Low Voltage on Curt Wiring Harness When Lights On

Updated 05/21/2025 | Published 06/21/2015

Question:

I have 2012 Ford Escape and have a Curtis part # s0612u-r1 shows 12v with headlights off then put headlights on voltage s to 5-6v

asked by: Brendan D

Expert Reply:

What you are describing seems to be common in some Curt trailer wiring harnesses that use a powered converter. If you were to pull the fuse out of the fuse holder it should not show power any more (with the vehicle on or off).

It sounds like you have a weak ground, loose butt connector or perhaps a crimp in a wire. Check all your ground wires to verify they are securely attached to a clean, bare metal surface. These checks should include the ground connector on the tow vehicle, the ground connector on the trailer, and the ground connection at each tail light assembly. Also check the condition of the wiring in between.

expert reply by: Chris O

Normand B.

7/9/2024

I have the same issue with a Curt 56035 T-Connector on a 2013 Toyota Matrix. I have 5.75V on the brown wire going to the marker lights whether the lights on the car are on or off. Signal lights, brake lights show the proper 12V. Voltage is lost if I pull out the fuse for the signal, brake lights and also the marker lights. This seems to indicate that the 12V supply from the battery is OK. I checked the converter ground connection and it seems OK. I probably would not get a 12V for the signal and brake lights if the ground connection was bad. The convertor is new and worked fine for two tows. Any recommendations?

Bud M.

7/9/2024

@NormandB The only real test you can do is to verify the 5.75v is all you are getting out of the brown wire leaving the converter on your Curt # C56035. If you tested at the 4-way connector there is a chance the wire between the converter and the 4-way was internally damaged and is causing the low voltage. If the voltage is the same there then you know the converter itself is bad.

Normand B.

7/9/2024

@BudM I will verify. I assume that if the convertor is to blame that I could wire the inlet brown wire to a relay coil with the other terminal to ground, then pick up 12V from the black wire from the battery on one side of the contact and the outlet brown wire to the other side of the contact. That would increase the load on the marker light circuit by a few milliamps while the trailer marker lights would still get their power supply from the black wire.

Bud M.

7/9/2024

@NormandB In theory that sounds correct if there is enough power to activate the relay.