BrentH

Well, I bought my truck last year as a custom order last year - It is a gas version but here is some of the things I added or that I like. I tow a 36 foot travel trailer, a dumper trailer, lots of building materials: Plywood, dry wall etc etc 2500 HD, 4x4, Long box, crew cab 18" steel rims and the better tires: I detest how aluminum rims corrode and then typically leak once you change tires, plus the steels up the load capacity Convenience Package I and II I opted for the leather interior with the bench seats (so you can lift the middle console and it is a seat) - under seat storage package for the rear - keeps things organized. The storage behind the seats is nice for work gloves, safety glasses - some cold weather gear, bungee cords etc Extra Battery and higher capacity alternator: Cheaper to add as a factory install. The extra battery only supplies a 30 amp source to the trailer. Can be handy if you boon dock or run a fridge through your trailer batteries. Some trailers have interior lights etc that you may wish to have operational when you park your truck. There is the Advanced Trailering system that may be included in the Convenience package. If so add the tire pressure monitors - cheap to add at build time - you still need to put them into your trailer tires afterwards Spray in Bed Liner - I did this after market but can come with the truck. All weather mats - they are not bad - Weather Tech would be better but also more cost I have a cap on the truck so I did not add the rear view mirror/bed view camera - I think the rear view mirror can use the tailgate camera and would have been handy. Trailer mirrors : if you upgrade you can get them with the rear lights that come on and the curb light - I did not do this and it would have been nice to have in certain times. I have the up/down tailgate - it is very nice option, especially when you are going to load things or someone else is and you just need to push the remote instead of running back to the truck. I have the upfitter switches - I added this outside the truck build and for some reason got it very cheap. It is, however, an option you have to install yourself. It is not bad, but does take some skill and farting around. I use it for a rear bed radio, LED lighting in the cap, air compressor and thoughts on a small fridge.... I added the front camper springs as it was inexpensive. Not sure if they will help for the camper per say, but a quality increase is always nice. I do not have all the safety stuff, like the proximity alerts, back up alerts or the birds eye camera stuff - I did not feel it necessary and I find the alarms etc to be very annoying (my wife has these things in her car) If you are thinking on something put it out there, I know I looked into a lot of options but it was tricky finding out how and what they affected - some things you add and they take away other things you may want

As I recall when last I towed, the app cycled through and then restarted itself and would cycle until I pushed the stop test button on the screen. If you have your trailer plugged into the advanced trailering app, I believe I just get a no faults found response. Typically I will get "cannot locate previous trailer brake settings" and I will acknowledge that and a few seconds later it will say that it located the settings and the gain is ok. I have received error messages like left trailer brake light not working or there is an issue with the trailer connector. I have not received an "unable to complete" alert other than when I was activating the tire pressure/temp sensors for the trailer. That took a bit to get working and I had to do the install thing about 6 times - works now without issue.

@03 Cobra: sorry I have been away for a while. Hopefully you got your truck ordered ok. The Package was offered for the LT package and was in the build and price. My Sales guy must have messed up on the order - or it was dropped by GM due to a lack of wire harness parts (this was mid covid crisis and there were supply issues with electrical parts) On my truck there are a few odd ends - like the heavy cable coming off the main battery that is not fused and I think is for a cab pre heater if the truck was a diesel. I also have the remote tailgate lower and Lift function. The Lift part is not an option for the LT model but my truck has it? No complaints - works great. Sorry I did not get back to you sooner. you should be able to order the extra battery package. There may have been changes to the build and price since I bought my truck. I was essentially able to build the Z78 truck into an LT for about 10 grand less - the only thing I am missing is a couple cameras and the skid plates as far as I can tell.

@clubba: I was only going by what the original factory installation has listed saying 220 amp alternator with the dual battery setup. This could be due to the loading if you have extras like the 120volt outlets or maybe it is linked into the charging management system in the truck? I would assume 170 amps would be sufficient? It maybe worth a bit more research .......

For anyone wanting to install the second Battery and has a 6.6 gas 2500HD with a main fuse block (the one on the main battery) that has a 250 amp fuse here is a wire diagram: this is the connection to the control relay from the BCM (drivers side, left under the dash). Clip wire and solder in a splice that will control the relay (2 in the drawing). This violet/brown wire applied power after the truck is started. It also runs a relay for the HVAC system. the spliced in red wire runs on the firewall at the same place you put in the wires for the UpFitters switches: this is the auxiliary battery connection with the fuse block and you can see the isolation relay. The yellow connector is pictured below. Hope this helps out someone. The control relay is the grey/green one with the red wires. Yours might be black.

