Car Headlights

My headlights went out, one at a time, prematurely - after just having them replaced two months ago. I strongly believe there is an electrical problem (based on having a history of little problems with them.) Anyway, I don’t totally understand why the brights work fine, but not the dim headlights. I picture the filament for the brights being in parallel to that of the dim filament; however, the rest of the wiring is all one circuit, (i.e., common to both filaments) correct? If I am correct, wouldn’t any problem with the circuitry affect BOTH dim and brights filaments?

Maybe someone can post a simple schematic to give me the right mental picture.

What make, model, year? Sealed beam?

Assuming you have dual filament bulbs the filaments are not parallel with each other, if they were, they would both be on at the same time. Instead they are two separate circuits.
On the back of your basic dual filament headlight bulb (sealed beam or replaceable bulb type) you have 3 connections.

  1. power for low beam
  2. Power for high beam
  3. Ground for both bulbs.
    The power for the two circuits may come from the high beam / low beam switch or from a switching relay. Also in some cars the low beam circuit is energized at the same time the high beams are on, in others it is an either high or low, but never both.

Are the bulbs completely dead, or just dim?

If there’s only one bulb with two filaments it works like this:

A wire runs from the battery to the headlight on/off switch.

A wire runs from there to the high/low selector. From there, a pair of wires run to the bulb. One wire to the low beam filament and one to the high beam filament. The other end of the filaments are hooked together and tied to the vehicles ground.

With the low beam selected the power runs through the on/off switch, through the high/low selector and on through the low beam filament and on to the ground to complete the circuit.

With high beam selected it’s the same except that the high/low selector has routed the power to the high beam filament instead of the low beam filament.

So in effect the two filaments are in parallel, but only one receives power at a time.

The bulb will have three contacts. One each for high beam filament, one for low beam filament, and one for ground.

The power is of course sent to the other headlight in the same way.

Some cars have individual bulbs and reflectors for high beam and low beam. It works more or less the same in principal.

I had a customer this AM that was complaining about this same thing, although his were the brights. online parts BP9005

The last bulb popped and did a number to the inside of the headlight lens assembly.There is a yellow smoke stain.

I tested his elect. system and alt looked fine, although this has been a left side high beam problem for the most part.
Well I installed a new bulb given to him at cost with the thought that maybe he was rough with the bulbs when replacing them without removing the 2 bolts that hold the assembly in place as I could not even begin to get my hands in that small area.
2 weeks ago I had customer come in with broken bulbs. each time he attempted to install a bulb it broke. Well I found the retainer clip was not aligned just right and reset its position and all was well for him and I gave him one new bulb even though it surly wasn’t CarQuests problem. A happy customer is what we want!

Will be very interested to know if any other’s have solution to your problem:dubious:

To give some feedback:
a) It’s a Honda CR-V 2005, and I am surprised since this car has been soooo good. This is our second one, and the first we hardly had to fool with in 200K miles!

b) When people say the brights are on a seperate circuit, maybe it’s a matter getting my lingo correct? Suppose I have a double pole, double throw (DPDT) switch. Is that defined as two circuits (because the DPDT switch gives two options), or is that all one fancy circuit? I guess I could see one claiming that’s two circuits (since they work independently).

c) The dim (regular) headlights are clearly out, and not just dimmed.

d) I should add that, in the past, a bulb has come back on after a good long while. If it’s a loose connection, it’s well hidden as the light never flickered as one would expect on bumpy roads, for one.

e) I should ask: Someone mentioned a relay, but what is a relay? I know they’re used on circuit boards as logic “gates”, right? But, is there an analog equivalent? And if so, what is it’s purpose?

f) I should also ask: Do the dims and brights have seperate fuses? If the dims and brights are on seperate cirucits, then I WAG they have seperate fuses? If the dim circuit’s fuse blew, then maybe the bad (dim) filament in one bulb is shorting out (and blew the fuse) in the dim circuit?

Thanks again…

A friend may help me diagnose today. I’ll let you know what we find. BTW, ironically the problem started with my left bulb as well…but the dim setting, not highbeams. (Would a halogen bulb create a yellow smoke stain, or only the older-style bulbs?)

