Blown Brake Light Bulb Mystery

02 Volvo V70xc. This car blows a brake light bulb generally every week. I'm rather baffled as to why.

Seems to be an over charge thing, not a vibration thing. The filament is normally poofed all over the inside of the bulb, just like an old flash bulb or such. Very rarely do I find one with a simple broken filament.

Brake light bulbs only, never tail light or turn signal bulbs. Only the side bulbs, never the center light.

Doesn't matter what bulbs I use. Cheap junk to expensive super duty Sylvania, poof!

I can't come up with anything that would be producing a transient high voltage to the brake light bulbs. There is never any indication of voltage spikes from the alternator. Nothing ever glows brightly, no recording of high voltage via the Torque app and an OBD dongle.

I'm at a loss, and just continue to replace brake light bulbs on a regular basis.

Any of you folk have some sage suggestions, experiences or off the wall notions?

Any chance of water finding it's way into the socket? I just went through something similar with WildScotWife's Focus.

Have you seen this thread? http://www.matthewsvolvosite.com/forums/viewtopic.php?t=48814

Gotta be a bad ground.

I would check for over voltage first. That's really the only thing that can make them burn out like that. I was blowing out both my headlight bulbs on a regular basis on my MKII GTI. I noticed driving home one night that my lights would occasionally get super bright and then dim back down to the yellowish glow. Replaced the voltage regulator and problem solved.

As mentioned earlier the brake light relay is a common failure on those too. I can't see how they would cause your symptom, but should be replaced on general principle. Hopefully it's not the light/electric module. Those aren't cheap and I believe require programming after replacement.

This is your rear lamps:

Though it appears to be one large lamp, it is really two lamps. So, your brake light is not really in the middle, it is at the bottom of the top piece. As such, if water is getting in, it then may be pooling and taking out the bulb.

I had similar issue on a different car. My answer was to not try to keep the water from getting in but rather, help the water get out. At the bottom of my lens, I drilled a couple of holes to allow the water to escape easily. I have not lost a bulb since. Like you, every time I replaced a bulb before figuring it out, the actual glass of the bulb would be broken not just a bad filament.

There is no water pooling in the lamps. Everything is dry and shiny clean in there.

I've no indications of overcharging. Nothing ever goes bright, and the data logging shows no signs of excessive voltage. This is only showing up in the two side mounted brake lights, no other bulbs anywhere on the car are doing this.

Ross, I did not know about that recall and change. This is very interesting! I've no idea if that was done on my car, but I will check it tonight. I also don't think the manual I have showed that circuits that way. With a relay controlling the side mounted brake lights separately from the top center mounted light, that's certainly a location to examine. Especially if I've the older combined relay. It's the most likely.

Though how I'd be getting an overcharge from anything there, I don't know. As far as I know (semi-dangerous assumption), everything is 12 volts there. So if it's doing a weird transient short, I don't see where high voltage would be coming from.

But... if it's sticking and holding the side brake lights on...maybe that could do it. I really doubt this is happening though. I've never seen any sign of the brake lights staying on, and I or a friend is often behind this car when driving.

There could be an over-current condition rather than an over-voltage condition.

RossD wrote: There could be an over-current condition rather than an over-voltage condition.

That's what I was thinking too. Is there anything at all in series with those bulbs? If so, and there was a short, the bulb could be behaving a bit like a fuse.

This may or may not be helpful, but with old SAAB c900s it was routine to solve a tail light problem by giving the tail light housings thier own individual ground wires.

A bad socket will pop a bulb on a regular basis as well. GMs are famous for this.

I'm not sure how you could feed a tungsten filament incandescent lamp too much current, they're self regulating in that regard, more heat=more resistance. They actually draw a ton of current at startup because of this.

I'm voting bad connection somewhere. I'll hazard a guess it would throw codes for a bad voltage regulator, and if it were that you'd see other bulbs fail.

Well, I found I do in fact have the old double relay that was recalled.

Double checked my manual, and it does not show this. It shows a more conventional wiring.

The replacement relays aren't expensive, so I'll try that.

Still don't see where the high voltage is coming from that blows the bulbs so dramatically, but...

The bulb sockets are excellent. I'm used to bad sockets causing failure to illuminate, not blowing the bulbs.

Glad to hear you figured it out. Troubleshooting electrical problems is a lifelong learning process.
1. My first lesson was working in a motorcycle shop in high school. A customer brought in a motorcycle that would illuminate the headlight when he applied the brakes. After some head-scratching and looking, I found the top element in the dual element bulb had broken loose at one end and made contact with the lower element. 2. I had an '83 Volvo wagon with intermittent stalling that I tracked to the lift pump in the tank. I checked the fuse and it was good. I started at the pump connector in back, started pulling panels all the way to the front, then found a loose blade connector where the hot wire plugged into the fuse box, under the dash...,

With loose wires, the system can have some capacitance (depending on length of wires, what they're run next to, and what else is on the circuit) that can build static when the circuit is open, then discharge when it makes contact.

jstein77 wrote: Gotta be a bad ground.

