‘21 3500 SRW Gas Mods

[Joker421]

Picked up a few handy tips from this site, so figured I’d post my mods to possibly help out the next guy. We pull an 8k lb travel trailer with the whole fam and went with a 3500 for all that glorious payload to bring more toys along.

Started the mods with a K&N Cold Air Intake - the first mod I’ve put on all my trucks for squeaking out a little extra mpg:

My truck didn’t come with the aux battery option, but it does have the 220a alternator. I actually prefer installing my own aux battery setup because I wanted to isolate the aux battery when the truck is off so I don’t burn down the starter battery (factory aux battery configuration just joins the starter and aux batteries to make one big battery). So I installed a 500a relay (overkill) and all the cabling (0 gauge) and put my aux battery (cheap deep cycle lead acid) in the oem tray. The key in this configuration is to run the negative from the aux battery to the negative terminal of the starter battery so the load of the aux battery passes through the hall sensor, keeping the smart alternator charging after the starter battery is topped off. Here’s the relay that isolates the two batteries:

Also ran a switch inside the storage pocket next to your right leg in the drivers’ seat so I can override the isolation manually. If my starter battery dies, I can flip that switch to connect the aux battery and jump the truck. The aux battery is connected to a bus bar on the firewall, and I ran a 2awg cable back into the bed, following existing wiring, to power accessories:

I added a topper on the bed - A.R.E. CX HD. This topper has a metal skeleton under the shell that supports the bars above so they can hold 500 lbs dynamic. Also had them install toolboxes on both sides that each hold 250 lbs, and had all doors made of metal with single T-handle latches. The aux power runs to a fuse box mounted on the side of the driver toolbox, and that distributes power to the air compressor, SUP inflator, lights, and aux outlets:

Ran LED strips from that fuse panel in the roof and in both toolboxes on prop switches, and then added LED flood lights under the corners of the rear bumper for better visibility backing the trailer at night at wide angles. Next step is to install lock actuators on all topper doors so they can be controlled by the factory fob and I can ditch this extra key.

Here are a couple more random views of the bed/topper:

 

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[Joker421]

Have another project on the books, but haven’t quite figured out how it’s going to play out - I’ll update once it’s complete. Love the topper, but it has blocked the rear view mirror camera that’s mounted in the 3rd brake light of the cab, and since I went with metal doors, there is zero visibility with that mirror. I want to move that camera under the 3rd brake light of the topper, but the oem camera in the truck compensates for the length of the bed, extending the view too far for the new mounting location. I ordered the same camera from a 2021 Yukon and a Fakra Z extension cable. I’m hoping 20’ is enough to route from old to new locations. I’ll have to drop the headliner to connect the extension cable, and I’m not sure yet how I’m going to mount the new camera. Looks like it’ll need some sort of waterproof housing since it normally lives in a brake light enclosure. If anyone has done something similar I’d love to hear about it…

 

[Ohiost90]

That is a nice-looking setup. I'll have to take a look at the K&N as well as the extra battery.

 

[Joker421]

Caught an mis-price on Amazon on the K&N one night, so I bought 4 of them. Listed the other 3 on ebay, and there are two left for $200 ea, new in unopened boxes. These fit the gasser, not diesel, but they’re a really easy install for a 5% horsepower bump and a lifetime filter, and claim to not void the warranty. I always do this first so it can start paying for itself at the pump.

 

[Joker421]

Another mod I’m seriously considering is an oil catch can. I noticed while installing the cold air intake that the PCV vacuum line had some oil in it, and I don’t love the idea of that being thrown into my intake. Anyone else have experience running a catch can on the 6.6 gas?

 

[Joker421]

Added some back-up LED’s to the corners of the rear bumper for backing the trailer at sharp angles at night. Spliced them into the reverse, and they come on anytime I put it in reverse OR when I unlock the truck and it’s dark outside. We’ll see how they hold up mounted behind the rear tires like this, but they were a cheap addition with just a little effort fabbing mounting brackets.

 

[fieldgeneral]

got any more details on the dual battery setup? brand/model/parts/wiring tips, wiring/mounting points, etc
wanting to do something similar in a 1500 at4, was looking at redarc as an isolator but that's about as far as I got

 

[Joker421]

Sure, I have LOTS of details. I looked at the RedArc first too, but ended up liking this better. There’s a good writup on a similar setup here:

Dual Battery Setup on My Silverado for Camp Power - Andy Arthur.org

Important Update: Make sure to connect the negative terminals of the two batteries. Grounding to the frame for the accessory battery will not excite the hal-effect (current) sensor, which is located on the negative terminal between the starting battery and alternator on the truck. Not making...

andyarthur.org

The relay mounted to the firewall is a Stinger 500amp:

https://a.co/d/3C79dqm

Bus bar mounted to driver’s side of firewall:

https://a.co/d/bQZ9jjl

1/0 AWG red 10’ cable:

https://a.co/d/29Cjr4S

1/0 AWG black 10’ cable:

https://a.co/d/7UzYEAV

Wire loom for 1/0 AWG cable:

https://a.co/d/aqiZLJo

Diode for override switch:

https://a.co/d/btYV7hH

You’ll also need a fuse tap for the ignition switch, and various ring terminals, heat shrink tubing, zip ties, and silicon terminal boots.

Here’s a wiring diagram for what I did:

…and then the wiring diagram for the fuse panel in the bed:

 

[fieldgeneral]

awesome! thanks for all of that, kind of a newbie in terms of this level of complexity in wiring (i've done stereos, simple light bars, etc) - what are the switches/diodes in your diagram for? presumeably the bottom one is to the fuse box to indicate to the isolator that the truck is running, but why is that joined back to the bus bar?

 

[Joker421]

I put a small switch in the storage pocket next to your right leg in the driver’s seat. That switch is wired from the aux battery (bus bar) and goes to the diode. The other input in the diode comes from a tap on an ignition fuse. The output of the diode controls the big relay between the batteries.

So, when I start the truck, the ignition tap closes the relay, joining the two batteries and charging both from the alternator. When I turn the truck off the relay opens (with an audible “thunk”), isolating the two batteries.

But what if my starter battery dies for some reason and I’m out in the woods (and the aux still has life)? That switch allows me to override the relay, connecting the two batteries so I can start the truck from the aux. Basically a set of jumper cables in a switch.

Wasn’t sure if sending the relay 12v from two sources would be bad, but I mainly didn’t want to send current from the aux battery into the ignition tap by just connecting the 3 wires together.

 

[Joker421]

I used another diode for the lighting inside the bed - the back corners and the light strips in the ceiling of the cap. One input into the diode is controlled by the bed light switch in the dash. The other input is a prop switch on the gas strut of the back door of the cap, which gets its 12v from the aux battery. When I push the bed light button in the dash, it powers all the bed lights from the truck, but when I open the rear door they’re all powered by the aux battery.

Didn’t need diodes, but I wired the side toolboxes the same way with prop switches and led strips. They come on when you open the door, and are powered by the aux battery.