SupermanWV

I just installed a no splice Tekonsha trailer harness on my 2018 GLC300. If the car is in accessory mode the signals and brake lights and running lights work like a charm on the trailer and on the 4Pin test light. But as soon as I start the engine, only running lights work on trailer or on tester. All the functions still work on the car… just the signal is not making it to the trailer or the tester. What could be he issue?

I have the harness kit grounded to a post in the car with additional brown wires grounded to it. Should I ground it some other way for a better ground possibly?

Thanks for any info you can give. I tried searching and such but couldn’t find any info.

SupermanWV

Also…..

I have also looked for a tail light wire diagram for the GLC 300 with the LED premium tail lights. They have a different connector than the incandescent tail lights. Does anyone know how I can go about finding a wiring diagram for them. They have a square blue connector. Rather than the flat connector.

Thanks

Clark

streborx

Did your GLC come with a tow package installed or did you add an aftermarket receiver hitch?

SupermanWV

I installed aftermarket hitch and wiring.

streborx

Not sure I understand why you tied a ground to the car's frame. The 4-pin connector that your adaptor harness plugs into has a ground connection in it, and this is the ground you should use. There's a ground current sensing mechanism built into the car that disables some features such as blind spot monitoring, lane keeping, and stability control (dangerous if they kick in when towing) whenever trailer lights are plugged in. If you've bypassed this by using a separate ground, it might cause circuit malfunctioning. I'm just guessing here - you might need to consult the dealer.

SupermanWV

“The 4-pin connector that your adaptor harness plugs into has a ground connection in it, and this is the ground you should use.”

I’m not sure what 4 pin connector you are referring to in this sentence.

I tried to find a factory type harness that could be just plug and play. And without LED lights I could have used one. But the LED lights have different connectors than the incandescent bulbs. So….I tried to use the harness that just clips over the wires using magnets to pick up signals to turn lights on and off. And if I used the trailer without turning the car ignition on it worked flawlessly. But when the car was running that system wouldn’t work. Not sure of the constant signal coming through the wires with the car running was not changed enough by the low draw of the LEDs or what but it did not pick up the difference in off and on to turn the blinkers on. That is when I determined the only way to get it to work was to splice the wires into the preexisting taillight wiring. And low and behold it worked. Still working. No issues yet. I don’t have any sensors in the rear bumper so I don’t know that having a trailer hooked up is gonna set off any warnings or anything. So far it has not.

so please explain what 4 pin harness your are speaking about above.

I just followed the directions of the aftermarket 4 pin kit and it said ground to same place the other brown wires are grounded. You can watch the video on YouTube that I watched to see what to do. I believe their install was on a 2017, by etrailer.com.

streborx

My bad -- when you said "no splice", I thought you meant that you had located a connector into which to plug your trailer receptacle harness, thus avoiding splicing (some vehicles are equipped this way to facilitate dealer installation of tow packages). I've never used the Tekonsha no-splice product -- there seems to be an application for this type of installation (perhaps), but avoiding the installation of wire splices in the overall scheme of things really seems to be the smallest task. I have no guess why everything would work in accessory mode but not in ignition running mode -- there shouldn't be any difference in current drawn by trailer lights. If you've reverted to the wire splice type installation and all is working, you seem to have solved it. Hope you're able to return the no splice kit -- it's not inexpensive.
What is it about your trailer LED lights that made them incompatible with an OEM wiring harness? I have a couple of utility trailers that over the years have gone through several sets of tail lights. I actually keep a spare replacement set in my garage 'cause I never know when it might be crunch time. I buy them at Harbor Freight and they're LED. They come with a standard wiring harness and flat 4-pin connector, and I've have never had any problem with them working with any of my vehicles.

SupermanWV

The problem wasn’t the trailer led lights. The problem is the LED lights that the 2018 GLC300 came with as taillights. They either came with incandescent bulbs or LEDs. And each had different connectors in the vehicle. If you don’t have the stock LEDs then you can buy a trailer harness kit that is plug and play. Has a flat white connector. The stock LED lights have a blue square shaped connector. Almost like a double thickness of the white connector. But they don’t make a plug and play for them.

I’m not a whiz with electronics but I’d say the reason the no splice kit didn’t work with the stock LED taillight wires when car was running's is because of all the noise in the electrical system kinda drowns out the small difference in voltage that happens due to the LED lights not drawing much on the line. So the magnet just doesn’t pick up enough difference between running lights being on and then the blinker coming on. If that makes sense. When car isn’t running. There isn’t as much noise in the system so the magnets pick the difference up a little easier. That’s my complete guess. Lol!

streborx

Leave it to MB to sell an SUV designed for towing but for which no tow light cable is available. LED lights do often have regulation circuitry that stabilizes their illumination, and I think you're correct that electrical noise from this and other sources could interfere with these clamp on current sensors. Thanks for taking the time to describe all this. I'm always interested in learning more about automotive engineering shenanigans.

SupermanWV

No problem. It took me a while to get through it all. I had no intentions of splicing wiring in. But MB forced my hand so to say. They would wire the car. But it was gonna be about $2500 or so with labor. And they would remove most of the interior they said to rewire a lot of the car to make it work like a factory system. I wasn’t down for that. So I just rolled up my sleeves and went to work. Seems to work fine. I only row every now and then. So I’m sure it’ll all be fine. My car doesn’t have any sensors of any kind in the rear. So I don’t think having a trailer attached to the car is gonna set any safety triggers off.

Icemaster

Do you mind a quick rundown on you did this? I was just about to hit "Order" on a wiring kit then found your thread - I'd love to hear how you did it and what wiring kit you used.

Thanks!

SupermanWV

I used this kit:


It wasn’t easy. Had to tilt rear floor up and pull entire area where a spare tire would be out. I then had to undo the two side panels of the rear cargo area out from the sides to get behind them. I traced the wires using test light to figure out which ones were brake light/turn signal (they are the same wire, and running lights. After finding those two wires on each side. (With my cars configuration the wires were same color on both left and right sides of vehicle.

I then had to cut the wires and splice in the wiring kit. Not hard if you are skilled with doing this type of thing. If not. You may want to consider having someone else do it. You don’t want to ruin something that would be expensive to have to replace it fix.

Then you do the reverse to re-install the cargo floor cover and your done.

To install the hitch I watched many videos on YouTube and had to completely remove the rear bumper and bolt it in. Then re install the rear bumper. Not to hard. Just be careful not to brake any of the tabs off the bumper. Just go slowly and carefully and all should be fine.