What's the inc/dec button for?

Increase/decrease idle . The switch below is for checking your ecm codes.

 

Yeah, I'm thinking it's a Freightliner as well, mid '90s possibly, and they did idle up automatically in cold weather. I drove one of those for almost 10 years and the dash looked exactly liked the one I drove in. It even had air start as well, which was a pain in the ###, because if the previous driver didn't leave enough air in the tank, the truck wouldn't start. Plus the wipers, mirrors and windows were air as well. You couldn't even drop the air bags when unhooking from trailers.

 

I doubt it's for anything. Usually the ECM takes care of all that. Your year model was very similar to mine, and usually they hook up the ECM underneath with the key switched on to power the laptop that they use and they control all aspects of the truck, such as governor and gear ratio. It MAY be used to clear the check engine light, which again can be done through the ECM.

 

My wild guess for the eng check switch would be to read out stored codes from the ecm. Simpler than some of the newer engines.

 

The switch underneath is to check your ecm codes that are active.
If your check engine or shut off engine lights come on.
With key off.
Then toggle switch down. Key on don't start motor. Cel lcomes on and blinks 3 numbers. 2 blinks pause 3 blinks pause 1 blink is code 231. This sequence will repeat. If more than 1 active code you will get more numbers.
This doesn't give you inactive codes you need to read those direct from ecm

 

Also the ck. switch can be held in the up position to override the eng. shut/down for long enough to make your truck move off to side of road when eng. shuts down because of low coolant or whatever your truck is still set up to shut down on?

 

The final flash code will be 2 5, that means the blink cycle is complete. The blink codes on the red engine light are active codes, the yellow light codes are stored codes.