Going back a page or two I mentioned that the kit was listed on the Canadian GMParts site for $65 and just had the title “switch”. My dealer ordered it in with the part number for the jet black interior- keyless start. It came in complete. I have it all installed and pulled 4 conductors out the fire wall for my options. All in it was something like $63.99 plus tax CDN. Must be a pricing glitch but I will take it!! Took about 3 hours to get it all in. Thanks for all the posts!!

@Rozz522: for all intentions the second Battery is doable if you can make a few changes to the original trucks charging system: 1. If for some reason (like mine) you have a 250 amp alternator fuse connection at the main battery fuse block you can upgrade a 170 amp alternator to the 220 amp. I did this - it was not a big deal. 2. You need a second battery that matches the original battery to keep charging reasonably equal 3. On the Auxiliary Battery you need an isolation relay that cuts out the Aux Batt during starting. 4. You cut the 30 amp connection from the original under hood fuse block and rout that to the Aux Battery through a new 30 Amp fuse. 5. The auxiliary battery positive connects from the isolation relay, through a 100 amp fuse to the under hood fuse block where the 175 amp feed comes from the alternator I Installed the factory fuse block (X50E) on the auxiliary battery as it looks good and came with the isolation relay, the 100 and 30 amp fuse. I will post my control connections once I have it installed. Far better to have it done from the factory (as was my original order they messed up ) - for a new truck (Canadian dollars) it would be 160 (batt) plus 175 (alt) for a total $335. With some salvaged parts and new Alt, Batt and relay box I am at about $550 and I have a spare 170 amp alternator.

So after wasting a lot of time looking for a wire needle in a haystack: I took the X50A fuse block all apart (this is the one under the hood). F79 for the Auxiliary Battery relay is 10 Amp and is connected to nothing. No wire comes from that terminal out of the fuse block. There is no pin for the the fuse to connect to and the fuse is just in there holding a spot. Same with fuse F80. I am no longer looking to hook up the aux battery isolation relay control to anything “original” as the truck does not have the wires necessary. I am simply going to run the control relay from an “ignition on” 12 volt power source and the aux battery isolation relay from the auxiliary battery itself. So for anyone looking to add the Auxiliary Battery in a 6.6 gasser the factory wiring will NOT allow you to connect it up “factory”. I think when I am done my install will appear pretty close to a factory install but there will be a few differences with the control wires. I checked my neighbours diesel and it has the 2 batteries and just a simple two fuse block on the second battery.

Here is the mess - no connection or even the wires according to the drawings. There is some confusion as the power from the under hood fuse panel (X50A) is from fuse F79UA and the wire is red/white but one drawing has it as red/blue - argh! Going to tape it back up and will Find another power source for the isolation relay. the next pic is what I have set up with the power as per the other drawings from the upfitters and the parts for the truck: I still have to hook up the 30 amp fuse but would like to have the battery charging properly first.

Working on the “then” right now. Not looking encouraging on my part. Just got the loom peeled back. Was hoping to see a couple (3) wires terminated right there in a small coil. It is a tough spot. Will post a couple pics after. the white arrow in the 30 amp feed from your Aux battery to the trailer plug. if you do not have the Aux Battery the feed comes from the engine compartment fuse block and is “on” all the time. You could kill your start battery if you leave your trailer plugged in and it has some power consumer on board.

No worries @wxman. Pics are super. You must be very flexible to get in there. your second pic is excellent- I will explore that connection. It is totally not where I would have checked. thank you !!!!

