Mercedes electrical issue. Non urgent help needed

[Jeremiah]

Hello

Hopefully someone out there can help or point me in the right direction. The 2008 gl450 that I have will not activate the left blinker/brake light. I have tracked the problem to where you hook it up to the trailer and tested the wires behind. The wires behind are not sending the signal so it is not the 7 pin female plug by the tow hitch. I put my hand up behind the bumper feeling for lose or broken wires and can’t find anything.

Anyone have suggestions, besides the obvious trip to the mechanic or the nose in the air dealership?

 

[Seadeals]

Hello @Jeremiah so you are saying the left blinker/brake pin is not passing 12+ volts at the vehicle receptacle where you plug in the trailer harness? Have you checked the fuse box for any blown fuses and is the left tail light/brake light on the vehicle working?

 

[Jeremiah]

It works in the vehicle just fine. And the answer to your first question is yes.

 

[Seadeals]

Sounds like next step is a closer look at the wires on the vehicle receptacle with a multimeter. As a last ditch effort you could shmooey it up with some of this and connect/disconnect several times to clean up any corrosion.

It sounds like the lead wire feeding the yellow trailer wire is compromised somewhere (most likely in the receptacle if your fuses are good).

 

[veedubtek]

I don't speak Mercedes, but I'm assuming you've got some form of active bulb monitoring system. Typically, when the system self-tests on initial key cycle and sees a fault, it will quit sending power to that circuit. IOW, say you have a headlight bulb out. With the key on, you can't just swap the bulb and expect it to come on - it needs a key cycle to run the self test again (or a computer to clear the fault) before it will start sending power to that headlight again. Same deal with your current trailer light problem I'm guessing. If you've got a fault in the trailer wiring/bulb, it's only gonna blip power back there for a second to test the circuit. If it sees a problem, it won't be sending power anymore, so this makes it virtually impossible to test the circuit like you are trying.

 

[PeterB]

Trailers with led lights will sometimes fail to work on vehicles with trailer light monitoring systems. The led lights don't draw enough power for the system to recognize. Have you run this trailer with your gl450 before?

 

[Jeremiah]

I used a bunch of electrical contact cleaner everywhere I could. Checked all fuses, no issues there. The weird thing is it gets the running light voltage on the left but not the brake or blinker.

 

[Jeremiah]

I ran a different trailer and had the same problem. No led lights are being used. The problem is not the trailer. I have a visual tester you plug into the receptacle. Again it’s not getting left brake/blinker.

 

[swatski]

@Jeremiah I think @veedubtek may be right, as usual. Do you have a separate trailer module in you harness?

My Q7 had an issue with with trailer electric and I ended up needing to replace the whole package to get it fixed (the hitch, harness and module...). At a dealer. Yes, it hurt.
Tried everything before throwing in the towel.
Not gonna tell you what it cost me, it was ridiculous.

--

 

[adrianp89]

This is why I stay away from luxury cars. The TSBs and recall bulletins for any single Mercedes model/year is three times as long as other car manufactures entire line up for a given year. The issue are just insane with those cars.

 

[swatski]

Yeah, there is some pain, no doubt.
Yet, I'm not planning on trading my Q7 TDI in... I don't really care how much they offer just because I'm not going to find another car like that again - easily. If - at all.

--

 

[Jgorm]

Does the blinker work on the car? If not you might need a new bulb. If it works, then try adding an additional ground from the chassis to the trailer. Jumper cables work for testing.

 

[spatty99]

I had a similar problem with a 2011 Mercedes ML. If I remember right it kept showing a trailer turn signal lamp failure. It finally resolved when I added a ground across the folding trailer tongue (a common trailer issue I wish I knew before swapping out LED lights, completely rewiring and troubleshooting forever).

 

[Jeremiah]

It’s not sending the signal to the plug on the hitch. Well before it gets to the trailer. I’m switching out the plug on the Mercedes today. Will keep you guys posted.

 

[Jeremiah]

No luck

 

[Sbrown]

It's possible there's a problem with the rear SAM. It's a module under the rear seat and it controls signal output to the trailer, among other things. One of three things may have happened with it. 1. It's susceptible to corrosion, good news is it can be taken apart and checked visually. If corrosion is present, it can be cleaned up carefully with a fair chance of fixing the problem. 2. The circuit board is prone to solder cracking, once again visual inspection and fair chance of successful repair. 3. The SAM detected a short circuit at some point on the specific circuit you are currently having a problem with. If it did detect a short, it may have shut that circuit down and may have set a fault code in the SAM. It will not power it up again until the fault code is erased and the SAM reset. This should be able to be done with any professional level diagnostic tool, but it may require a factory one, not 100 percent sure.

Anyway, those would be my guesses, going off of what you've said and assuming you've done all your diagnostics correctly.

 

[Jeremiah]

I give up. Depending on my mood I will drive the trailer with one brake light indefinitely and try not to make any left turns or take it to the dealer and bring the KY with me.

Decisions, decisions.

 

[Jeremiah]

It’s the Sam unit and they want $1400. Can buy the part but it needs to be programmed to my Vin by the dealer. They won’t do that and let me do the install.

????! What choice do I have here?

 

[Seadeals]

Sam unit must stand for “Surrender All Money”!

 

[Jeremiah]

So what about jumping the right blinker to the left?

Super bad idea. What’s the worse that can happen?