Maestro

Hello all,
Jumped from my ol' '96 to a 2012 back in May, been lurking around here ever since, and I've finally got some time to crack open the tool cabinet. This seems to be one of the only forums around that has a good amount of discussion on the GMT900s. And I need it, making my 18 years of technology jump...

I'm wondering if anyone has any information on adding to the main under-hood fuse/relay box. I'm going to be installing some lighting, and want to put the fuses and relays in the stock box. It still has space for 3 micro 280 relays and plenty of fuses, but it looks like it will require some very particular terminals to actually make the connections between fuse or relay terminals and the disconnect blocks on the bottom of the box. Has anyone done this or have suggestions on where to start?

Thanks!

01WS6/tamu

iTrader: (36)

Don't touch the stock block. Use high quality aftermarket relay's externally and grab power from the AUX fuse output studs on the underhood block or directly off the battery.

Maestro

No offense intended tamu, but it doesn't help me at all to simply say "don't do what you're planning". WHY not touch the stock box? Because no one knows how to do it or because people have tried to do it and fried something?

I know I'm new to this forum, but we can skip the save-the-newbie-from-himself type responses. I have something I want to do and I'm going to try to do it. If I fail I will be back here telling folks what not to do, if I succeed I'll be back showing how to do it.

I know it's not common practice to integrate with the stock box. I know that--done improperly--the repercussions can be significant. However, there's likely no reason that accessories can't be wired into the stock box while staying electrically separate from the stock wiring. If necessary, ground and supply straight from the battery could be run into the box to stay completely isolated.

SLCviaAK

I'm assuming 01WS6/tamu is trying to save you some heartache. There have been many trucks, modified and stock, where the connectors on the bottom of the fuse block have caused really weird electrical gremlins. Part if the reason is because it is a "quick connect" type connection. If you disturb it, it may never be the same again. Just come off the accessible hot post, to a relay to power your accessories. Don't cause more problems by unnecessarily disturbing things that are working correctly right now.

Just my $.02

01WS6/tamu

iTrader: (36)

As stated above But if your hell bent on it I'll be on here somewhere when the I have a gremlin threads start to show up.

It is in your and your trucks best interest to not mess it with. From a gm tech to you my advice and not a message board know it all. Proceed at your own risk.

The UBEC is comprised of a visible block assembly. This sits on top of a printed circuit board that carries power ground and serial data comm signals in and out of the UBEC assembly to the power train and forward lighting and rear lighting and fuel pump control module. Just because you see a relay on top and or fuse spots available they generally are not landed on the printed circuit board underneath rendering them inaccessible to make any physical connections without hacking the UBEC.

Welcome to the boards and enjoy lots of reading and post often we are all more than happy to help.

01WS6/tamu

iTrader: (36)

FYI. The MBEC is located under the dash by the EMER brake assembly on the firewall. I have had trucks in on a wrecker and not covered under warranty by improper connections for adding additional equipment. It can be done you just have to get juice at the correct places or the wrath of the computer gods will result with very odd results sometimes.

INFORMATION
y
Bulletin No.: 08-08-45-004C

Date: April 08, 2011

Subject: Installation of Electrical Aftermarket Accessories - Battery, Ignition and Ground Feeds - Do Not Splice into Wiring Harness (Install Diode to Solenoid/Relay to Suppress Voltage Spikes)
Models:
2007-2012 Cadillac Escalade, Escalade ESV, Escalade EXT
2007-2012 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2008-2012 Chevrolet Express
2007-2012 GMC Sierra, Sierra Denali, Yukon, Yukon XL, Yukon Denali, Yukon Denali XL
2008-2012 GMC Savana

Supercede:
This bulletin is being revised to add the 2011 and 2012 model years. Please discard Corporate Bulletin Number 08-08-45-004B (Section 08 - Body and Accessories).

Installation of a Diode to Suppress Voltage Spikes

When an electromechanical solenoid or relay is de-energized rapidly by a mechanical switch or semiconductor, the collapsing magnetic field produces a substantial transient voltage in its effort to disperse the stored energy and oppose the sudden change in current flow. These voltage spikes can occur at the positive terminal when the solenoid or relay is de-energized (keyed-off). If a solenoid or relay is wired onto the Run/Crank circuit of the vehicle to control aftermarket equipment, the spikes can be transmitted onto the circuit. The spikes can permanently damage the internal circuitry of the sensitive electronic components and/or control modules that are on this bussed circuit. To prevent damage to these components, the solenoid or relay MUST have the control circuit suppressed with a diode.



