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Cattle Trailer with Electric Brakes

Remember the Car Talk show where the Brothers could not answer the question of “Why the electric brakes activated when the hazard warning lights were on, ie when the truck and trailer were climbing a grade slowly.”

I wonder if there was any suggestions or discussion on the matter.

Might not be the suggestions or discussion you are looking for, but there’s this philosophical treatment

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I thought I would make a possible diagnosis. It is likely that the electrical current for the hazard flashers is making its way back to the towing truck through the brake actuator coils through the controller. In operating conditions, the brake pedal actuates a connection from the B+ of the battery to the controller which regulates the current flowing through the brake servo coils to pull the frictions up the face of the trailer drums, actuating the drum bbrakesl. When the trucks brake pedal is released the voltage is removed allowing the servo coils to release.

Ordinarily when the hazard lights are active, the battery current goes through the lamps to the ground.

Now consider the case where the ground is defective between the truck and the trailer. I considered this unlikely considering the weight of the loaded trailer. Later, I have seen the rusted condition of trailer loops the hooks they are coupled with. If this truck and trailer have been expose to weather conditions, it likely the ground path is questionable. The usual work around is to have a ground bond wire going from the trailer to the truck in the connector plug. If the bond wire is broken or its terminals and corroded, there will be no ground.

In this case the current from the hazard lights will find a ground back to the truck through the brake servo coils, hence through the controller to ground. This would cause the trailer brakes to activate, causing the observed situation.

I think this was read in the last month or so on the podcast. What a letter!

Part of what made Tom and Ray’s show such a joy was the fact that they could receive and even broadcast a letter like that and enjoy being skewered, The Melissa Peterson incident is another example.

I am already missing the anticipation that there would be another such incident - and while listening to the older broadcasts helps, it’s just not the same.

Electric brakes are coming up on the podcasts? Great! I’m about 6 weeks behind, so now I have a reason to get busy and get my podcast player loaded up with new shows! Can’t wait to hear the electric brake incident again … lol … electric brakes, the only unknown scientific principle left to discover in the universe… lol …