Hissing sound from headlamp
 

Hi all, I've got a real curveball for ya'. Hoping someone has some experience with this!

My '99 LS400, with self-leveling Xenon headlights, makes a loud, pronounced hissing sound from the right (passenger) side headlamp. It's not subtle. It sounds like leaking compressed air from a quick-change fitting that isn't quite snug. It only occurs when the light is on and is even audible when the engine is running.

The light works fine; the self-leveling function works fine; it has not blown a bulb.

I had a weird event where the battery went almost completely flat very quickly yesterday when I was switching the power on and off a few times (and the headlights were coming on) to mess around with a new gadget that plugs in to the cigarette lighter. After about 6 or 8 power-on power-off cycles, less than half an hour after I got home and parked the car (and the battery SHOULD have been fully charged), the battery was so flat that when I turned the key to position 1 (power on), the fog lamps were extremely dim, barely lit, the driver's side headlamp was on, but the passenger side headlamp was NOT on. The car obviously wouldn't even turn over once. It turns out that the bulb was fine in the passenger-side lamp; after I jumped the car and got it started, the alternator started charging the battery and all the lights were normal. I can't imagine why one headlamp would work and the other would not in a low-battery condition, but the car has made this hissing sound ever since we bought it two years ago.

It might also be worth noting that this battery is pretty much new, no more than a couple of months old. I'll be putting it through its paces today to see what kind of current it carries when freshly charged (on the trickle charger) and after a day without charging but I REALLY don't expect to find any problem with the battery itself.

Any thoughts what this hissing sound might be? Is the self-leveling function pneumatic?

Thanks for your thoughts!! :woot:

Being careful to avoid moving parts, you might try probing and listening with an automotive stethoscope to exactly locate the source of the noise - I bought mine many years ago for a few bucks at an auto parts store ... exactly like the one in the attached photo. It's sometimes difficult to locate a noise source. About the first time I used my stethoscope was back in the 80's to locate a nasty buzz I thought was in a dash board -- turned out to be a loose setting in a ring I was wearing!

I do have a stethoscope, thanks... I do know that it's related to the headlamp electronics or self-leveling for the reason that the hissing sound occurs whether the engine is on or not, and shuts off immediately when the relay shuts off the headlamps (after the car has been turned off for, I guess, 60 seconds).

I suppose I ought to remove some covers up there though and see if I can zero in on it a little better, though... you're right. I'm just wondering if anyone might be able to give me some idea what type of thing in the headlamp cluster even COULD cause a leaking-compressed-air sound like that! Weird.

Might want to look for any vacuum lines in the area that may not be secure while the covers are off.

Yeah, I'll check - it couldn't be engine vacuum leak though since it continues after the engine is off. That's why I'm wondering if the self-leveling headlamps are electric or pneumatic. I might dig in a little tomorrow, see what I can find.

ballast. Mine make the same noises on both sides. head lights leveling is electronic.

+1 for ballast noise.

OK - that makes sense, I guess I never thought about the fact that HID lamps need balasts. Never had any problems where I had to mess with the headlights on any of the cars that had 'em.

Is it a problem? Do you guys think think it might cause an unusual power drain, esp. after the engine is off and not charging?

usually it's indicative of bad grounding/dying ballast, but no it won't cause extra drain or anything like that.

Sounds like the ballast lost the initial high voltage control function.

A xenon bulb only requires a high voltage when turning on. There are two separate circuits in the ballast. One for the initial high voltage generation to start the arcing and the other for about 80V generation to operate normally.

Once the arcing is stabled, the high voltage is no more needed and the initial high voltage generation circuit stops the function. Your ballast may be generating it continuously after the bulb is turned on. It is usually needed for 10-20 seconds or so.

Hmm... maybe so. I'm assuming it's a relay that's going to switch power off this high-voltage circuit? Is it an external relay? I'm guessing it'll be internal and not serviceable.

It's not a relay but a semiconductor switcher. Dealers always just replace the whole unit because they are not allowed to touch inside of the ballast. It is very dangerous to service inside too.