@Wxman You are awesome! so the grey relay in your second last pic and also in the second pic has 3 wires coming into it : red/white, black and a violet/brown The violet/brown comes from the BCM and energises the grey relay once the truck is started. The red/white comes from the fuse block right there beside the battery (X50A) and once the truck is running the grey relay closes and White/Red powers the isolation relay in picture . The black is just ground. There are a pile of errors in the GMUpfitters electrical drawings. Your assembly contradicts the wire harness from the parts options - no issues, just adds more confusion. Looks like I will need to splice into a few connections. My truck has the "auxiliary Battery" 10 amp fuse in the engine compartment fuse block (X50A) (red/white) and has a violet/brown coming off the body control ....where did they run it?? I also have an electrical heater in the cabin for a diesel engine that is not connected....WTF Here is your pic with a few questions. If you can locate the end of the connector spice, I "should" be able to chop open the wire harness and possibly find 2 ends with a seal on them. Your pictures give me hope! thank you for response. If I can get this mod nailed I will post how to do it. Terrible access to some of the stuff, had the wheel well protectors out 5 times now.....alas Regards, Brent

Hello, So i have everything installed except for the one control relay that is supposed to come off the body control module to allow charging to take place after the engine is running. A wire from the fuse panel is for the power to close the isolation relay. Some where in the wire harness should be a plugged 4 conductor with three feeds - I cannot find it anywhere - "should be" under the auxiliary battery tray.....frustrated....... I installed the upfitter switches - that job was a couple hours, everything clicked easy. If anyone has the auxiliary battery fitted in gasser 6.6 can you post a pic or two of the wire harness tie in after the first small relay?

My son just checked the truck and the battery connection to the lug I was asking about in the previous post-it is dead - no power going to it. So Hmmmmm....What is somewhere in the truck that has no power and then why is it there?

Hey @Rozz522, I ordered part number 84663447 - J Block Auxiliary Battery, 84471670 Junction Block Cover I still have to get under the hood (I am away till next week) and clarify a few things: "Supposedly" under the battery mount could be the connector X136 which would then allow you to run to the relay KR81 (Auxiliary Battery Relay 1) from this relay you go to the KR79 (Auxiliary Battery Relay 2) that supposedly comes included in the J Block Auxiliary Battery? I ordered the 220 amp alternator so I have to wait for parts to arrive. I have some questions still to answer about the fuse block X50D as I have a wire coming off it that is not identified in the Upfitter wire diagrams. I plan to make the battery hold down parts as they are expensive for what they are. A new positive cable just has a pig tail off the end connection so I will also make that verses buying a new cable - same with the ground. I downloaded a pic from this site where someone was discussing the connections. This is what it would/should end up like: Trying to figure out where the unfused terminal above the blue connection goes to? It is not on any drawings other than the Diesel engine where it would go to an auxiliary heater? I have seen YOUTUBE videos and it is not on some of the trucks?? Maybe it runs to the Invertor for the 120 outlets?? From the Upfitter site material the Auxiliary battery will charge when you are in tow/haul mode and pulling a trailer, or I assume in that mode. Still figuring out if that is the whole gist of it?

@Gr8FrozenNorth : did you ever complete this operation of adding the auxiliary battery and increasing the alternator capacity? I just purchased a 2021 2500HD LT and they "forgot" to install the package or couldn't due to missing parts. Anyway I have the parts ordered and a new 220 volt alternator on the way and will be doing this mod in a few weeks when everything arrives. My truck arrived with the 170 Amp Alternator but has a 250 amp fuse for the alternator on the main battery. For a 170 amp alternator this would normally be a 175 amp fuse. For some reason my fuse block is from a 1500 Diesel with a 220 Alternator - for me, that is a good thing I purchased the " J Block Auxiliary Battery" 84663447 - this part comes with a 30 amp fuse, a 100 amp fuse and a 175 amp fuse. The 100 amp fuse is connected to an isolator relay all as one assembly. The cover is Junction Block Cover 84471670 I am re-using the cable from the main battery positive to the terminal on the X50D (175 amp) it runs from the main battery over to the X50A Fuse Block (as per normal) and then a short pigtail will run from that connection to the 100 AMP fuse on the auxiliary battery J Block (X50E). this is the fuse attached to the isolator. The isolator is supposed to be powered from a secondary relay that wires into a connector X136. I have to find all that on the truck first but I am away on business so it will take a bit. right now I am still in research mode but it looks like that is pretty much it other than connecting the trailer 30 supply power wire up. From reading the battery relay will energize if you switch to tow/haul mode and it will charge the aux battery. maybe someone with the factory install would have more intel. I will post pictures once things get underway.