The only electrical problem I can think of that would cause headlights to fail prematurely would be a voltage regulator. This would cause the bulbs to burn out because the higher voltage would make them brighter/hotter and burn the filaments up quicker. The high beams would not be affected as much since they are not on very long. You can test a voltage regulator by measuring across the battery terminals while the engine is on. I do not know the current specs on your voltage regulator but anything over 16 Volts would likely be trouble. Check with a real mechanic if you think the Voltage regulator is a problem.
That said, I doubt the voltage regulator is the problem. Much more likely is that you got some poorly made bulbs. You said your headlights went out one at a time. How much time between the first headlight failure and the second?

When installing your new bulbs avoid touching the glass part of the bulb, I was told this could cause hot spots that shorten the life of the bulb, another doper will correct me if I am wrong. Also avoid jarring the bulb as that can weaken the filament. Make all your motions 


When you take out the current bulbs look at the filament. If it is broken your problem is  almost certainly just a bulb.

yes DT on a switch would be two circuits

a relay is an electrical or electronic switch which is controlled by another switch. used in this case where the relay passes higher current than the controlling switch. there are a number of relays in an automobile.

separate circuits doesn’t require it have separate fuses.

And I should have included that in my post as well, yes avoid touching the bulb with bare hands. It is the oils that burn and reduce the bulb life.
A clean cotton glove works very well or I use disposable nitrile gloves also.
Everyone, well almost say, Ya I know that and well, Errr thats not reallity, they just do not want to admit to ignorance.

It might just be how my mind works, but I have always considered the low beams and the high beams as seperate circuits for fault tracing.

OK, that leaves out a bad ground or othe poor connection causing a voltage drop

A busted filament can come back on when the two ends of the busted part make contact. Makes for fun fault tracing when the lights are all working when the car comes to the shop.

A relay is an electromechanical or electronic switch that uses a small(er) amount of current to control a large(r) amount of current. This allows the car maker to use smaller wire inside the car and lighter switches. Modern cars are full of relays. The relay is probably not your problem.

Quite often yes. I have seen headlight circuits covered by none, 1, 2, or even 4 fuses. You car probably has either 2 or 4 fuses covering the headlights.

Thinking about what could be causing your issues, here is what I came up with (this is assuming that your car has separate replace bulbs inside the headlights)
[li]Improper mounting of a bulb inside the headlight. Causing mechanical shock[/li][li]bad connection inside the headlight (where the wires plug into the bulb. High heat from resistance might cause the connector to deform, or arc and either way no light)[/li][li]water leakage onto the bulb (hot light bulb + cold water = dead light)[/li][li]Fight prints or other greasy dirt on bulb (do not touch the glass while installing. If you do, clean with rubbing alcohol)[/li][li]poor quality bulbs (try a brand name)[/li][li]over voltage (not likely as I would think you would have other very obvious symptoms that would be noticed, but easy to check)[/li][/ul]

If voltage regulation is a problem, what other symptomes might I notice? Although the car is from 2005, I have 150K miles already from a lengthy commute. So, please think along the lines of what problems an older car may have with voltage regulation.

Also, someone asked how far apart in time were the two bulbs failures. I’d say between 10-14 days.

The only way to look at or think about an over voltage problem truely is with a volt meter.

Well for one thing the battery will boil and sulfate. Basically it smell horrible. A very strong smell of sulfur.
Depending on the design of the systems, you could also have various electronic systems shutting down due to too much voltage.
Snnipe 70E nailed it to check for an over voltage issue you need a volt meter.

My friend says there was no power to the low beams, so I’m off to the shop for a diagnosis. I’ll let you know what they find…

I had this very thing, where the regulator in the alternator was charging the battery to 14.0 V (it should be 13.4 V with the engine running, 12 V with an engine that’s been cold for a few hours).

Reduced bulb lifespan apart, it actually improved a number of things, notably a more solid bass on the stereo, and a stronger spark to the plugs.

An earlier car of the same model had just the same problem as Jinx; lost the dipped beams one night during a 1 hour drive to meet some friends for a gig. Initial investigations by the side of the road were not fruitful, so not wanting to be late I decided to pull out behind the next car that came along, tuck in behind it at a safe distance, and parasitise the headlights from that. Alas that freaked out the occupants of the car in front, who were wondering why a big black car with no lights was tracking their every move - I could see them talking worriedly, then speeding up/slowing down etc, all to no avail, I wasn’t losing their precious lowbeams. I discretely peeled away once we were in a streetlit town; wish I could have explained myself, but I’m sure their imagination was much more interesting than the truth.

