Brake lights - what does DOT require?

On this 359, the trailer harness suffered a meltdown in the past.

OEM wiring according to the manual shows independent brake and directional signals. There are 3 switches that control brake lights - a service switch, a parking switch and a trailer breakaway switch. Plus, the directionals are independent, with the tractor using a shared turn/stop light, and the trailer using separate lights for directionals and stoplights.

As the truck is wired now, they have one stoplight switch that operates both the tractor and trailer stop lights. They mounted separate brake lights on the bobtail. And they have only 3 wires connected to the directional switch to control L/R and 4-ways across both truck and trailer.

So, will this pass DOT? Will they test that when in park the brake lights will be on? Will they simulate a breakaway and test to see if the lights come on?

John

 

Additional data - the wiring diagram on supermiller shows a different wiring situation. My manual is a 1978 manual and my truck is a 1978 model built in 11/77. The supermiller schematic is not dated that I can find.

Supermiller schematic shows a service brake switch that runs the tractor with a line to the spring break switch that runs the trailer. No parking break switch.

So it appears that I may just need to install a breakaway switch. This is a switch that is dual purpose I think. It takes the stoplight signal and passes it on to the trailer in normal operation, but also can supply power to the trailer brake wire in the event of a loss of emergency pressure to the trailer. That would cause the brake lights to come on as the spring brakes are starting to engage.

Am I correct?

John

 

yes. 3 paddles. I am going to rewire it like the newer 359 schematic. I got a Haldex breakaway switch at NAPA so I can now hook up the trailer brake signal correctly. I will then just wire the tractor switch direct to the back lights on the bobtail. I can then leave the directionals alone. Should work fine.

On this 3 paddle system, when in park, you use the third paddle to charge the trailer along with the Trailer valve. It then pops out when you push in the parking valve.

I am guessing they use this switch in the event of partial breakaway upon loss of trailer supply air. If the spring brakes are being applied I guess they want the brake light to be on automatically. The switch goes on at 55 psi. On a complete breakaway - #### happens.

John

 

Swaan, yes that is the way this truck was wired. Thanks for the info.