Andres Morejon

Hello, my dad's 2008 GL550 is having trouble with connecting to trailers. The rear SAM has been replaced, and I made sure it's been programmed via DAS to recognize trailers. When connected to a trailer DAS recognizes it, and brake lights and turn signals work. Running lights and reverse lights DO NOT work. I have fuse 72 installed and relay L installed as well.

Using a 7 pin to 4 flat adapter shows 11.5V on every pin. Confused as to what this could be and how to fix it. DAS even says trailer circuit 30X relay is not installed. Any ideas? Thanks

eric_in_sd

In general, the electrical system works by completing ground, not supply, so the voltage on the pins is not surprising.

I am relatively ignorant about trailer lights, but I don't recall a trailer having backup or separate running lights - don't they usually have three-pin (brake + tail light) bulbs?

edvaru

Are the trailer lights LED, then you need a curtis LED adapter to increase the current so the car senses a trailer wiring.

Tsumi

Does your trailer work properly with other cars? I'm inclined to think it's a trailer wiring issue if the brake lights are working properly.

DennisG01

As above, the first question is if your trailer has LED's. The car's computer can be re-programmed (supposedly) to recognize LED trailer lights - but you'd have to re-program it again if you use a trailer with incadescent. The more convenient way, if you have multiple trailers, is to just use the Curt 7 to 4 (or 7 to 5) adapter. In 2008, that's how it is done. In later years, supposedly it's a selectable function in the dash menu. Bit of an oversight on MB's behalf, I think?

You won't have reverse lights on your trailer with only a 4-wire setup. Are you sure about the reverse lights? You'd need another circuit for reverse lights - such as one coming off the "reverse" pin of the vehicle's 7-pin connector. That would give you a 5-pin trailer connector.

lyonsfin

Is there a particular Curt 7 to 4 pin adapter that allows the use of LED lights? Do other adapters work or only those produced by Curt?

Tsumi

The Curt adapter is the only one I know of.

DennisG01

Any adapter that advertises what we're talking about will work. I have no idea if there are other brands - you can certainly look for other brands if you want, though.

You'll find it easily enough on Curt's website if you want to look for it, though.

Andres Morejon

Hello, I'm back working on the vehicle and now we have brake lights and turn signals (which are the same bulbs) working. All the lights are incandescent and with xentry the sam is coded accordingly. But the running lights are non existent. I feel it's odd the other lights get power but not the running lights which are also incandescent

Tsumi

You have a wiring problem if running lights aren't working. The Mercedes does not check for running light function, only turn/brake.

vesiadog

Andres Do all the lights on the trailer work with other vehicles? Another words … do the trailer lights work?
How clean are the Mercedes plug at the bumper connector terminals ? Check closely for corrosion… then take your meter and start with the ground wire on the meter to GROUND on the vehicle… then positive wire to the appropriate terminals on the connector terminals.
If that works accordingly then trouble shoot the ground of the connector and make sure you have a great ground on the trailer all the way thru to the lights . I have worked on many trailers and vehicles with lighting and brake controllers… Grounding is 75% of the issues!
So again your questions are do the trailer lights work with another vehicle ? Are there LED lights?
While checking with your meter… as you are checking the positive terminals … brake lights, turn signals … terminal to a solid ground on the vehicle like the hitch…NOT THE GROUND TERMINAL..

Andres Morejon

Hello so I checked using a ground on the vehicle to the plugs, I have 12v for left brake light/turn signal, but 4v for the right brake light/turn signal and 4v for running lights. I also have plain 12v to the positive terminal on the plug. Plug A on the Sam was a little corroded so I'm going to de-pin and clean everything out, but having a plain 4v seems fishy to me. The bulbs on the trailer are all incandescent. It's worth mentioning the right brake/turn signal on the trailer isn't working either. I thought it was a bad bulb but I see not it's only got 4v

DennisG01

What does "plain" 12V mean? What "plug" are you referring to? It's a little hard to follow what you're writing.

But... let's start with something simple... verify that your trailer lights work properly. Simply put 12V+ directly to each pin on the trailer connector one at a time (and 12- to the ground pin on the connector). A battery jump pack will work - or, really, any old way you want to do it.... run wires from your battery if you want to.

Tsumi

The "plain" 12v is most likely the 12v accessory trailer power.

vesiadog

I would really like to get this one going but am giving up. We all have asked “ DO THE TRAILER LIGHTS WORK? Dennis just gave you another tip … use a power supply battery to test the trailer.
Corrosion all cleaned up????