DIY - Build and install a Bussmann RTMR Fuse/Relay Block

Up2NoGood · Nov 5, 2015

I just hope people don't start quoting the OP in their replies.... computer may explode lol

The pictures are great though as a visual aid.

tacozord · Nov 5, 2015

Up2NoGood said: ↑

I just hope people don't start quoting the OP in their replies.... computer may explode lol

The pictures are great though as a visual aid.
Click to expand...

Good point. And I think my brain may have exploded putting this together.

Iggy · Nov 5, 2015

Awesome writeup! Excellent photos, and instructions. Thank for adding the mention about my panels!

I've been reading your blog too! Good stuff.

rob1208 · Nov 5, 2015

This write up was awesome and came at the right time! I've been sitting on my bussman box and switches since I had no idea where to start. Now I have no excuse not to start and knock out my relay box. Thanks man!!!

TheTrooper · Nov 5, 2015

Wow

kitsym · Nov 5, 2015

tacozord[OP] said: ↑

Good point. And I think my brain may have exploded putting this together.

Click to expand...

And you didn't take photos of your brain exploding??? Disappointing.

ManBeast · Nov 5, 2015

Amazing write up.

beertimecontinuum · Nov 5, 2015

Gott damn dude. Just. Damn.
:slowclap:

JdevTac · Nov 5, 2015

Surprised you didn't just link to your website which has some awesome info btw (big 3 upgrade info too)

tacozord · Nov 5, 2015

JdevTac said: ↑

Surprised you didn't just link to your website which has some awesome info btw (big 3 upgrade info too)
Click to expand...

Yeah...I could have done that. But I'm not trying to promote my website or anything. I started that a long time ago just for fun, and I've used it to make notes of things. Recently I've started to add various articles about the truck. So it's a running blog mostly for myself, friends and family.

Also, the Big 3 upgrade and various other things that I've done to the truck are in my build page here on TW. So...it's in both places. I figured it'd be more effective here anyways.

tacozord · Nov 5, 2015

kitsym said: ↑

And you didn't take photos of your brain exploding??? Disappointing.
Click to expand...

Well...that could be arranged.

tacozord · Nov 5, 2015

beertimecontinuum said: ↑

Gott damn dude. Just. Damn.
:slowclap:
Click to expand...

Shit-gott-damn! Thanks!

Rattletrap66 · Nov 5, 2015

tacozord[OP] initial post: ↑

Part 1 - Introduction

This is Part 1 in a series of tutorials that will show you how to build a Bussmann RTMR fuse/relay block. If you’re not familiar with this product, it’s a simple waterproof enclosure that allows you to connect accessories on your vehicle through relays and/or fuses.

After reading this tutorial, you will have the knowledge and skills to build one of the above fuse blocks that you can then install in your vehicle. This will end up being the foundation that you can then add accessories to such as lights, compressors, radios, and more.

This part will introduce you to the Bussmann RTMR and hopefully answer some questions. By the end of this part, you should have a better understanding of what this device is capable of and if it’s something that you need. I’ll explain many details, but let me first outline the upcoming parts in this series.

Part 1 – Introduction

Part 2 – Parts

Part 3 – Tools and Techniques

Part 4 – Wiring and Schematics

Part 5 – Building The RTMR

Part 6 – Installation

Before proceeding, a word of warning. I take no responsibility for damage, accidents, or injuries resulting from the fabrication or installation of any electrical modifications to your vehicle. You do so at your own risk.

What is the Bussmann RTMR?

In simple terms, the Bussmann RTMR is a fuse/relay block. Although the manufacturer, Cooper Industries, which is part of Eaton, calls it a Power Distribution Module. This serves as a central connection point for various accessories in your vehicle. The device consists of fuses and relays to accommodate switched and non-switched accessories.

There is additional info at Cooper Industries website regarding the RTMR, most specifically within the datasheet. We’ll take a look at the datasheet as we proceed through this tutorial. For now, here’s the specific webpage for the Bussmann RTMR 15300 Series Power Distribution Module.

The RTMR is an acronym for “Rear Terminal Mini Fuse and Relay”. What this means is that the device allows you to insert terminals into the rear side with mini fuses and relays inserted into the front. It’s a weather tight enclosure(IP66), when used with appropriate cover, cable seals and cavity plugs. Additionally, there are several models with various configurations of internal electrical bussing. Out of the available models, there are really only two that I feel are the most versatile. I’ll detail the differences in Part 2 – Parts.

Why would I want one?

The primary reason to make one of these enclosures is if you want to add accessories to your vehicle, such as lights, compressors, radios, water pumps, etc. Of course, all of this is possible without using an enclosure such as the RTMR. However, as you continue to add accessories to your vehicle, the organization of the wiring can quickly get out of hand. Take a look at the following image. This is a vehicle with poor electrical organization. Although connected directly to the battery, the wires are all over the place, and it’s a potential fire hazard. The RTMR will clean this up nicely.

