bayer-z28

iTrader: (14)

I've been doing my homework on this for almost a damn month now and I never would have guessed these 4 stupid wires can be such a pain in the ***..

Anyway.. I've dealt with this at work a couple times, but mostly fixing clipped cables and repairing the connectors.. But never installing from scratch.

I took a job to fix a buddy's hacked up wiring because he was taking it out of state that evening. I gave him a temp fix to get him by but I'm not happy leaving it like that so I'm going to make it permanent and I have to do my truck now too.

I asked my uncle who was a GM master tech for quite a while and he told me how to do the OEM way of connecting the leads under the fuse block, install the appropriate fuses and then go in the cab by your left foot and connect the harness there, that goes to the trailer lighting.


So I popped the cap off the junction block and I'm sorry but I can't make heads or tails out of this and It doesn't even match anything. ???? Normally I'm decently good with wiring (been around that block many times) but this is just giving me a headache. It looks like a relay block diagram but it doesn't match this block.... at all..

Does anyone have a pic of this block where the LIGHTING connector would plug into and something that I can make out where each lead has to go for L,R, Brake and Marker. I have a converter box that's on his truck now. he has an 01 1500 Silvy and I have a 2000 2500 Silvy.


Here's my block and the cover. I'm sorry if this sounds stupid, but I'm losing my mind here..





I was basically wanting something like this to Tie into the junction block in the back. And just add bigger fuses up front.

thunder550

iTrader: (70)

gmupfitter.com has a lot of these types of diagrams I believe. That box down by your left foot is called the MBEC (mid bussed electrical center), might try to search for MBEC wiring pinouts or diagrams too.

1FastBrick

iTrader: (17)

See attached image this is from my 2000 silverado. This is were the brake controller plug attaches. The one that is not wrapped in tape is coming from the brake controller.

Edit: Stupid mobile turned it sideways, I corrected it putting it upright and put an arrow pointing to the harness plug for the Brake controller.

The Wiring from the rear 7 port round plug Pigtail goes to the under hood fuse box. Lug goes on top of a battery post there, and the other wire will attach to the blue plug that is not connected to anything.

Picture of blue plug stolen from here... https://www.performancetrucks.net/fo...-power-537466/

I also attached the GM conversion harness directions so you will get a better understanding of what's involved. That harness your looking at should have some instruction as well.

bayer-z28

iTrader: (14)

I don't need the brakes, but thanks still! Very helpful!

My 2000 Truck has the older 5 pin round lighting connector and the wiring (finally checked today) so I have to start from scratch on my buddy's truck...

thunder550

iTrader: (70)

After re-reading your post, I'm not sure that I understand what you're after. I don't recall there being anything other than a plug for the trailer brake controller under the dash...IIRC everything for trailer lights would be in the underhood fuse block only. There wouldn't be any reason for the light harness to route through the cab.

Am I understanding correctly that you're trying to add or activate circuits in the underhood fuse block for trailer left, right, and running lights, and that's it?

thunder550

iTrader: (70)

I think whoever explained the lights to you overcomplicated things. You need three power leads, one for left turn, one for right turn, and one for parking lights. The 4th wire goes to ground. There shouldn't be any reason to go to inside the cab at all. The OEM trailer wiring also does not pass through the rear junction block.

You should just be able to install these three fuses and run wires to the back of the truck...

bayer-z28

iTrader: (14)

I think whenever I mention "trailer" and "wiring" in the same topic people automatically jump to the "brake controller" default which requires that 30a Maxi-Fuse, connecting leads and using that MBEC. This is why a google search was getting so frustrating after three weeks.

Whoever had my buddy's truck before him hacked into the wiring at the tail lights. and it was just bare wire twisted together with a little bit of electrical tape over it. So I just got rid of that and used butt connectors for now... it was JUST for TEMP! Not intended to be permanent. So they ran power to the trailer from that but there was no ground.... ??

I think the only reason the lights were working in the first place was that they were grounding through the trailer ball into the truck. I added a new ground right to the frame. He had a slew of blown fuses and blown bulbs. so I fixed all that, added the converter and added 5 Amps to the OEM fuse recommendation to compensate for the added lighting. It lasted for a couple hours till later that day when he said he lost everything.. ??? WTF now?! ok.. Whatever. I'll deal with it later.. Everything worked when I left it. I even put a big block of wood on the brake pedal (working alone) to test the marker lights and brake lights and signal lights and the brakes.

So now I'm going to solder the OEM harness back to stock and start from scratch and get rid of the hack-temp job.

So, short story long.. he's just asking for the lights and I guess my truck already has that so I have a little bit to draw from. I was looking to duplicate the OEM setup. It has an adequate power supply from the fuse block under the hood and I can re-pin a connector to work with the MBEC to run a basic 4 wire lighting harness to the back and use the converter if need be.

I'm basically going to be using your idea though. It's just taking me an abnormally long time to wrap my head around this project. Like they say, though, it's the simple **** that can throw you off.

thunder550

iTrader: (70)

Gotcha....I still dont think the OEM trailer lights run to the MBEC at all though.

1FastBrick

iTrader: (17)

I don't believe they do.

It should all be under hood fuse box as the wiring diagram suggest.

I am pretty sure, Only the brake control goes through there for the trailer brake function.

bayer-z28

iTrader: (14)

^ My light bulb just turned on...... After three weeks.. lol