Installed Tekonsha Wiring # 119250KIT on My 2019 Mercedes Metris, now Issues with Blind Spot Assist

Updated 04/24/2025 | Published 01/22/2025

Question:

I installed a Tekonsha no contact towing harness on my 2019 Mercedes Metris cargo van. I installed it in the way your excellent and helpful video. The harness works properly. The vehicles blind spot assist became inoperative at the same time. Before I take it to the dealer for diagnosis, do you have any feedback of this unit interfering with the blind spot sensors. It is a Tekonsha Modulite ZCI. Thanks

asked by: Bill W

Expert Reply:

Hello Bill,

Thanks for the shout out to our video department, we take a lot of pride in our content and it's always nice to hear from customers that we are doing it right.

I am sorry that the Tekonsha wiring # 119250KIT is causing you issues with the vehicles blind spot assist. We have not ran into this issue. I do have some troubleshooting for you to try before taking it to the dealership:

-Check Connections - Inspect all wiring to ensure the Modulite ZCI is properly installed and securely connected. Verify that no wires related to the blind spot assist were disturbed during installation.

- Power and Ground Source - Confirm that the Modulite ZCI is powered and grounded using independent points, ideally separate from the blind spot system.

- Inspect Sensor Areas - Check for physical damage, obstructions, or misalignments in the blind spot sensors, which might have occurred during installation.

If all those things are good then it sounds like the dealership may need to recalibrate the blind spot sensors. That should resolve the issue.

Please let me know what you find out about the wiring and what was causing this issue, I am eager to know the true reason.

expert reply by: Kate F

Bill W.

1/31/2025

Kate, thanks for addressing my question. As there is no incident causing damage or moisture entry into sensor I am taking the Metris van to Meredes dealer to see if they can diagnose. I will report back after that appointment. Thanks,

Kate F.

2/3/2025

@BillW Anytime Bill. Please let me know what you found out at the dealership as I am eager to know what is causing this issue.

Bill W.

2/11/2025

Kate, the Mercedes dealer found the problem to be corrosion of the 4 pin terminals that plug in to the blind spot sensors in the bumper. Both right and left sides were getting no signal to the computer. Problem starting at the same time as trailer harness install appears to have been a coincidence. Thanks, Bill

Kate F.

2/11/2025

@BillW Thank for letting me know Bill, I am glad they were able to get to the bottom of it.

Kate F.

4/22/2025

@Bill That issue you're running into, where the trailer lights turn back on after you shut off the van, from my research I have found it is actually a common quirk when using the Tekonsha ZCI module # 119251KIT, especially on vehicles like the 2019 Mercedes Metris that have complex electrical systems. Even after you turn off the van and remove the key, the vehicle’s body control module (BCM) and lighting circuits often continue to send low-level pulses as part of a normal "shutdown" or sleep cycle. Since the ZCI senses electrical activity using clamp-on sensors (rather than tapping wires directly), it can mistake those pulses for an active lighting signal, and turn the trailer running lights back on. What makes it extra confusing is that the trailer lights go off like they’re supposed to… and then come back on a minute or two later, once the van shifts into a different power state. This is just how many modern vehicles wind down their systems, it’s not immediate. In daylight, it’s easy to miss, which makes it even more frustrating if you return to a dead trailer battery. The best fix is to install a simple 12V automotive relay that cuts power to the ZCI (or just the output to the trailer lights) when the ignition is off. The relay gets triggered by a power source that’s only live when the van is running, like a fuse tap off an ignition-switched circuit. When the ignition shuts off, the relay opens and stops any stray pulses from powering the trailer lights. It’s a clean, automatic solution. You can pick up one of these at your local Auto Store, or the Blazer 30 Amp Heavy-Duty Relay - 12V # BL63VR will get the job done. Alternatively, you could add a manual cutoff switch if you prefer something simpler that gives you direct control. Some folks also try inline delay relays or voltage filters to block those post-shutdown pulses, but those solutions can be hit or miss depending on how the vehicle behaves. Bottom line, this isn’t a defect in the Tekonsha ZCI. It’s just one of those odd side effects when modern vehicle electronics meet aftermarket accessories. A $10 relay can make it all work exactly how you’d expect, no more surprise light shows or battery drain. Annoying that you need to add an additional part to make it all work as it should.