Photo by @Shmellmopwho. He has since cleaned his wiring up with an auxilliary fuse block and circuit breaker.

Besides clean wiring, using an RTMR prevents you from having to branch off the OEM stock fuse block using fuse-taps. Of course this can be done, but you have to be very careful that you don’t overload a circuit, creating another potential fire hazard.

And finally, another safety feature of the RTMR is the use of relays. By routing the high-current accessory through a relay instead of directly through a switch, you’re minimizing the amount of current at your fingertips. If you don’t quite understand how relays work, I’ll get into more detail in Part 4 – Wiring and Schematics.

Why build it myself? Can’t I just buy one already made?

There’s an old proverb, “Give a man a fish, and he’ll eat for a day. Teach a man to fish, and he’ll eat for a lifetime.” I definitely fall into the latter camp, and I encourage you to do the same. By taking the time to read this tutorial and learning how this box is put together, you’ll learn a lot of the basics about electricity as well as how all of your accessories are installed in your vehicle. This will serve you well, especially if you’re out on the trail offroading and you need to trouble-shoot a problem. Additionally, you’ll save a few dollars over the long haul when it comes to installation of additional accessories. With the RTMR in place, your foundation will be setup. Adding a new accessory is practically plug-and-play.

Building a RTMR fuse/relay block is not beyond your ability. Anyone who puts in the time can build one. If you like working with your hands and solving problems, you can do this. If you’re willing to make mistakes and learn from them, you can do this. If you want to be self-sufficient, you can do this. Building one of these fuse blocks is not difficult. It just takes a bit of time and patience.

However, if you’re uncertain, hesitant, unskilled or impatient to take on this project yourself, you have options. Several vendors are selling this enclosure pre-built or selling something similar. Here’s a list I’ve gathered together.

SPOD

Trail Toys NSA Switch Panel

PMA 4×4

Painless Performance

Blue Sea (fuse blocks only, no relays)

Bussmann RTMR from I8Mud.com forum member

Bussmann RTMR from Tacoma World forum member, Yotamac

In Part 2 – Parts, I’ll outline all of the parts needed to build the RTMR.
Click to expand...

Up2NoGood said: ↑

I just hope people don't start quoting the OP in their replies.... computer may explode lol

The pictures are great though as a visual aid.
Click to expand...

Like that?

kitsym · Nov 5, 2015

tacozord[OP] said: ↑

Well...that could be arranged.
Click to expand...

Nah. Who would update your DIYs???

chenbro · Nov 5, 2015

AWESOME write up! I'll be referencing this whenever I get around to building my relay box.

floodedkiwi · Nov 5, 2015

Wow... Thanks for the write up.
One question I am trying to sort out. Why do you need to T-tap both the Green and Green/White wire for the light circuit?
I was thinking that just T-tapping one of them (not sure which one) and then the other side of the light to a body GRD...
and a little

kitsym · Nov 5, 2015

floodedkiwi said: ↑

Wow... Thanks for the write up.
One question I am trying to sort out. Why do you need to T-tap both the Green and Green/White wire for the light circuit?
I was thinking that just T-tapping one of them (not sure which one) and then the other side of the light to a body GRD...
and a little
Click to expand...

It's all explained in the write up. Net voltage is 0 when both wires are tapped which then allows you to utilize the factory dimming function for your switches.

tacozord · Nov 5, 2015

floodedkiwi said: ↑

Wow... Thanks for the write up.
One question I am trying to sort out. Why do you need to T-tap both the Green and Green/White wire for the light circuit?
I was thinking that just T-tapping one of them (not sure which one) and then the other side of the light to a body GRD...
and a little
Click to expand...

I'm going to assume that you haven't read that section yet, because I explain specifically why we need to tap both. If you're still unsure why, ping me again and I'll see if I can give it another go.

tacozord · Nov 5, 2015

kitsym said: ↑

It's all explained in the write up. Net voltage is 0 when both wires are tapped which then allows you to utilize the factory dimming function for your switches.
Click to expand...

Ooooooooh beat me to the punch. Well done!

kitsym · Nov 5, 2015

Here is Mike's explanation:

tacozord[OP] said: ↑

One might think that the brightness of the dash lights are changed by varying the applied positive voltage to the lamps with ground remaining at 0V. If so, then lowering the positive voltage would result in a lower potential difference between ground and positive, thus resulting in a dimmer bulb. But this would be incorrect. Although the principle is the same, the approach is opposite. By varying the ground voltage instead of the positive voltage, the potential difference between the two changes similarly, and that’s what makes the bulbs dimmer. Think of it this way: the positive source will remain at 12V and the ground is capable of changing from 0V to 12V. With the positive source at 12V and ground at 0V, the difference is 12V, and the bulb is at it’s brightest. With the positive source at 12V and ground at 12V, the difference is 0V and the bulb is off. With the ground voltage level anywhere in between, the difference between positive and ground is what defines the bulbs brightness level.
Click to expand...

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