Trailer ABS cycle not working.

That switch probably turned power on/off to the 7th pin....maybe for a trailer dump valve.

 

ABS computer in trailer might be shot.
Do you have power to the 7th pin when ignition key is on?
Do you have a switch in the dash that controls power on/off to 7th pin?
Is this your truck and trailer or the company's?

There was a fella one time that had a trailer and the ABS light would stay on. The NYDOT always like to stand back there and watch for the trailer light to flash on and off when the driver turns the ignition key on. Trouble is that light would come on and stay on. The fella wire the 7th pin to a toggle on the dash. Now the revenuer stands back there and he turns the key on and a second later he shuts the toggle off. This cuts the power to the 7th pin and the ABS light goes out. Everyone is happy.

 

Yea I know this is an old thread but, I got dot'd yesterday and received a violation for this. So I took to shop and traced problems to both ends of the pigtail. My light still doesn't come on with the key however, if I unplug the pigtail and then plug it back in it will then come on and do what it's supposed to do. My truck is providing continuous 12v power....is that a problem???

 

Freightliner?

My Cascadia did this and it turned out there was an incorrect relay in the black rectangular plastic box mounted on the drivers side of the firewall under the hood. There should be a label inside to tell you which relay type goes in each spot. There are two types. All of the relays are of the same type except for one and it can be identified by a black dot on the label on the top of the relay. If you open that box and don't see a relay with a black dot on it you have found your problem.

The trailer ABS system turns on the ABS light on the trailer and the ABS light on the dash for three seconds when it boots up. It only does this when it boots up. If power is not turned off to the ABS computer when you turn the key off, then the ABS computer will never shut down, so it will not reboot when you turn the key back on - QED. Pulling the pigtail out and plugging it back in will cycle the power and cause the ABS light to work properly, but only the one time.

The one relay that is different has the center pin missing. That's the only difference. The other relay type will fit in it's slot and that center pin, when present, will provide continuous power to the trailer ABS computer, which in turn causes the problem.

$19 for the relay at a T/A, and $80 for a mechanic to explain all this to me.

 

Yes I have an 07 Columbia..... I'm not seeing any type of relays under the hood unless I'm just blind!!!! And that is possible!!! And no if I unplug the pigtail and plug it back in it does it does it each time not just once.....

 

What I meant was it only works once when you plug the pigtail in, then doesn't work when you cycle the key off/on after that.

 

20170302_093048 by Antinomian posted Mar 2, 2017 at 2:40 PM

20170302_093027 by Antinomian posted Mar 2, 2017 at 2:40 PM

This is the relay box and the relay I was talking about. I lied. It's a white dot, not a black one. Memory is the first to go. Maybe you can find something similar on your Columbia.

 

Yea all my fuses and relays are all behind driver door or on the dash. Fuses are good.... I'm stumped!!!

 

Well in answer to your original question, you cannot have continuous power to the trailer ABS unit. It has to shut down when you turn off the key.

 

When I went to get repaired at trailer facility that i know are very strict on inspections.... they said some trucks are wired that way to provide continuous power