Hi.
I may be able to help.
But I first need to know what make and model sound decoder you're using.
And, of course, which of the many Bachmann On30 locomotives that you're working on.
I also need to know:
Is it factory (Soundtraxx)?
Or after market (TCS , Digitraxx,etc.)?
Is it going to be a sound decoder installation?
Or are you trying to simply reinstall the factory board?
Some pics would also be helpful.
If it's a factory decoder, and you are needing a wiring diagram, it will be extremely hard to figure out which wire goes where.
If you're dealing with broken plug wires, it may be very difficult to figure out the wiring.
The motherboard does have wiring call outs on it,but I've never seen a wiring diagram from Bachmann for one of these little locos.
The only thing they include in the box is a blown parts diagram.
But not wiring.
And most of my On30 locos were brand new, and factory sealed when I received them
To answer some of your questions:
A Keep Alive usually plugs directly into a Soundtraxx TSU-2200,or TSU-1100 decoder.
I'm not sure if any of the open slots on the factory board are labeled for one of those(I do know that TCS includes a built in Keep alive circuit in their complete decoder/motherboard kits such as their TCS DCC WSK-BAC-8 WOWKits  that I installed in my ET&WNC #12 4-6-0 a couple of years ago. It doesn't have any call outs for the rear heaglight. Bummer.).
The headlight has call outs on the board that are clearly labeled for it.
As far as most decoders(sound/non-sound) go,they can be joined to an 8 pin JST Plug.
Soundtraxx,TCS,Digitraxx,etc come with instructions to do this.(Soldering is required).
The decoder then can be plugged directly into the original motherboard in the tender.
The Keep Alive is then plugged into the back connection on the decoder,not the motherboard.
My DCC Mogul came with a 21 pin motherboard
And it included an adapter in the tender which allowed it to be converted from 21 pins to an 8 pin interface by using the method I described above.
There isn't a lot of space in the tender of a Bachmann On30 Mogul.
So,I opted to remove the adapter ,and place a TCS 1525 WOW 121 21 pin decoder in the tender.
This required a little modification(removing the screws holding the motherboard down in the tender,and then gently turning it around backwards so that the new decoder would fit!).That,plus a little electrical tape added for insulation.
Here's a link to the decoder I used(if you need a sound decoder for this installation):
https://www.ironplanethobbies.com/product/tcs-wowsound-wow121-steam-1525-21-pin-dcc-decoder/
I've installed decoders in 6 of my On30 locomotives which have ranged to a simple plug and play installation, to a full hard wiring job in a 2-4-4-2,and one of my 4-6-0's.
The latest one I did was the a fore mentioned installation on my 2-6-0 Mogul.
Initially, I didn't have the decoder seated quite right on the motherboard,which caused the headlight not to operate.
Honestly,I was feeling a little frustrated with this little 2-6-0 due to an unrelated issue with the guides on the steam chest.
So,I put it back in its' box,and posted about the issue on this forum.
Nobody had an answer,so I finally decided to look into the issue after a couple of weeks went by(I was by then deep into playing with my new Version 7 Big Hauler 4-6-0 with full DCC and it's Soundtraxx TSU-4400 sound decoder(it's pretty darn great! What with the sound ,and all.).
It took about 2 minutes for me to realize my mistake when I finally took the tender for the Mogul apart to look into repairing the issue.
The TCS 21 pin decoder was not quite all the way seated down on the motherboard.
There was a small gap between it and the motherboard.
I pulled the decoder off to check to make sure that the pins were all straight.
Took a moment to inspect all the wiring.
All was good there.
 I then carefully replaced the decoder on the motherboard, put it all back together,and then track tested it.
The head came on immediately!
So that was what the problem had been all along!
Funny how a slight misalignment like that caused such a problem!
Here's a link to the video of the now fully operational locomotive on my desktop test track :
https://www.youtube.com/watch?v=Opdtn--q7zk
Let me know if I can help!
Mike!

Ok.
Took a break from running my new Big Hauler Version #7 4-6-0 with DCC and  Soundtraxx TSU-4400 sound.
So, I finally decided to take an in-depth look at the mogul to see if I could figure out its' headlight issue.
Disassembled the tender and checked for a loose/broken wire.
All good.
Pulled the TCS 1525 21 pin sound decoder board to check for a possibly bent pin on the motherboard.
No problems there.
Had noted before pulling it that the decoder was not completely set all the way down on the pins.
Made sure that it was properly aligned and set on the motherboard after reinstall.
Removed all 4 wheel sets,cleaned all 8 wheels and lubricated the axles.
Reassembled tender.
Connected to locomotive and placed on tracks for testing with NCE Powercab.
Worked flawlessly, thusly answering my own question.
Mike
Thanks!