Install a diode, P/N 12112422, across the coil of the solenoid. It is important that the striped end of the diode be connected to the positive terminal of the coil and the other end of the diode be connected to ground.


Important
Be sure to insulate the diode with heat shrink tubing before installing as shown in the picture above.

Notice
Some solenoids/relays may only have a positive post and will get their ground through the mounting bracket. In this case, the striped end of the diode is to be connected to the positive terminal and the other end should be connected to the ground of the solenoid/relay.



Install a diode, P/N 12112422, across the coil of the relay. It is important that the striped end of the diode be connected to the positive terminal of the coil and the other end of the diode be connected to ground. Be sure to insulate the diode with heat shrink tubing before installing.

There are two different areas on a fullsize truck or utility vehicle that power and grounds can be acquired without having to splice or cut into the existing wiring. One is the MBEC and the other is the UBEC.

MBEC - Mid-Bussed Electrical Connector
The MBEC is located below the instrument panel to the left of the brake pedal (#4 in the graphic points to the MBEC).
MBEC Location:



The MBEC has 10 positions for connecting electrical connectors. One of these positions is designated for aftermarket utilization. Install a connector (P/N 20791502) into the open position identified in the following graphic.
Within this connector, there is a fused 30 Amp battery feed, a fused 15 Amp battery feed, a fused 10 Amp Run/Crank feed, a 300 milliamp RAP (Retained Accessory Power) feed and a ground.
Location of Connector for Aftermarket Utilization:


Important
Connector P/N 20791502 comes with a one wire lead installed. This lead will need to be removed before the connector is pinned for use with aftermarket electrical devices.

The pin out of the connector is as follows:



UBEC - Underhood Bussed Electrical Center
On vehicles not equipped with an Auxiliary Battery (Auxiliary Battery RPO TP2), there is a stud that could be used for a fused battery feed at the UBEC. The UBEC is located on the left side of the engine compartment (refer to graphic).
Connecting Aftermarket Electrical Devices On Vehicles Not Equipped with TP2:


Important
A J-case fuse (1) with a 40 amp maximum rating MUST be inserted into this position (1) for the stud (2) to be powered. Outboard M6 stud (2) can be used for a fused battery feed.

Wire Gauge Selection

For any of these powered connections to be used, Circuit Protection Guidelines must be followed to assure that the circuit gauge is selected appropriately so that it will be protected by the fuse in case of a short circuit.



Parts Information

Maestro

Hmm, that is interesting info 01WS6/tamu. I generally do not like to wire to the run/crank circuit anyway, but who knows where my modding might take me, so good to know.

I do appreciate the warnings on the prior issues with even the stock box and possibilities of "gremlins". I suspected that the UBEC had some sort of board in it from my examinations to this point, and the quick disconnects did seem somewhat sketchy to me.

Even still, I like a challenge (some might call it stupid, but what's a little semantics between friends). My plan is to find an extra box at a wrecker and mod it to suit my purposes, leave the stocker alone. I intend to set it up so that the relays and fuses I add are not connected to any of the stock wiring; I'd pull the box then have to disconnect another disconnect with all my new wiring. I'm hoping I don't need two additional boxes just to get the additional terminals I'll need.

Don't worry, I won't be back with a gremlins thread. If the truck goes haywire or I get into it and decide it is indeed a dumb idea I'll let you know and you can both say "I warned you"!

RacinRust

All I can say is wow!

cam

iTrader: (9)

There is a lot of integrated systems with these trucks so tread lightly. Definitely best to get your factory wiring diagrams and be sure to use isolated circuits and you'll be fine. It is uber easy to add new "stand alone" feeds too though. Probably faster too but to do a right good job of it you need to buy weather pack or metri pack compliant blocks/relays and connectors. Make sure the grounds are good and your alternator is up to par for the new loads

Cfd3124

Realizing this is an old thread im still hoping for help....i received a lot of info here and am skeptical on my thoughts now. Just now learned about the MBEC...have seen before on a vehicle but didn't know what this was, now i do! I have an air compressor that needs a 15 amp switched circuit on my 2011 Silverado....thought about using the 15a battery feed switched by the 300 milliamp RAP feed to relay....do i still need the diode as mentioned in service bulletin or is this only a threat on the crank circuit which i will not be using?? I certainly dont want to risk anything over this and im aware this thread is old so maybe something has changed.....and finally if it is ok to use can anyone tell me which spot it is in on the mbec?
My original thougnt was simple, at least i thought: was going to wire in relay using the trailer brake wire under dash that is connected into fuse panel under hood with 15amp fuse and find any switched fuse from interior panel and go from there but the mbec solution seemed cleaner.....thanks