Why do Headlights Stay On When Ignition Off?

[TromboneAl]

I've had this question for perhaps forty years:

This concerns regular old cars, like our 1999 Toyota Tacoma.

When I switch off the ignition, the radio turns off but not the headlights. For decades this has been the case, leading to millions of dead batteries and announcements in stores ("To the owner of a red Volvo, license plate 5Z1234: Your headlights are on.").

So, why have cars been designed that way for decades?

 

[justlikebike]

So you can party around campfires!

 

[RobbieB]

They stay on for a minute or so to light your way to the door so you avoid tripping and bashing your skull in.

 

[easysurfer]

So you can party around campfires!

+1

Old school artificial intelligence .

 

[njhowie]

They stay on for a minute or so to light your way to the door so you avoid tripping and bashing your skull in.

He's talking about older cars with the manual headlights lights that you turned on and were required to manually turn off. If you didn't turn them off, they'd stay on until the battery drains to nothing. I believe that some Hyundai's are still equipped with them as I remember being caught off-guard with a rental one time when it was dinging the bell as I was getting out of the car. There are also some models which do not have the warning/reminder bell and just let you leave with those manual headlights on.

 

[HarveyS]

My question has been along the same lines. My 1974 AMC Hornet had a headlight alarm that would emit a continuous beep if I left the headlights on when I turned off the ignition. No other car I have had since then has this feature. (Although now my cars have an auto-off feature).

 

[MRG]

They stay on for a minute or so to light your way to the door so you avoid tripping and bashing your skull in.

Doesn't work when you are drunk.[emoji848]

 

[CaptTom]

I've had times when I wanted the lights on, but the engine wasn't running. Probably many more the other way 'round. Not only to be sneaky (as a kid) but also, for example, pulling into or out of a driveway at night and not wanting to light up someone's house. Or coming in to work before dawn and not wanting to blind the security guard at the gate. I had to snip a wire on my last car to disable the automatic headlights for that one.

The first vehicle I had with auto-off headlights gave you about 5-10 minutes delay. I didn't know that. My kids went to a birthday party out in someone's back field. When I came to pick them up after dark, I pointed the headlights out across the field and started walking out to get them. Halfway there, everything went pitch black.

I really hate cars which try to do the thinking for you.

 

[Sunset]

My question has been along the same lines. My 1974 AMC Hornet had a headlight alarm that would emit a continuous beep if I left the headlights on when I turned off the ignition. No other car I have had since then has this feature. (Although now my cars have an auto-off feature).

I had to add a beeper to one of my early cars, after draining the battery twice

 

[Sunset]

To OP's question,
Besides the conspiracy theory of selling more batteries, the more likely reason way back when was: Want a light on, turn it on. Just like a house light you would turn on/off, or a cigarette lighter (the red glowly thing) , turn it on when you want it on.

Why else would you have to do something different ?

 

[TromboneAl]

I always figured it was either:

1. The headlights drew too much power to be routed through a circuit that is switched off (doesn't make sense).

2. It was some safety issue in case someone needed the lights on for an emergency (also doesn't make sense).

But there's got to be an answer. Even if you're burying a body in the desert, you can leave the keys on.

It couldn't just be a bad design decision—over the years, someone would have said, "Hey, wait a second!"

I'll bet most of you have left the headlights on at some point in your life.

 

[TromboneAl]

To OP's question,
Besides the conspiracy theory of selling more batteries, the more likely reason way back when was: Want a light on, turn it on. Just like a house light you would turn on/off, or a cigarette lighter (the red glowly thing) , turn it on when you want it on.

Why else would you have to do something different ?

Two problems with that:

1. There is a very good reason to have it go off when the key is turned off.
2. The same reasoning would apply to the radio or cigarette lighter.

 

[jimbee]

The first answer that pops in my head to these types of questions is 'it costs less".

 

[W2R]

Al, this is one of the Grand Questions of the Ages.

I have always wondered the same thing! When I drove a rental car on work travel, the lights would always stay on like that. I have always thought that was weird and wondered why on earth they would stay on.

I think that possibly there may be a setting to keep the lights from staying on because the lights on my Venza don't do that (and I would have adjusted the setting so that they wouldn't, given the option).

 

[Walt34]

I suspect that in the "early days" pre-1980ish it was one less thing to go wrong by leaving a switch out of a safety item circuit. Faced with headlights that don't work when needed and a need to go home right then, most [-]idiots[/-] people will drive home anyway thus endangering themselves, which is okay if they want to, and everyone else, which is not okay.

 

[Keim]

Having lived in a cold climate I'll throw out an unmentioned possibility: Old batteries didn't have good cold cranking amps. If the lights turn off with the ignition, they will also turn on when you go to start the car. This would quickly drain the power if the motor didn't start right up.

I agree with a prior commenter, I like it this way. If I turned it on I want to be the one turning it off. I don't like cars trying to out think me.

