2005 Dodge Ram Trailer Relay Replacement

— on July 25, 2019 by in Repair

I love my truck, but I’ve put up with a problem almost since day one. The right trailer taillight doesn’t work. It isn’t a trailer problem. The pin on the factory 7 PIN RV connector is just dead. No signal… Now that I’m towing a teardrop camper on a regular basis, I need it fixed.

Diagnosing the Problem

After checking the basics (i.e. fuses) I concluded that the problem was inside the sealed fuse box assembly. Dodge, for some reason, soldered the three relays for the trailer lights onto this board and sealed them inside a plastic housing. Why not just socket them like the other 10 relays? It’s not like there isn’t space… But I digress.

My original thought was that I had damaged something installing the trailer brake controller. I crossed wires when I installed it way back in 2005 and burned a fuse out. I just assumed I’d burned out part of the circuit.

The broken contact tab from my defective trailer relay on a 2005 Dodge Ram 2500.

The broken contact tab from my defective trailer relay. You could hear it rattling around with the fuse box disassembled.

It turns out I was wrong. The relay was defective. The contacting tab was broken off, probably from the factory. I fixed the problem by soldering a new mini-automotive relay of the higher amperage capacity and equal coil resistance into the board. The only catch was that I had trouble getting the old relay off.

Installing the Relay

The factory relays are essentially micro automotive relays as used on many cars today (in particular Japanese cars like Subaru and Toyota) with one difference… They are designed to be soldered on instead of socketed. The terminals on their bottom are smaller. An off the shelf relay (at least that I could find easily) won’t slip in to the same spot on the board.

Instead of fighting with that, I took a standard spare relay I had lying around (a spare from our Subaru Forester). It has the same resistance on the coil (80 ohms) and is rated for a higher amperage (30 amps vs 20 amps for the original).

To remove the old relay, I basically broke it into pieces and cut it away. I was EXTREMELY CAREFUL not to damage the other relays, the board or any thing else.

The broken contacting tab from the factory trailer relay on my 2005 Dodge Ram 2500.

Destructively removing the old relay from the circuit board.

Once the bulk of the relay was removed, I could desolder the pens. Here I was a little impatient and ended up removing one of the solder pads from the board. That left me unable to solder one of the leads for the coil into the original location. Instead I ran a lead back to the pin header (tracing the trace on the board them ohm-ing it out).

Wiring a replacement trailer light relay on my 2005 Dodge Ram 2500.

Alternate wiring for one side of the relay coil due to a damaged solder pad.

I soldered the relay onto the board and then used hot glue to provide at least some basic vibration mitigation.

Replaced trailer tail light relay on a 2005 Dodge Ram 2500.

Access the Inside of the Fuse Box

Dodge designs these fuse boxes to be non-serviceable. If they go bad, you just throw them in the garbage and install a new ($400) box. That is BAD DESIGN.

Once all the fuses and relays were out (take a picture so you can put it all back sanely!), the fuse box can be disassembled. There are three plastic “welds” that hold it together. I drilled those (carefully, drilling only as much as needed) using a 1/4″ bit.

2005 Dodge Ram Fuse Box disassembly - Plastic Weld Locations

The red “circles” identify the three plastic “welds”.

With the plastic welds out of the way, the top of the box is held in by tabs on the side. Some gentle prying with a screw driver allowed me to pull it apart. From there, it takes a little more disassembly to pull the rubber bus mats out of the way. Just be careful not to bend anything.

The rubber bus mats from inside a 2005 Dodge Ram 2500 fuse box.

There are several layers of these rubber bus mats on top of the circuit board for high amperage circuits.

Assembly was a bit of a PIA. It took a few times to get everything aligned and in the right hole. Just be careful. You don’t want to bend anything or have a missing connection. With the box snapped together (held by the tabs on the sides of the upper cover) I installed three pan head screws into the old plastic “weld” locations.

Small pan head screws holding my repaired fuse box together on a 2005 Dodge Ram 2500.

One of the pan head screws holding the fuse box together.