Brakes grabbing

MFL wrote:
Hey JR, this is a common issue, but normally takes at least a light touch of pedal, that can result with braking similar to pulling the pin. Very aggressive, tires chirp/maybe smoke. As Grit mentioned, turning the gain way down will help, but braking will be diminished.

It starts mostly intermittent, due to the wiring moving/bouncing inside the tube.

Quick fix..cut the wires going through tube, and run new cross wire outside the tube.

Hi Guys,

I might learn something new here, but please help explain how the brakes can grab hard when the hot wire in the axle tube shorts?

Here is from my background to help the cause. The chaffing is real. What I cannot sort out is how the brakes grab real hard close to lockup as you are stating.

My findings:

Yes, as the trailer ages and the towing miles increase, the double wire lamp cord style cable inside the axle tube creates a wire insulation chaffing problem. First is, the insulation gets brittle over time. Circular cracks in the insulation happen, mostly where the wire flexes but it is not limited to just the flexing areas. Moisture can then get to the conductor given the right conditions.

The middle area of the axle tube is prone to the wire bouncing and rubbing over time. The wear is not selective to which wire wears to expose the conductor.

Here is one of the many I have found while restoring older campers.


A close up showing the tarnished copper exposed


The hole area at the ends of the tube also can have exposed conductors also. This wear is different then the mid tube area. The brittle wire cracks where the wire rubs the tube hole edge, starts to wear/vibrate into the conductor.

Now, when the exposed conductor touches the metal frame, it is sort of a 50/50 good luck or bad luck which conductor touches the tube.

If the chassis ground wire wears through and touches the tube, this issue may not be found under normal operating as one side of the brake coils are tied to chassis ground. I have not seen this be a problem short of the wire wearing all the way through and creating an open ground on the brake coil. Or a weak ground and the magnet not getting full power.

If the brake hot tire wears and touches the tube, when the brakes are applied here is where problems can start. And where I need someone to help explain how the voltage and current above normal braking to get high enough to create hard breaking on all wheels or even one of them.

The average brake coil on a 10" x 2 1/4" all the way to a 12" x 2" or 12 1/4 x 3 3/8", drum ID x show width, brake has an magnet coil ohm reading around 3.2 ohms. Its not much but it can create 3 amp draw on a full 12 volt power circuit. There is a load there and not a dead short.

When the chaffed wire touches the axle tube, that can be close to a dead short or a less pending how much corrosion resistance is on the tube.

How does the truck brake controller send full voltage and current similar to a emergency break away switch activation with only a light tap of the brake pedal? What tricked the controller to know to do this?

On a time based controller that only uses the brake pedal switch, it is a timed response for when full current is reached according to the gain setting.

On a proportional controller, well this depends on the brand and type. The pendulum ones, need the truck angle change to create high power.

The newer truck integrated ones which us the master cylinder pressure, that is something different too.

What an I missing on how all 4 or 3 of the 4 go to full power when one is grounded out on the hot wire?

My original thought was the controller senses the dead short and tries to protect itself, maybe shutting down the power to not have it burn out? What happens next I'm not sure. Help fill in the details on how the full power is created like you are stating.

Thanks

John