Trying to identify a key-on switched circuit, dashboard fuse box, '13 3500HD LML

Pullshard

New member
Nov 27, 2015
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hey folks - as the post title states, Im looking to find a key-on switched circuit for a UHF/VHF radio

I have a mini-fuse ATO (add a circuit), and I was popping it in/out of various circuits (the radio wants a 10A, so all good)

1 rear seats (not sure what this circuit actually does, but it has constant power)
2 rear acc. power outlet (constant 12V)
13 rear climate controls (don't have the rear climate controls, no power)
16 accessory power outlets (assume these are the 2 outlets on the dash, no power)
19 rear seat entertainment (don't have, no power)
23 rear wiper (don't have, no power)

the rear circuit surprised me with 12V even with the key out…no idea what it does...
Do I need to go thru the firewall to the engine compartment fuse box, to find an open circuit?

Ive attached a document that has all the '13 3500HD pickup fuse blocks for anyone that wants a quick reference (both gas & diesel)

EDIT: I was chatting with my brother, and realized I was doing all this troubleshooting with the driver's door open, which MAY affect the ON-TIME, given some accesories stay powered on after key out for minute - maybe not with the door already open, but it's worth checking when it's not a windchill heading into the teens...oh well, ever forward!
 

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Ron Nielson

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Oct 11, 2009
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Berryton, KS
wiring diagrams in the ELECTRICAL MANUAL for your truck and model https://www.gmupfitter.com/pages/body-builder-manuals

I didn't look to see what is available for your truck, but on my 2008 mode there is a fuse block near the brake pedal called MBEC and there are likely open. Also, look for Retained Accessory Power (RAP) in the wiring diagram There should be a radio circuit connection, I THINK. I wired my lift pump with a connection in the MBEC.
 

Pullshard

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Nov 27, 2015
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hi Ron - ty for the advice. do you mean this one? (found a thread on another forum where somebody wired in their seat heaters)

GM_pickup_MBEC1.PNGGM_pickup_MBEC2.PNG
 

Pullshard

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Nov 27, 2015
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found some MBEC connectors on ebay, and a terrific write-up on another forum


not sure of the etiqutte, so see picmbecusagethread.PNG
 

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