^^^ this

Weird I had an issue with my buell where I had a busted filament in my dual filament brake light, go over a bump and the thing would blow a fuse because the filaments met, that was fun to diagnose. I also vote ground issue

Oh I haven't "figured it out" by any means. I've simply found I've an old combined relay that has a known problem with sticking. Mine is not doing that. There is no indication that anything is wrong with it. But, it's a location where I now know there is a clear separation of the lower brake light bulbs (that keep blowing) from the upper center lamp (that never fails). Cheap enough to replace the unit with the newer setup of two single relays, and see if somehow that makes things happier.

Ohms law still applies to this car. I=V/R. This is why a bad socket or shorted ground will generally not blow a bulb. Oh, it'll cause one to not light when it should, or light when it shouldn't. It can also cause fun dim illumination as the current travels through two filaments (1147 double filament bulb typically, I've got 1146 single filament bulbs). But they won't cause a bulb to blow the filament apart.

I'm not willing to state absolutely that it's an over voltage (and therefore over amping) situation. But the evidence strongly suggests it. Almost every bulb is metal fume coated on the inside, exactly like every bulb I've seen where I know the lamp has failed from over voltage, either deliberately (flash bulbs, etc), or accidentally (12 volt boosts on a 6 volt vehicle, etc).

Now where the heck that high voltage is coming from is rather baffling to me. As far as I know the canbus connection the double relay is connected to (which is not shown in my repair manual) is still a 12 volt circuit.

That said, I've also been around enough to see things happen that shouldn't. For example, I have fried a motorcycles electrical system with a car battery. Ohms law says that cannot happen, it's 12 volts, just like the bike. None the less, it happened. Experience has shown it can (rarely) happen, and this is the basis of the generic caution about jumping a motorcycle from a car.

All of which reminds me, I need to see who has those Volvo relays in stock.

In reply to foxtrapper:

First thing. I need a clarification. Is this just 1 brake light bulb (on one side), or is it bulbs on both sides that are blowing?

If it is just one side all the time, the problem can be isolated to things that feed or touch that bulb. If both sides, the culprits expand to include anything that is common feeding both bulbs.

Does the power to light these bulbs come from something simple like a switch or relay or from a "controller"?

The 3rd brake light might be separately fed from the side lights, or it might be an LED affair which could be internally current regulated and thus pretty immune to voltage surges that could take out the incandescent bulbs.

There is one 1146 bulb on each side, either of them will blow quite frequently, and sometimes both will blow. I do not think it has ever blown both bulbs at the exact same instant, though it does sometimes blow them within hours or days of each other.

There is a canbus controlled relay that controls the power to the side lights. This is separate from the center third brake light, which appears to be an LED strip. I did not know this, as the Haynes manual I have has a wiring diagram for the tail lights that does not show this at all. The link given by Ross taught me about the separation and the relay that I did not know was in play.

Volvo originally used a combined relay for both the side mounted brake lights and the rear fog lights (another set of 1146 bulbs, mounted separately in lower sockets). I've no idea why Volvo went with a double relay, but they did. This relay was problematic and Volvo did a recall to fix the ones that were acting up. Acting up was either sticking closed or not closing (brake lights chronically on, or never coming on). Mine does not suffer from either of these problems and does have the original double relay.

The recall involved simply removing the double relay and replacing it with two stand alone relays. I've ordered those relays and will install them and see what happens.

When this car first starting blowing bulbs I just figured the cheap $2 boxes of bulbs I was buying at car shows was the reason. Though I did note the problem was only with this Volvo. The other cars I use the exact same bulbs on do not blow them out this quickly. After going through two boxes of cheap bulbs, I started trying more expensive Sylvania bulbs and such, with no improvement.

As it stands now, the 1146 bulbs in the side mounted brake lights will blow like a flash bulb every few weeks. No other bulbs on the car blow out like this, or regularly. There is no indication of voltage spikes or over charging via lights brightening while driving, or via the voltage recordings I have done via the OBDII port and the Torque app.

As far as I know, there is no high voltage circuit in this car. That is one of the things I had suspected, that there was/is an intermittent short on the hot side of the circuit to a high(er) voltage circuit. But as far as I know, there is no 18 or 24 volt circuitry on this car. There is no higher voltage tap on the alternator, and I see nothing indicating a step up transformer for anything on the car, except for the ignition coils.

About the only thing which can cause voltage spikes besides the charging system itself is "Back EMF"  from an electric motor or a voltage spike from an inductive load, for example a relay coil or solenoid. Just the first couple paragraphs of this article will explain what I'm talking about. 

Particularly if there is any sort of intermittent connection or arcing in the circuit,  an inductive load can create HUGE voltage spikes. Does this car have a brake light failure monitoring system (dash light which indicates when a bulb is blown.)

Some brake light monitoring systems use a relay coil in series with the bulbs to monitor the current. 

If the car is an automatic, sometimes the shift interlock solenoid is wired to the brake light circuit.  

Sometimes these inductive loads will have internal clamp diodes (which may or may not be shown in the schematic.) It would be an unusual failure for a clamp diode to open up, but it could happen. More likely, a bad connection somewhere in the circuit or a bad contact on a switch is momentarily interrupting power to an inductive load and creating spikes. 

-check all connections in the circuit for loose connections. Make sure the brake light switch itself isn't flaky- any flicker of the brakelights as the pedal is slowly depressed? 

Maybe the recalled relay has flaky contacts in it. Changing it would be a good first step. 

 Good luck!