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Trailer Light Receptacle Wiring

fastback86

fastback86

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Central Florida
I just picked up a 1998 GMC 3500HD 2wd 6.5L turbo diesel with a Knapheide service bodyand I need to install a trailer light receptacle. The truck already has a receiver hitch installed but no sign of a trailer light receptacle. I assume there's a connector where the service body plugs into the truck for the lights. Does any one have a photo of the connector or know what color the wires are? Also, how do I know if the truck has a towing package?
 
You should be able to go down to the local auto parts store and order a T-connector.

If not,

white is ground
brown is parking lights
green is right
yellow is yellow

Welcome to the Truck Stop!
 
Is this the right T connector?

4

http://www.autozone.com/autozone/parts/_/N-9cj54?itemIdentifier=70596_98695_0_


If it is we carry those connectors at work and I can make it on my lunch break. I picked this receptacle up and it looks like it plugs into it.

4

http://www.autozone.com/autozone/accessories/Hopkins-4-wire-flat-to-7-RV-round-and-4-wire-flat-vehicle-to-trailer-connector-adapter-Vehicle-side-OEM-no-splice/_/N-25wp?itemIdentifier=237220_0_0_
 
pictures of the truck please! welcome to the site!

If its what I think it is, you might have a 3500HD. the real one with 19.5 rubber and leaf springs and disc brakes at all 4 corners.

the t-harness should work, and you can either get a plug and play 7 pin, or cut off the 4 flat plug on the t-harness and wire it into a plain 7 pin connector.
 
Here ya go...

IMAG0506.jpg

It's a little beat up but it seems to go down the road ok. It used to belong to the Mouse House apparently (I can see where some Disney decals had been).
 
GM Guy said:
pictures of the truck please! welcome to the site!

If its what I think it is, you might have a 3500HD. the real one with 19.5 rubber and leaf springs and disc brakes at all 4 corners...
Click to expand...

Thanks and yes, though I haven't verified the disc brakes yet.

GM Guy said:
...the t-harness should work, and you can either get a plug and play 7 pin, or cut off the 4 flat plug on the t-harness and wire it into a plain 7 pin connector.
Click to expand...

So the one I have pictured is correct? As I said I have the receptacle already that has the flat 4 pin trailer connector and the round RV 7 pin. So assuming I don't have the towing package I just have to run wires up front for the brake controller and the auxiliary power.
 
Really nice wiring diagram, color coded even. Thanks!!

I printed it off, stuck it in a clear sleeve, and put it in my info notebook.

Don
 
Thanks for the diagram. When I wired the Tahoe the diagram that came with the plug was wrong and when I search the internet I got several different configurations when I expected to only get one.

I ended up checking my truck and wiring the same as that. I knew it was correct as I have pulled other peoples trailers and everything worked.

I've found that you can't necessarily trust somebody else's work, when they wired their vehicle and trailer at the same time. Everything works for their set-up but won't work with anybody elses
 
fastback86, you have a true blue 3500HD. congrats!

as far as wiring, the two items you have pictured look identical to what I have, so it should all work. and then yes, run two wires for brake and aux power.

and for 98, use a big lug on the back of the underhood electrical center. the stud is metric, so get a metric nut to clamp down on the terminal. as far as a brake signal, probe around on the pins on the under dash electrical center, you should find a pin on one of the bottom rows IIRC (If I Remember Correctly) that is dead normally and hot when the brakes are applied. no cutting or splicing is needed on these trucks to wire up trailer connectors.

Finally, you havent told us what transmission the big girl has! add that to the signature as well! run your VIN number at compnine.com to find out the gear ratio for the rear axle and see what other cool options you have.

good luck!
 
GM Guy said:
fastback86, you have a true blue 3500HD. congrats!

as far as wiring, the two items you have pictured look identical to what I have, so it should all work. and then yes, run two wires for brake and aux power.

and for 98, use a big lug on the back of the underhood electrical center. the stud is metric, so get a metric nut to clamp down on the terminal. as far as a brake signal, probe around on the pins on the under dash electrical center, you should find a pin on one of the bottom rows IIRC (If I Remember Correctly) that is dead normally and hot when the brakes are applied. no cutting or splicing is needed on these trucks to wire up trailer connectors.

Finally, you havent told us what transmission the big girl has! add that to the signature as well! run your VIN number at compnine.com to find out the gear ratio for the rear axle and see what other cool options you have.

good luck!
Click to expand...

Looks like I have a 4L80E trans and a 5.13 in the rear axle. I was under the back the other day looking for that harness connector and I do have disc brakes all the way around. I found the terminal in that block under the driver's side dash for the brake signal too. I'll probably run the wires up front today for the receptacle and brake controller so they're ready when I get a controller. Thanks for your help!
 
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