Because in this case I do not see the point of using relays. Just have directly the switches instead of the relays.

Or maybe there is something I am missing. The point of using relays is to command with a low current a high-voltage portion of a circuit. So there must be coming low currents from the existing harness and not directly 14V. I'm not sure.

Or maybe you know another better circuit that too includes a battery input, switch, dip switch that I would want to know.

#### bwilliams60

Joined Nov 18, 2012
1,363
You haven't told us much about your intentions with this circuit or what you plan to use it on. This circuit would be used on any vehicle where a multifunction switch or a dimmer switch is used. Power comes from the battery, through a headlight switch, through a dimmer switch and then on to your headlight relays. That is a typical system. You are using a low current circuit to control a higher current circuit through the relays. Those two wires "from exiting wiring harness" are form the vehicle's dimmer switch, generally speaking. Tell us more about what you want to do with it and we can make it happen for you.

Joined May 19, 2013
214
You haven't told us much about your intentions with this circuit or what you plan to use it on. This circuit would be used on any vehicle where a multifunction switch or a dimmer switch is used. Power comes from the battery, through a headlight switch, through a dimmer switch and then on to your headlight relays. That is a typical system. You are using a low current circuit to control a higher current circuit through the relays. Those two wires "from exiting wiring harness" are form the vehicle's dimmer switch, generally speaking. Tell us more about what you want to do with it and we can make it happen for you.
Like in the file that I attached? I drew the rest in Paint. (I also added two new fuses like I saw from some other sources)

I don't want to do anything with it, I just want to understand how an headlight system looks like and works.

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Joined May 19, 2013
214
I found here the reason for using relays.

And because of this, I guess that the switches have high resistances in series with them so that the current through them is small, while the headlamps have very small or even non-existent resistances in series with them.

But still I do not see exactly where the headlights switch and dip switch are placed. I know that how I placed them is a viable option, but I dont know if this is the case in reality.

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#### bwilliams60

Joined Nov 18, 2012
1,363
First of all, when someone is trying to help you, sarcasm is not called for. I am simply trying to understand what you are asking and what the basis of the question is. This circuit is used quite heavily on a lot of vehicles and although I am not sure why there is a DIP switch, the rest of it is straightforward. A relay is used when you do not want to run high current through as switch, which would require heavier switches and heavier wiring, taking up a lot of space under the dash. So the headlamp switch and dimmer switch would be placed under the dash or in the steering column, and this "control" circuit would be wired to a relay, usually located in an area near the battery in the engine compartment. That way large wiring is short to the battery and the load, and smaller wiring is run to the switching components. There should be no resistance "in a perfect world" in the switches and relay contact area.

Joined May 19, 2013
214
I'm not sarcastic. I've been told this before when talking (on facebook, for ex) and i still don't get why you all would think that. English is not my first language.

Anyway, thanks for the answer.

The dip switch is the dimmer switch. Two names for the same thing. I think it's us vs uk or maybe it has something to do with history.

Dip not as in dual in port, but as in 'dipping', whatever that means.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,394
I'll try with an example:

One vehicle I had used 4 fuses for the headlamps. Each light/beam was fused. the fuses were in the engine compartment It was a single headlamp dual beam system. Headlamps take a lot of current. 10 Amps at 12 V is 120 Watts, Not unreasonable.

A wire is a very low resistance and wire guage (based on cross sectional area) used is selected so the voltage drop isn't large. So, it makes sense to place a relay near the headlamps and the power source.

The dimmer and the headlamp switch is part of the multifunction switch on the steering column. The contacts only have to carry a fraction of the headlamp current. Say 500 mA or less. The contacts can be smaller and there's less risk of fire.

In another vehicle, there was a pull switch on the dash for park and lights. the high/low switch was operated by your foot. The headlight switch contained a thermal breaker. One night when i was driving, my headlights started to blink. One of the beams shorted. In this vehicle there were no relays. The switches directly operated the headlamps.

Now, we see different kind of head lamps like HID which are Xenon arc lamps and now LED lighting.

