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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4055
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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I have exactly the same problem as described in this post:

Customer Question

Michael,
I have exactly the same problem as described in this post: http://www.justanswer.com/mercedes/7sioo-ml320-hi-having-trouble-pin-trailer-hitch.html#re.v/173/
Can you check my VIN to see if the rear SAM is programmed correctly. ? 4JGDF7DE2EA277678
Submitted: 1 year ago.
Category: Mercedes
Expert:  Lou P. master tech replied 1 year ago.
Hello my name is ***** ***** i will do my best to help you , i strive for EXCELLENT service and nothing less. What kind of lights are on the trailer ? Led? Or regular ? And I will also check to see if codes correctly.
Customer: replied 1 year ago.
It is not a traler. It is just a reverse light (not LED) because the GL550 reverse lights are useless. Did you read the prior post I provided in my question? My issue is identical to the prior post. I need to know if my rear SAM is programmed.
Expert:  Lou P. master tech replied 1 year ago.

It is programmed.

Expert:  Lou P. master tech replied 1 year ago.

The issue if you will let me explain

Customer: replied 1 year ago.
I don't understand your response Lou. Are you waiting for permission to explain? Please do.
Expert:  Lou P. master tech replied 1 year ago.

The rear Sam looks for proper resistance so if you are not hooking up a trailer with regular type bulbs the Sam will not produce voltage

Customer: replied 1 year ago.
The bulbs are regular, not LED. How do you know my rear SAM is programmed? If it is, what next?
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 1 year ago.
I hit the live call button accident. You can call me if you want though. If not, just send more messages. I don't feel like we are getting anywhere.
Expert:  Lou P. master tech replied 1 year ago.

Give me a second I'm on a call

Expert:  Lou P. master tech replied 1 year ago.

Ok my apologies. No need to pay for a call we can get this sorted.

Expert:  Lou P. master tech replied 1 year ago.

The Sam looks for proper resistance from a trailer hook up on all the wires. If they are not showing resistance the the Sam will kill power to the wires. For a check , hook up the GL to a trailer with regular bulbs and you will see it will work.

Expert:  Lou P. master tech replied 1 year ago.

When we code a Sam the vehicle option sheet changes in our computer system.

Expert:  Lou P. master tech replied 1 year ago.

I will upload the codes so you can see

Expert:  Lou P. master tech replied 1 year ago.

Code 550 is the proof

Expert:  Lou P. master tech replied 1 year ago.

Let me know if 6 total links showed up

Customer: replied 1 year ago.
You have my VIN number? You checked my car? I hooked the light to my jeep and it turned on immediately. My jeep is so much smarter than my GL550 in so many ways. I'm very frustrated with this GL550. It is a simple light. It should not be complicated.
Expert:  Lou P. master tech replied 1 year ago.

You gave me your vin number remember lol

Expert:  Lou P. master tech replied 1 year ago.

Believe me I understand. They made something so simple over complicated

Expert:  Lou P. master tech replied 1 year ago.

You know how many customers blamed me because they bought a 10k$ trailer with led lights and it won't function ?

Expert:  Lou P. master tech replied 1 year ago.

Give me a second to check something that may help you.

Customer: replied 1 year ago.
I got all 6 links. So now what? I don't have LED lights!
Expert:  Lou P. master tech replied 1 year ago.

I was sayin that as a example

Expert:  Lou P. master tech replied 1 year ago.

I will never mention led again

Expert:  Lou P. master tech replied 1 year ago.

Do you have a test light , to test the pins ?

Expert:  Lou P. master tech replied 1 year ago.

I was looking for a adapter to help make the lamp work. The only option is to have the trailer socket tested at a dealer. Faster way is to plug in a trailer. If you want a external lamp you can get fancy and run a wire and relay to activate the lamp. But I'm not sure how much you want to stray from oem. But everything looks good on the coding side. Sorry I don't have a better answer other than they overcomplicated a simple system.

Expert:  Lou P. master tech replied 1 year ago.

