vinny

I would like to thank everyone for your input and helping me solve this problem. You guys are great , Thanks again

Check the stoplight switch. if it's stuck on, the stoplights will be on obviously, but you'll also have power coming back to the stoplight wire of the brake controller too.

Also, if you have a "trolley valve", make sure that isn't slightly applied, which would cause the stoplights to stay on.

Also check the foot brake treadle to make sure an accumulation of dirt or a corroded anchor pin isn't causing the foot brake to stay on slightly, which would also cause the stoplights to stay on.

You are the winner, It was the trolley valve. My brother thinks he may have leaned on it when he put the dash back together. Thank You so much, It was driving me crazy.

How did you wire the plug? Did you install a different 5,6,7 pin out back? Sounds like you might have something crossed up(if it's 7 pin, did you wire in the contant hot?), this could be backfeeding to brakelight switch?

I suppose this happens with NO trailer hooked up?

Another question: is this an air/elec unit? or plain electronic unit like used in pickups?

My air/elec unit has an adjustment for "power level". If you adjust it up too far, it will leave the brake lights on. I ended up disconnecting that wire, as it was there so "IF" you actuated the brakes by the unit only, it would operate the brake lights. Safety gizmo I suppose? I was not worried about it.

I had the trailer hooked up and put new plug which was the same and every worked fine until the next day. even checked it with another trailer and it worked fine. I will check if something is crossed. Thank You

A couple things to check. Many Macks light the stop lights when the parking brake is set so that is first to check. Did you install a new trailer electrical socket? If so perhaps a strand of wire has come loose and crossed circuits. Is the trailer connected while you are noticing this? That could be giving a feedback from the breakaway battery.

Chuck

Thank You Chuck, I will check that out

Check the stoplight switch. if it's stuck on, the stoplights will be on obviously, but you'll also have power coming back to the stoplight wire of the brake controller too.

Also, if you have a "trolley valve", make sure that isn't slightly applied, which would cause the stoplights to stay on.

Also check the foot brake treadle to make sure an accumulation of dirt or a corroded anchor pin isn't causing the foot brake to stay on slightly, which would also cause the stoplights to stay on.

I disconnected the wires from the switch thinking it was a bad switch but even when disconnected the cold side has power all the time causing the lights to stay on. I also disconnected brake controller with no luck. I will check the trolley valve and foot brake treadle. Thank You

hello, can anyone help. I have a 1983 mack r model 685. I installed an electric trailer brake unit yesterday, everything worked fine untill today.the lights are staying on all the time. I traced the problem back to the brake light switch wiring. For some strange reason now the cold side of swich has power all the time and the hot side is dead even with the wiring disconnected from the switch. what would cause that? I disconnected the trailer brake unit still no luck. A wiring diagram would be helpfull to trace back the wires. Thanks