What is this button for?

For night driving mode

 

Top is power divider, bottom is a neutral switch. Dad had one in his Mack years ago and I'm pretty sure he used it when he was connected to a conveyer that was milling the road that pulled/pushed the truck

 

That's two buttons.

 

The top button is for when you can’t get a four wheel burn out, so you press that button and it will lock the axles, then roll through the bleach water and gas it hard. That will heat up and clean the tires for maximum traction.
The second button is for the nitrous gas control. You should only use that when you’re getting ready to launch, maybe a shot for a burn out. Thats not something you want to have turned on when just rolling around. You gotta have lots of throttle when you use that. Could blow a motor like that.

 

The last Mack I drove was 1984 R-Model..... " No Nitro Button".

 

if I had to guess Neutral switch, turn it on with pto place trans in 4th gear and put her up, I haven’t been stuck inside that new of a pile of #### though