Why are Diodes Needed for Lighting when Flat Towing a Vehicle

Updated 08/28/2017 | Published 08/26/2017

Question:

This statement is in the description: Note: You will need to install a tail light isolating diode system 38955 and 2 additional diodes RM-792 with this tail light converter to prevent electrical feedback when flat towing a vehicle. Vehicles with separate turn signals and brake lights will need a total of 6 diodes. QUESTION: My tow vehicle is 2 wire and my towed vehicle is a 3 wire. Why are any diodes required IF I disconnect my towed vehicle battery ground wire whenever in tow ie, no power onboard towed vehicle?

asked by: Bennie W

Expert Reply:

When a vehicle is being towed it cannot operate its own lights. The diodes are a gateway with 2 inputs and one output. One input is the regular wiring on the vehicle. The other input is from the RV. Since vehicles being flat towed will not have working lights they will have to get power to function from the RV. The diodes allow for 2 inputs so that you don't have to disconnect the vehicle lights to plug in power wires from the RV.

The tow vehicle lighting system only matters if the tow vehicle does not already have a trailer connector to plug into. If it does have a trailer connector then the wiring is already converted to a 2-wire system. What converter # 118158 does is take the combined turned signal/brake light functions and separate them out so that you can have separate brake light and turn signal functions on the toad while it is being towed. It isn't a requirement by law to do this but some people like to have the separate turn signals and brake lights even when the vehicle is being towed.

I have included some helpful links on tow bar wiring for you to take a look at as well.

expert reply by: Michael H