As it happened my lights worked OK on the journey home, and investigating them properly at my leisure later revealed the problem to be corrosion issues where the relay plugged into its socket. A brush-up and a smear of Vaseline put this right.

This was a lesson in wetting current (AKA whetting or sealing current), where an electrical contact requires a certain amount of current to flow to make a good low-resistance contact. This is normally specified for relay and switch contacts by the manufacturer, but also applies to any breakable electrical contact, including terminals and connectors. Problems arising thereof can be very difficult to diagnose due to their intermittent nature and non-linear characteristics.

Most of the electrical items in a modern car are switched by relays, it saves from routing heavy (and expensive) copper wires to the dashboard, and enables the use of light-duty (and small and cheap) switches, or electronic control. But it makes tracing the wiring thereof an absolute pig.

Almost in keeping with your EXACT experience, I have learned:
a) Two friends with same model CR-Vs (2005) have similar complaints, and a complaint is on record with the NHTSA…as-is my own complaint.

b) My friend helping me diagnose found (alleged) no power to the low beams. Consulting a friend of his works on Hondas who says the circuit is “cut” (i.e., no voltage) once the bulb dies. Must be some kind of “smart circuit”, but for what purpose? …Or, is this the nature of a relay? …Or, today’s onboard computers?

Might this statement above explain the bulb’s (or relay’s) need for what you call a whetting or sealing current? (As in wheatstone bridge, perhaps?)

c) Ironically, my previous CR-V (2002) of 200K miles hardly ever needed anything done…rarely even a bulb change!

These numbers are totally incorrect.
Your alternator had better damn well put out more than 13.4V and if your 12 battery only has 12V in it you will be walking soon.
Each cell in a 12 V battery is 2.12V X 6= 12.72 volts when fully charged. Anything above 12.6 is considered fully charged. A battery will take a surface charge of over 12.72 that will dissipate after a few hours.
At about 12.45V the battery is considered 3/4 charged
At about 12.25V the battery is considered 1/2 charged
At about 12.0V the battery is considered 1/4 charged.

Now the battery also has an internal resistance of about .2V per cell. That means that to charge the battery the voltage has to be greater than the state of charge of the battery by .2V/cell. .2 X 6 = 1.2V. So to full charge a battery you need 12.72 + 1.2 = 13.92 Volts minimum. So an alternator that is putting out 14 V is doing exactly what it should be doing. BTW some systems run as high as 15.5 volts in normal operation.

Rick, on that surface charge;
I see that often when I test a charging system at the store. How do you eliminate this before testing to get a true reading?
Is it more prevalent with a dirty old battery vs a new one, or is the surface discharge that I am thinking about unrelated?

Thanks for the information.:slight_smile:

I’ve never heard of a lighting design that cuts a circuit to a blown bulb; doesn’t mean to say they don’t exist, but I can’t think of a particularly good reason why this should be done. Maybe the fragments of fractured filament could bounce against the contacts and cause an overcurrent situation, but this is a bit tenuous on my part here. Often there will be some sort of sensor to warn of a blown bulb, but this just detects lack of current when a voltage is present.

As for the diagnostics, it’ll be very difficult to faultfind without a wiring diagram, some sort of indication as to where the key fuses, switches and relays are, and a multimeter (a cheap $10 job would do here). Alternatively it might be a known common fault that could be fixed in a jiffy by someone with experience in just this very thing. Have you tried Honda CR-V forums? Someone might have sweated spinal fluid to solve this problem in order that others might benefit from their pain.

Rick, I do apologise profusely if the numbers I pulled out of my butt don’t meet your cromulence expectations, and you’ll forgive me my rounding error on the 12 V quote. The lack of decimal places here is a clue to the given accuracy. But you will find that in-car electronic systems are rated at 13.4 V, which is the average system voltage with the engine running. It can’t be too much higher than this or bulbs will start blowing; too low and the battery won’t charge fully, or will take too long to charge and won’t reach full capacity. BTW, did you mean an internal battery resistance of 0.2 ohms per cell? In which case, the actual cell voltage will depend upon the charging current. As well as the battery temperature, the integrity of the plates, the pH of the acid, and so on, there are many variables. For the record, I earn a living as a freelance electronic designer producing circuits for things like battery chargers and alternator regulators, and I take care to retain my hard-earned BS-free reputation, as the job market says I’m only as good as my last design.

All the details of sealed lead-acid battery charging can be found at the excellent