Hi. Good decision! It's trash.

Hey everyone!
I bought a Bachmann D&RGW #138 Mogul brand new on Ebay a few months ago.
It is the newer model with DCC.
So, for $100.00, it was a great deal.
The DCC board is a 21 pin model made by Soundtraxx.
Decided to put a sound decoder in it using the factory board.
I bought, and installed, a TCS 1525 Steam 121, 21 pin decoder in the tender.
Just like the Bachmann sound module, it is basically drop in.
Except for one thing:
You will have to turn the factory board around in the tender due to limited space.
After that, you're good to go.
And ,for those who don't know, the TCS decoder is far superior to the Bachmann sound module as it has far sounds and features at a price that is generally lower than the sound module is(if you can find one other than on the Bachmann website).
It's pretty fantastic!
I don't know what I've done , because the headlight no longer comes on.
I'm not certain what might be going on as I've checked out the wires in the tender,and they all look good.
I installed a TCS sound decoder just like this one in my 2-6-6-2, and it works flawlessly.
I'm thinking that I might have loosened a wire somewhere,but I don't see anything.
Any ideas what might be different with this one?
Thanks!

Will do!
Thank you !

Hi. That's very kind of you.
I'll probably take you up on that offer if I don't hear from the seller by Sunday evening.
Are you sure you don't need it?
They're very hard to come by.
And I don't want to leave you short should you need it.
I'll PM you by 6PM on Sunday if I haven't heard from the seller by then.
Can I send you any money for it? Or at least pay for the postage?
Let me know .
I highly appreciate this!

Tender coupler pocket.
You have to remove the rear truck, and then a cover that is held in place by  small screws.

More Photos!

Let me be the first to introduce you to the new Bachmann Big Hauler tender coupler(?).
I hope this will clear up any confusion about this item.
The shaft is metal while the coupler itself is plastic.
The coupler is removable from the shaft by removing the screw and popping the coupler off the end of the shaft.
I have never seen a coupler like this one in over 20+ years of running G Scale trains.
I believe that this coupler is unique to this locomotive and tender.
Not even Kadee makes a direct replacement for it.
At least not yet.

Thanks for the idea!
Last night I bought a set of Kadee 1901 Type E Centerset Couplers on Ebay.
They only cost less than $12.00, they're center set ,and they have a slight downward tilt to them.
They're also a direct drop in to the Kadee #831 gearbox.
I'll see how well they work,and make a decision on what to do if they don't work as I hope.
Thanks again!

Hi.
Thanks for the idea,but I don't think that it would work with the new 4-6-0 tender.
The coupler that comes with it is unique.
It mounts to the frame under the truck,not directly to it.
So, therefore, some modification would be required to mount any other coupler to the tender.
Personally, I don't understand the need for Bachmann to have changed it so radically.
It just makes for a giant pain if you don't get both couplers in the box with this thing.
Especially if all your cars are equipped with Kadee #831 couplers,which are just a little too high to mate properly with the low coupler.
I'm still waiting for the seller to reply whether he found the high coupler or not.
I may be able to modify one of the cars with a low coupler from Kadee(if they make them),but I would prefer not to spend any more money on something that she should have came with.
Of course, I bought mine with the Soundtraxx TSU-4400 already installed in the tender for only $458.00.
And she runs perfectly, and sounds great.
So I don't really have any room to complain!
LOL!
Thanks again!

This is what the TSU-4400 looks like in the tender of the new 4-6-0.
I did not install it. It was installed by the previous owner.
I would recommend contacting Bachmann, or Soundtraxx for more information.

Nope.
Kadee lists their G Scale coupler for the Big Hauler 4-6-0 as the #791.
It is a truck mounted coupler with gear box.
How is that going to be mounted on the new tender for the 4-6-0?
The tender rear coupler set up on the new version 7 tender is radically different from the one on the original Big Hauler tender.
https://www.kadee.com/shop-by-category/couplers/g-scale-couplers/791-g-scale-large-offset-centerset-coupler-with-pilot-pocket-gear-box-truck-mount-gearbox
I've messaged the seller, and he promised to look for it.
I guess that I'm out of luck if he can't find it...

Hi,Mr.Bachmann!
I recently bought a very slightly used new model 4-6-0.
On page #9,figure #14,the manual describes 2 different knuckle couplers.
One high.
One low.
The seller only included the low coupler installed on the tender.
The high coupler is nowhere to be found.
I need the high coupler,but there are no parts listed for the new model yet.
What do you suggest?
Thank you!

I have that exact same model.
It has a Soundtraxx TSU-4400 DCC sound decoder installed in it.
Sounds,and works great!