Hello X164 Gurus,

I'm in a bit of a bind. I bought a 2008 GL450 from a used car dealer in Houston TX recently, knowing that the driver low beam bulb was out. The headlights are halogen ones, by the way. I figured it would be a relatively easy job - new bulb, right? I looked inside the headlight and found that the wire sheathing fell apart. Ordered a new wiring loom, disassembled the headlight, replaced the wiring to new, put back together and installed in the truck. Everything works: High beam, turn signal, but not the low beam. I checked the fuses - the ones under the passenger seat next to the battery (front SAM), all good, also the ones in the fuse box in the engine compartment, also all good. I then checked voltages, connecting my voltmeter prongs to the jump starter locations to see how much was at the battery - 11.96V. I then checked the voltage at the low beam bulb socket - also 11.96V. Taking a known good bulb and hooking it up to the jumper terminals, the bulb illuminates. Taking that same bulb and plugging into the low beam socket, no illumination. What gives? I don't think it's the front SAM, as turn signals and high beams are working and ground connections are tight. So what else could it be?

I'm STUMPED, can anyone give some advice on what to do next? Is there something I'm missing? Thanks in advance.

Did you check to see if the control unit is fully functional. Some time the low beam part stop working do to the fact it get used more often.

Which control unit, the front SAM under the passenger seat? And how do you check if it's good?

Still stumped on the fact that the low beam is getting voltage.

The low beam is getting voltage try changing your bulbs around take onw of the high beams out and put it in the low beam spot. I know that sound bomb put try it and see what happens

Tried that, along with like 3 other bulbs and still no go.

Wait - so you got 12v at the socket but when you plug a known good bulb in it doesn’t illuminate?

I bet you have resistance in the wiring. When you have no current flowing, like with a voltmeter, you get voltage, but once you try to run current, you get a large voltage drop elsewhere.

Put the bulb in the low beam socket, but unplugged enough so the terminals are accessible. Measure the voltage from each terminal to ground. Check the voltage across the terminals as well (I bet it is near zero).

Wherever you have respectable voltage to ground, the circuit is good up to that point. If the voltage at the bulb terminals is high, that means your open fault is on the ground side. If the voltage at the bulb terminals is low, the open fault is on the supply side.

Keep probing; you'll find it. I bet anything it's a low quality, possibly corroded, connection somewhere in the line.

Thanks for the advice. I was actually thinking that I hadn't tried measuring while the bulb was "plugged" in, but ill give it a go and let you know. Thanks!

UPDATE:

I found the problem - it's the front SAM. I managed to find the wiring diagram for headlights and traced the wire back to connector A on the SAM. Testing that wire before taking out the SAM, I found that the issue persisted - a short to ground. I initially suspected that the wiring between the SAM and the headlight was to blame, considering that the battery tray compartment under the seat looked to be clean and dry. Well, as a last resort I popped out the SAM, took it apart and found that one of the chips nearest to connector A (corresponding to the driver headlight) looked to have been burned. Some other resistors on an edge also have corrosion from possibly water intrusion at one point.

In any case, thanks for your suggestions.

In regards to replacing the SAM, can I pick up a used one without having it programmed? Or is this a component protection thing? Is a dealer visit required? I come from the world of Audi and Volkswagen where things like this would have component protection and dealer-level programming is required. Please advise.

Unfortunately, they're tied to the VIN so you can't just buy a used one and slap it in. I have a front parking light that starting staying on full-time once some water was spilled under the front passenger seat and got on the SAM. I opted for unplugging the bulb rather than replacing the SAM. I think there are people who can code them outside of a dealer, but I'm not sure how the process works.

I had same problems, i got the SAM out and cleaned every single pins entire board. Water corrosion caused pins connected together. My Front left parking light has been working good ..

Check the condition of the wires inside the headlight. The wiring insulation tends to degrade over time and mine were missing the insulation and shorting so I replaced both headlights. Hate to get a new SAM and it get fried again. May or may not be the issue but do check the condition of the wiring insulation.

Sound advice. When you have a component that failed after many years of normal functioning, you must find the root cause. A partial short could easily over-tax a switching circuit.

The sensitivity of this vehicle to water intrusion is alarming.

Hello AlaskanAudi! My 2012 GL 450 has the same issue as yours. Where is the front SAM? Based on your posting and some internet search results, it must be next to the main battery under the front passenger seat. When I checked the battery compartment, I saw a fuse box next to the main battery. Is the fuse box also the front SAM? Did you reprogram it at a MB dealer after getting a new one? Thanks for your help in advance.