What I think is correct the the 2500HD LT for the Aux Battery circuit: On this drawing though, F79UA 10A is misnamed and actually called the AUX BATT/SCRPM in other locations of the electrical schematics

So here is the crazy thing: On my truck: 2021 2500HD 6.6 gas LT I requested the 220 amp alternator and aux battery - It didn't come that way - no aux battery and a 170 alternator BUT: My fuse panel on top of the regular battery is NOT the correct one for the set up as fitted (according to GM parts) my fuse block is part number 84646133 this is a fuse block from a 1500 diesel with a 220 amp alternator. So my fuse block (X50D) has the following F1 - 250 Amp connected to 170 Amp ALT, F2 - 175 Amp connected to Fuse Bock X50A F3 - 400 Amp connected to Starter Motor The next 2 fuses are 60 amp and run to the interior fuse panels and then I have a 1 more wire in the supposedly F4 position that is not fused and is a decent size wire. No idea where this runs to and does not jive with any wiring diagrams on the GMUpfitters. From the wire diagrams it "should" be easy enough to install the GM Fuse Block Auxiliary Battery 84663447. The wire from F2 175 amp would run to the 100 amp fuse in the X50E aux battery block and then back to the X50A fuse panel. The Auxiliary Fuse Block may already have the battery isolation relay (KR79) that is supposedly controlled with a simple connection to the body control module. (connector x136). This would be a very simple install and follows the jist of the parts diagrams for the auxiliary installation. The GMUpfitters electrical seems to be riddled with mistakes or errors , probably created by the lack of parts during covid - my truck is supposed to have the main battery fuse block X50D as part number 84646132, which I suspect has a lower rated 200 Amp F1 Fuse, not the 250 - it will make a 220 alt upgrade a lot easier though. I have an after market isolator, but for simplicity and looks under the hood it would be just as easy to purchase the required X50E OEM part. I tried to load the wire diagram and pics but exceeded the limit and then it won't let me try again...

For you guys installing this, I have a 2021 2500 HD and things look just a bit different on the electrical side and I have a question/favor to ask if any of you have the second battery: Can you post a picture of the open fuse box on the 2nd Battery? The GM part # is 84663447 and I cannot get a clear answer whether the box contains the relays and fuses or it is just bare and you need to add. From the cost it seems it should have the guts. Also, I went to order the kit 84942073 for the 9L7 ( upfitter switches) and it seemed way to cheap - like it was $65 ? that doesn't sound like a "complete" install kit so I am awaiting word back on that. I would like to install the upfitters with the 2nd battery for trailering and I work out of my truck bed at times so I would add some lights, radio etc I had requested a few options when I sent in my build but I guess some of the wiring was not done due to some harness availability out of Mexico? Thanks to anyone who can help me out - great write up and contributions all around

The GM Upfitters site has the instructions I do believe. There is a hole to drill for sure and it does provide a guide to the connector. for some reason I cannot get to the site right now due to a privacy thing from google

Is there anyone out there that can take a couple pictures of the second battery installation? Perhaps the cover off the fuse block and along the wire run? I ordered my truck to come with the second battery and they did not put this in, I instead got the powered lift and drop tailgate. This feature is not available (don't think so?) on your typical LT model so it was a bit of a surprise. The added battery is so I can power things in the back cap while I work out of it when doing on site carpentry type things. I have been through the GM Upfitters and the auxiliary battery is not yet a bulletin, also their electrical schematic manual has some mistakes - or there are some differences in my trucks wiring from what they have listed. The fuse block (X50E) that connects to the positive on the aux battery is about $165.00 the guy at the parts counter doesn't have one in stock and with covid you cannot go and look anyway - I want to know if the isolation relay and control relay are actually included in this part (GM 84663447). If not, $165.00 is ridiculous for just a piece of ABS plastic. There is no listing for the relay on the parts page so that is also confusing. The tray is in place already and the wire harness looks like it is just a short pigtail added to the main connection back to the X50A fuse box - just do not know where the isolation relay power comes from (supposed to be connector X136) so will have to find that. Any pictures would be great....thanks

That is the problem, you need to have a z71 or higher package+ to get that option. I have an LT and could not get the option unless I have the rear cargo camera - useless for me as I have a truck cap - not worth paying for. GM sorta shooting themselves in the foot as this would be a sweet "standard" edition to the LT plus packages

Looking for the same - will post if I can get some information