I'll stop bothering you now. I know your frustrated with the car. Please do not take that out on my rating if you choose to rate. If you need anything else let me know. I am here to help. Hopefully you can see that. Have a good weekend. Lou

Customer: replied 1 year ago.
I don't have a test light. I have very simple light that works in my jeep and not my GL550: http://www.amazon.com/gp/aw/d/B000OOMQDG/ref=mp_s_a_1_1?qid=1439606630&sr=8-1&pi=AC_SX200_QL40&keywords=Blazer+C8020+Back-Up&dpPl=1&dpID=41T5hWaXPgL&ref=plSrch
Customer: replied 1 year ago.
Of course, I don't hold you responsible for the design of the vehicle. What do I do next? Take it to a dealer? What can I tell them to move things along?
Expert:  Lou P. master tech replied 1 year ago.
I double checked the wiring diagram. There are no fuses involved. It's all from the trailer control module. Code 550.
Expert:  Lou P. master tech replied 1 year ago.

Well if you took it to the dealer

Expert:  Lou P. master tech replied 1 year ago.

It would only be to verify that the trailer circuits are fine

Expert:  Lou P. master tech replied 1 year ago.

I'm gonna tell you right now that that light will never work with the factory wiring. And they will tell you the same.

Expert:  Lou P. master tech replied 1 year ago.

If you want a light like that you will have to jump a wire to the reverse wires at the tail lamp.

Expert:  Lou P. master tech replied 1 year ago.

So at this point to be blunt , send that light back or leave it on the jeep your gonna beat a dead horse with this issue.

Expert:  Lou P. master tech replied 1 year ago.

The factory trailer wiring will only work with a standard bulb trailer UNLESS you or the customer buys a add on resistor that uhaul makes for trailers with Non standard bulbs ( I did not say that word).

Customer: replied 1 year ago.
Am I wrong? Is my light led? Is that why it won't work? I do appreciate your help. Don't take my frustration with this truck personally.
Expert:  Lou P. master tech replied 1 year ago.
Actually the description says it's a standard bulb not a led. It is not the bulb. It's the fact that the bulb setup probably only touches 2-3 pins and not all of them. So the resistance does not match so it will not trigger the trailer module to activate the wiring.
Customer: replied 1 year ago.
Oh, I see! Yes, it does only touch 3 wires. So what part do I need to buy?
Expert:  Lou P. master tech replied 1 year ago.

None. It won't work with your vehicle.

Customer: replied 1 year ago.
I thought you said I can buy a resister from uhual?
Expert:  Lou P. master tech replied 1 year ago.

You are not fully reading my posts.

Expert:  Lou P. master tech replied 1 year ago.

Uhaul makes a adapter that adds resistance to the trailer socket so the led TRAILERS can be used. Not some reverse light off of amazon. I will opt out. I don't know how much clearer I can be. And I don't take offense I know you just want this darn thing to work. Hopefully I'm wrong and you find a way for it to work off of the factory plug. Let's see if another expert can help better. Sorry I could not provide you with the answer you wanted. I wish you luck. Lou

Customer: replied 1 year ago.
No need to pass it on. You helped me understand the situation. Just close out this issue. Thanks.
Expert:  Lou P. master tech replied 1 year ago.

Well if you feel my service is satisfactory I will only get credit once you rate me. But if you want a refund the refund also affects me so please do not accept my answer if in the long run you will just ask for a refund but those are the options. And I wish you the best. If you ever need anything in the future I will do my best to help. Sorry again about this issue. Take care. Please take a second and click the box and rate my service GOOD OR EXCELLENT SERVICE if you feel I have been helpful and please request me in the future. If you feel I have not been helpful please DO NOT RATE BAD OR POOR AND WASTE YOUR MONEY , I will opt out to see if another expert can provide more or better assistance so you're not wasting your money!!. Thanks for choosing just answer. Lou. also , if you feel i deserve one , BONUSES are GREATLY APPRECIATED ! 😉

Customer: replied 1 year ago.
I appreciate the thought, but there is no way I should ask for a refund. You are a little rough around the edges, but you know what your are talking about. I will rate excellent. Take care.
Expert:  Lou P. master tech replied 1 year ago.

Haha, sorry about being rough. I promise I'm not always like that it was the only way I feel we were connecting. Take care. And thanks. I owe you one 😉