 

[cooch96]

Having lived in a cold climate I'll throw out an unmentioned possibility: Old batteries didn't have good cold cranking amps. If the lights turn off with the ignition, they will also turn on when you go to start the car. This would quickly drain the power if the motor didn't start right up.

I agree with a prior commenter, I like it this way. If I turned it on I want to be the one turning it off. I don't like cars trying to out think me.

+1. It forces the driver to turn off the lights so you have enough juice to start the car again next time.

And there are several scenarios where you want to sit for a few minutes, or even hours, with the engine off and the radio on. Most of my thoughts go to lovers’ lane with your honey, under the stars, smelling her perfume, and listening to recent hits on the FM radio.

 

[meierlde]

By 2011 my chevy cruise had a variable delay that you could set thru the drivers info center. Before this beyond a possible alarm the issue would be what would the user interface be without an lcd screen? But more generally this is because a lot of what were options on say a 1960s Cadillac, such as light minders and light dimmers, as well as of course AC, and power windows and door locks are now standard on all cars. As this minimizes the different wiring harnesses required. For example the doors may now lock at 5 mph for example. Actually modern GM vehicles have retained auxillary power systems that keep the radio etc on for 10 mins then you have to reset the clock. One big safety feature in parking lots is the ability when the key fob is in range to unlock the doors without having to fish for the keys.

 

[Al in Ohio]

I always figured it was either:

1. The headlights drew too much power to be routed through a circuit that is switched off (doesn't make sense).

2. It was some safety issue in case someone needed the lights on for an emergency (also doesn't make sense).

But there's got to be an answer. Even if you're burying a body in the desert, you can leave the keys on.

It couldn't just be a bad design decision—over the years, someone would have said, "Hey, wait a second!"

I'll bet most of you have left the headlights on at some point in your life.

Nope. And I turned them on when camping or helping others to see better when my car wasn’t running many times. Never drained a battery by not turning them off seriously. I liked that feature.

 

[The Cosmic Avenger]

Remember the days before computers and digital assistants? People expected everything to work mechanically: on was on, and off was off, that had been the default for many decades. I'll bet the idea of "smart" auto headlights would have upset people in the 60s and 70s. People didn't trust computers, probably for good reason back then, as the decision-making would have been incredibly crude compared to today. But we've become accustomed to that now, so it seems weird to NOT have it.

 

[TromboneAl]

Googling found:

Why were car headlights ever designed to stay on when the key's removed?

The first cars had gas powered headlights and many early cars that had electric lights the lights were an extra cost option (in as much as anything was standard at that time). So it wasn't that they were designed seperate as much as they were seperate or installed later.

Once lights became standard the draw of the lights was much higher than the switched ignition components. Because the contacts required were so much larger it made sense to keep them discrete from the ignition switch.

And there were other reasons to enable lights to come on independent of the engine running state. EG: turning your lights on was how you called for car hops.

But mainly automotive electrics live a very hard life. They are exposed to vibrations, extremes of heat and cold, extremes of humidity; and all sorts of harsh chemicals. Engineers had to spend a lot of time getting them reliable (not always successfully *Cough* Lucas *Cough*) and any sort of integration with other functions is/was troublesome. Keeping things separate was easier and more reliable. This has only recently changed with microprocessor control of car systems. Even then it is often advantageous, for example, to have components be vacuum actuated rather than electric.
posted by Mitheral at 2:09 PM on November 30, 2015 [4 favorites]​

and

Before the driver's station trunk release was invented, you often needed the keys for the trunk. Sometimes you wanted the lights to stay on while you did that.​

 

[ERD50]

I always figured it was either:

1. The headlights drew too much power to be routed through a circuit that is switched off (doesn't make sense). ....

It actually does make some sense.

Headlights are ~ 55 Watts each, so 110 Watts for the pair is about 9~10 Amps running. Double that if the high beam are on. And the surge current of a filament bulb can be about 10x that, so the relay needs to be able to handle a very hefty switching current, and reliability is a huge factor for headlights. And relays of that size/reliability are not cheap.

And it would need to be a relay, your old ignition switch could not switch that high current. The ignition switch would just switch the much lower coil current of the relay.

Today, they may be using an electronic switch, and of course the delayed turn off is a feature most people like, so it makes more sense and is probably cheaper to do it today than way back when.

-ERD50

 

[harley]

I've always had the opposite question. Why don't car headlights automatically come on when the car is started? And then they could stay on for a minute or two after turning the car off, then shut off automatically. As a safety feature it seems it would be cheap and easy. It doesn't drain the battery to have them on when the car is on.

And my second question: why don't the lights come on automatically when the windshield wipers get turned on. Many (most?) states have a law that when the wipers are on the lights should be too. Again, it seems like it would be an easy and cheap method of increasing safety.

 

[jimbee]

So basically, it cost less.

 

[ncbill]

I like the auto-off feature.

Just wish there was a reasonably-priced aftermarket solution for HID upgrades.

Only one of my vehicles has HID headlights and they are fantastically better than the headlights on the rest.