The headlamps are being controlled by the BCM (Body Control Module). the amount of daylight turns on the lights automatically, Some cars dim the headlights for the Daytime Running Light function. Interior lights can slowly dim which is controlled by the body control module.

Same deal with the window motors. Controlled by the body control module. Some of the relays have been replaced with more sophisticated and monitored control.

Now, you might find the lights being operated off a data bus (CAN) with one large wire carrying power to the trunk area, The computer can determine if a lamp is burnt out.

#### Tonyr1084

Joined Sep 24, 2015
5,574
A switch requires a manual operation (someone to turn it on and off) A relay can be controlled by the car's computer. My wife's Hyundai and my Taco(ma) both have headlight relays. If I drive at night with the headlights on, then park and shut the engine off the headlights go off because the car realizes I don't need headlights any longer. In the morning when I start the car - regardless of ambient light, the headlights come on. In short, they're only on when the vehicle is on. My old 72 Chevy Nova - I could turn the headlights on and leave them on all night with or without the key. In short, I have had many a dead battery from forgetting to turn them off (in the morning after arriving at work).

My Chevy S10 pickup had an ambient light sensor. If I set the headlights to "Auto" the computer would sense the level of ambient light and turn my lights on and off for me. I never had to worry about turning lights on or off. Relays allow the computer to control that part of the functioning.

Some cars have time delay shut-off. You can park the car in your driveway, turn the key and get out while the headlights remain on for another 30 or 60 seconds - or however long they are set to remain on, then turn themselves off automatically. This is a security feature designed to give you light while you enter the house. Again, the relays allow the computer to control the lighting.

Other reasons for relays can be for reduced weight from heavier gauge wires running into the cabin of the vehicle along with heavy duty switches. This is HARDLY the reason why they put relays into cars. Nevertheless, relays control heavier loads while allowing the interface of much smaller much lower power switches. AND the switches will last longer.

That's my take on it. Somebody correct me if I'm wrong or missed something.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,394
A Toyota I own will turn off the headlights when the door is opened and the car is off. You have to realize that.
It does incorporate an ambient light sensor.

A Chevy can be programmed how the headlights behave, but it keeps all accessories on until the door is opened. It also will turn the interior lights off if you forget to and the car is off.

In one vehicle of mine, I wired the door switch (diode OR'ed) to the dome light. Buzzer sounded when the driver's door was opened and the lights were on.

What I would like on the Toyota is a parking brake on buzzer, but tied to the Odometer/distance sensor. The tilt steering obscures the parking brake on indicator. e.g.If the parking brake is on and the wheels moved about a foot sound alarm for x amount of time + other details.

#### Tonyr1084

Joined Sep 24, 2015
5,574
Regarding my Toyota headlights - yeah, they shut off when I open the door. You're right about that. I was speaking in general terms regarding how the computer controls the relay that activates the headlights. Semantics I guess, but my Taco does not have an ambient light sensor, nor does it have an "Auto" setting on the MFS (Multi-Function Switch). My Taco doesn't even have intermittent windshield wipers. Either momentary, or low or high. Base model.

My S10 - I don't remember exactly when the lights went off but I don't think they stayed on for any length of time. The Sonata we have does not keep lights on for any length of time.

My comment about the reason for having relays was just a light comment about the subject. The computer can control them. As for the exact reason why they have relays - I don't know - maybe some engineer had a bet with a boss and either won or lost a bet. All I can say is that the computer can control the relay whereas the old pull knob with the high beam stomp button on the firewall - they didn't even have computers in cars back then.

Oh, and anyone born in 1960 is likely 56 years old. Just a guess on my part. And as to why I responded to an older thread - sorry, I screwed up (again). I didn't realize it was an old thread.

Please, may all the gods of Egypt forgive me.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,394
The computer can control them. As for the exact reason why they have relays - I don't know - maybe some engineer had a bet with a boss and either won or lost a bet.
To accommodate those funky muti-function switches on the column that seem to do everything. Wipers, washers, low, high beams, tail, and headlights.

Technology has changed and the DRL headlights can be implemented by PWM dimming.

HED arc lamps are a totally different story.

and we are starting to see LED's as headlamps.