Troubleshooting Tekonsha ZCI Not Functioning Correctly
Updated 09/29/2025 | Published 09/15/2025
Question:
Hello. I have a 2012 Mercedes ML350. Im having issues with the Tekonsha ZCI kit. Ive used a 7 way tester along with a test light to confirm these findings. With the kit installed, when I turn on my left turn signal, both the right and left trailer turn signals flash. Right turn signal works fine. When I hit the brakes, the left brake light comes on, but right is half dim. And sometimes the trailer running lights come on even without the Mercedes running lights on. All the lights on the Mercedes function properly. Ive tested the Mercedes wires to confirm Im hooking up the correct signal wires. I have good clean ground. Ive done the relearn procedure probably a dozen times. The sensors are pointing towards the lights. Any ideas what else to check? Thanks!
asked by: Briley R
Expert Reply:
Hello Briley, thanks for reaching out. I have heard of this happening on certain vehicles that have a few different wires that have a signal when the turn signal is applied. Some of these wires can have a different pulse width which makes it hard for the module to read. I would see if there is another right turn signal wire that has power that you can move the sensor to, as that will likely fix the issue.
You can try moving the sensor that is not working to where another sensor is installed. Then see if that function now works. If it works, the original wire used was wrong, if it doesn't, the module is bad.
If this does not fix the flashing issue, it is possible that there is an issue within the Module and will need to be replaced. The modules going bad is rare but not unheard of.
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Jesse M.
9/29/2025
Hey Briley, did you have a chance to try moving the sensor to a different right turn signal wire or swapping it to the left side? I'm curious if that helped resolve the issue with your trailer lights. Your experience could really help others facing similar challenges.