Piston Slap: Affalterbach's A-faltering Headlight!
Martin writes:
Hi Sajeev,
I’m writing you because I’ve searched and asked model-specific forums, and mechanics, to no avail. I have the last of the 1st Gen SLK AMGs. I love this car, and I’ve loved it since the first non-AMG launched in the late 90s. Overall, it’s well maintained – a trend which I continue – and I’ve had it for a few years. I have one major issue.
The Xenon headlights will blink out randomly – the issue is solved by flicking the lights completely off, and then back on…it almost always happens on my passenger side headlight, but I’ve seen it happen on the driver’s side as well though this is rare. It usually happens on a bump, or on impact of some kind, like a speed bump, braking, or closing the hood, and can occur every few minutes (usually in wet weather – thought doesn’t ALWAYS happen in wet weather) or not at all for several months (usually dry weather).
Mechanics have diagnosed it as anything ranging from a bad ballast (doesn’t make sense to me as ballasts either work or the don’t) to a faulty bulb. One mechanic put some kind of lubrication on the contacts with the bulb and the problem went away for several months – even in wet weather, but I’m not sure if this was a solution or coincidence. Due to two factors – higher incidence of occurrence in wet weather – and the presence of condensation in the passenger’s side bulb (the worst offender) – I think there’s a short somewhere. I’ve checked the wiring and it seems ok. No one can give me a convincing reason as to why I should just replace the whole headlight assembly (an expensive proposal) – and although I realize AMG cars are pricier to maintain and I don’t mind spending, I also don’t want to do it unnecessarily just to discover that it’s a short in some kind of control module or peripheral piece.
Have you ever heard of this? Looking forward to your input.
Sajeev answers:
Not an easy question, but luckily you want what’s best for the car. Which isn’t cheap for a German car of this era. I still have nightmares about attempting to fix anything on my Father’s former 1996 BMW 750iL…beautiful, glorious nightmares I tell you!
Proper Mercedes-Benz shop manuals for this car are a must…but first…give this a shot:
A problem this intermittent, normally happening on one side means there’s an easy diagnostic route: switch headlights (first) and ballasts (second, assuming there are two, so RTFM) between left and right headlights and see if the flickering pattern changes. If so great…you probably found your offender.
If not…well…
I am somewhat confident that voltage irregularities in failing ballasts can cause this, but the bulbs themselves aren’t free from guilt. I worry because you flick’d them off/on: hot re-strikes on many older HID bulbs is a big no-no. BIIIIIG no-no, as I learned when converting the horrible headlights on my 1995 Mark VIII to the HIDs of the 1996 model: this shortens HID lifespan significantly.
If the HID bulbs are original, perhaps they need replacement after the hot re-striking and from age. Or maybe the ballasts are no longer up to par internally, perhaps a lighting specialist can load test them to verify. I doubt you have wiring problems, but who knows…I haven’t checked myself!
Who really knows how to arm-chair this query? What say you, Best and Brightest?
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.
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Many, if not most of the electronic modules have a feature called undervoltage lockout. If the voltage dips for a certain time-volt period, the unit resets to "protect" the assembly or the vehicle. Unfortunately, old vehicles are full of those glitches. Traditionally, with an old fashioned incandescent lamp, those glitches would be all but invisible, and at worst the lamp would yellow a bit. Not any more with electronic assemblies. This is similar to analog vs high def TV. If you remember, with a poor reception analog signal, one would see a snowy horrible picture, but an image could still be seen. With HDTV, the picture is perfect until it reaches a certain signal level where it will blank out completely. Glitches have become such a gremlin in automotive electronic assemblies that most car manufacturers are now specifying full module functionality with glitches lasting up to 5 milliseconds. 5 milliseconds may appear to be very short time period, and indeed it is, but to a microcontroller that is a loooooong time. Glitches are notoriously difficult to find, even with the correct test instrumentation.
I didn't see mentioned if you get a bulb out error in your instrument cluster when the lights begin to flicker. I would think its a ballast or connection to said ballast that is causing the issue. Mercedes kill the bulb output when it senses an error due to lack of load presented on the individual lighting circuit. The controller may attempt to get a functioning light by turning it off and on multiple times. Example: if you replaced a Mercedes marker bulb with an equivalent led bulb (sans load correcting circuitry) it will light briefly followed by flicking off and on and an error message about a light out in the instrument cluster.