Identifying a switch on the dash of my 389

It’s a trailer hot line switch. It provides power to the center post of your pigtail. It’s used to supply power for your trailer Abs.
Newer trucks have this post hot all the time and don’t have this switch normally.
The reason it lights up when you hit your brakes is because the brake circuit provides power to the abs if the center post is not hot.
I could be wrong, but I don’t think so.

 

I would run with it on. Because that way you’re getting power to your ABS on the trailer at all times. Otherwise, the only time he gets power is when you step on the brakes. A good way to check would be take a test light to that center post of your pigtail coming out of the tractor and see if it gets hot when you flip that switch on and goes dead when you turn the switch off.

 

That switch in my truck will raise and lower the front axel on our tanker trailers..

 

With that switch turned off your trailer ABS light should come on for a brief second when you press the brakes .That will also verify what that switch does.

 

Since he mentioned, the light comes on when he touches the brakes that made me assume it was for the ABS hot wire. It could be used as a spare in certain situations like yours.

 

The switch of the left is for the tanning bed