When I was in school they had this ancient cabover with 3 knobs on the dash. The third was blue and the lettering was worn off. What was the 3rd knob for?

Here is a screenshot from a movie (Armed and Dangerous) so you know what I am talking about.

 

I drove an old 81 r model tractor with the same thing i dont know what the blue on was for :?:

 

maybe it was an ejection seat.

 

I think it is an idiot light, just throw some electrical tape over it and forget it. :lol:

 

This has been covered here before, but here is the info:

 

The yellow one controls brakes on all axles, the red is only for trailer, and the blue is only for the truck unit. It actually helps to have that blue one, because in winter time you'd want to have the trailer brakes released so that the residue water doesnt freeze in the air lines.

 

in the winter just set the tractor brakes and release the trailer brakes to keep the trailer brakes shoes from freezing to the drums when you park

having air in the trailer lines will do nothing to stop THEM from freezing, i've had the trailer lines freeze going down the road.

 

On the subject of brakes ... does anyone know what the trailer brake handle, inside the cab, is for? I only used it to check the trailer brakes before I get going, but I dont use it while driving, because it might lock the trailer axles and cause a skid.

 

It's for tugging to make sure you have a good connection on the 5th wheel, and sliding trailer tandems. That's about it. NEVER engage it and leave the cab of the truck.

 

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How long you been driving, Bud???

The yellow one only sets the tractor brakes. A really good idea in winter so that you leave the trailer brakes released and don't have to use a hammer on the shoes to release the trailer brakes before you can move. I've seen guys pull into a fuel island, when the weather conditions are right, and after they fueled up, they had to have a service truck come and warm up the trailer brake drums so they could leave the fuel island. It can save you money and grief to pop the yellow one only, and leave the red one in so the brakes don't freeze to the drums.

Now, I don't know about the newer tractors, but on some of the older ones, when you set the tractor brakes, you only set them on one axle. If you wanted to have both axles holding you, you also had to lock the differentials. The new ones may be different, but some of the old ones only had one set of "SPRING BRAKES" on one drive axle. If they froze to the drums, it was a lot easier to get them to breake loose than the trailer.

Another good winter tip...

Get a bottle of "AIR-LINE ANTI-FREEZE" and put a shot glass of it into the red glad-hand anytime the weather gets below freezing. It tends to keep the air moving in the trailer where you need it. Otherwise, you can find that the trailer brakes will not release because of ice blocking an air line.

Now the BLUE BUTTON on some of the "ANCIENT" trucks had to do with an earlier form of ABS from back in the late '70s (If I remember right). The ones that when the state trooper would aim his radar at you, the brakes either didn't work at all, or they would lock up and if you were lucky, you had enough time to get it onto the shoulder before you stopped moving. They were a ROAD HAZARD!!!!!