Need help - intermittent connection between truck and 351M

[NFurno]

Today I purchased an AC/DC clamp meter, contact cleanser, dielectric grease and a spare emergency brake switch. Earlier I pulled the plastic pin on the emergency switch and went to see if I could hear a hum people mention when the magnets engage the brake linings. I could not. I used the meter to verify current was flowing through the switch and that there is current at the brakes although quite a bit less. After I put the pin back in place, I connected the truck to the trailer and the brake controller in the truck sensed the trailer and everything was working as it should.

At this point, my thoughts lean to the problem being temperature sensitive as the temperature was in the 80's when the problem occurred. It's much cooler now. As I will be headed further south by the end of the week where the forecast is for hotter weather I should know if this is the case. I do plan on replacing the umbilical cord and cleaning up the wiring when I reach my destination.

[geotex1]

Today I purchased an AC/DC clamp meter, contact cleanser, dielectric grease and a spare emergency brake switch. Earlier I pulled the plastic pin on the emergency switch and went to see if I could hear a hum people mention when the magnets engage the brake linings. I could not. I used the meter to verify current was flowing through the switch and that there is current at the brakes although quite a bit less. After I put the pin back in place, I connected the truck to the trailer and the brake controller in the truck sensed the trailer and everything was working as it should.

At this point, my thoughts lean to the problem being temperature sensitive as the temperature was in the 80's when the problem occurred. It's much cooler now. As I will be headed further south by the end of the week where the forecast is for hotter weather I should know if this is the case. I do plan on replacing the umbilical cord and cleaning up the wiring when I reach my destination.

By what you just stated, you brake issue is not in the umbilical. If you aren't seeing nearly the same current that is at the breakaway again at each brake, your problem lies between the two. You've got a partially open or shorting circuit.

[NFurno]

By what you just stated, you brake issue is not in the umbilical. If you aren't seeing nearly the same current that is at the breakaway again at each brake, your problem lies between the two. You've got a partially open or shorting circuit.

Thanks Rob. I'm not sure where to start with this. Maybe checking the wiring to the brakes in the morning. Hopefully it will be a bit drier under the rig by then. If I don't find anything. I will take apart the connection in the junction box where the wiring for the emergency brakes connect to the truck. I'll let you know.

[geotex1]

Thanks Rob. I'm not sure where to start with this. Maybe checking the wiring to the brakes in the morning. Hopefully it will be a bit drier under the rig by then. If I don't find anything. I will take apart the connection in the junction box where the wiring for the emergency brakes connect to the truck. I'll let you know.

Always fun chasing intermittents... With trailer brakes, the most common electrical issues are at connections, especially those right behind the backing plate, which are rarely sealed well and corrosion wicks up the wire. The other is a bad magnet(s).

I would test the actuation wire first. Pull the breakaway and measure outgoing voltage and then the voltage at each backing plate splice. Work quickly as you don't want to overheat the magnets.

[NFurno]

Always fun chasing intermittents... With trailer brakes, the most common electrical issues are at connections, especially those right behind the backing plate, which are rarely sealed well and corrosion wicks up the wire. The other is a bad magnet(s).

I would test the actuation wire first. Pull the breakaway and measure outgoing voltage and then the voltage at each backing plate splice. Work quickly as you don't want to overheat the magnets.

Rob, would I be able to do the same measuring current with a clamp on amp meter? I found a Lippert manual which indicates the current draw for four brakes (two axles) should be 12 amps. When I tested this today, initially I had 10 amps at the breakaway switch and this dropped to about 8 amps by the time I checked the current at the backing plate splice. I check the driver side rear brake current and it read about 3 amps. I read pretty much no current at the passenger side rear brake backing plate. What makes this harder is that I'm doing this by myself. Kinda hard to apply the truck brakes and check the meter at the same time but I have figured that out. And I will make sure to do my checks as quickly as possible to avoid over heating the magnets. Thanks for following this thread and for your input. It's much appreciated.... Wayne.

[geotex1]

Rob, would I be able to do the same measuring current with a clamp on amp meter? I found a Lippert manual which indicates the current draw for four brakes (two axles) should be 12 amps. When I tested this today, initially I had 10 amps at the breakaway switch and this dropped to about 8 amps by the time I checked the current at the backing plate splice. I check the driver side rear brake current and it read about 3 amps. I read pretty much no current at the passenger side rear brake backing plate. What makes this harder is that I'm doing this by myself. Kinda hard to apply the truck brakes and check the meter at the same time but I have figured that out. And I will make sure to do my checks as quickly as possible to avoid over heating the magnets. Thanks for following this thread and for your input. It's much appreciated.... Wayne.

Wayne, I'm trying to take you down a path that is process of elimination. If you rule out supply, then you know your issue is at the axles.

Measuring current requires completing a circuit, and your testing now sounds like your issue is at the rear axle breaks. Now to determine connection, ground issue, or magnet.

[CoChris]

... I used the meter to verify current was flowing through the switch and that there is current at the brakes although quite a bit less.....

The current through the switch is the sum of all the brakes. For a dual axle, with four wheels/brakes, you will have roughly 4x the individual brake current at the switch.

You do want/should see very similar currents at each brake (typically 3~4 amps for a fully charged battery). If you see a significant difference in current between brakes (more than 1/2 amp?) with (and this is important) the same battery voltage, that is a problem. Obviously, if you find one (or more) brakes with 0 current, that circuit (wiring and/or magnet) is broken.

The brake wiring is run from the junction box under the king pin, down one side of the frame to the two brakes on that side, then through smaller gauge wires in the axle tube to the brakes on the other side. These wires are the minimal size to work. Which means they many not work that well. You may see significant difference in current from one side to the other.

Chris

[NFurno]

The current through the switch is the sum of all the brakes. For a dual axle, with four wheels/brakes, you will have roughly 4x the individual brake current at the switch.

Rob and Chris, between your posts and a review of basic electricity I think I have a better understanding of what I should be looking for and why. In simple terms, because that's all I can wrap my head around, it sounds like the trailer braking is wired as a parallel circuit. If I had a constant 12V source I should see 12V anywhere in the circuit that I test. And, if I had, as the LCI manual states, 12 amps at the source, since I have 4 magnets, one at each brake, I should have 3 amps at each backing plate as Chris stated.

Since this is the real world, as Rob suggests, it will be easier to detect a drop in voltage than a drop in current in a circuit that has a partial open. I think, thinking always gets me in trouble, I can rule out a partial short since I didn't measure a higher than expected current at the breakaway yesterday.

Well the sun is about up. Time to go roll around in the mud.

Wayne

[NFurno]

I'm embarrassed that it took me so long to catch this but I have a bearing failure. I was so focused on a braking issue I looked right past this. I did check the voltage going into this brake and it was about 8 to 9 volts but nothing on the return wire. I'm hoping I can clean up the magnet and still use it. Trying to order a new bearing kit and brake replacement kit. May be stuck in GA a few days longer than planned. I'm keeping my fingers crossed that this was the problem all along.

[powerscol]

I'm embarrassed that it took me so long to catch this but I have a bearing failure. I was so focused on a braking issue I looked right past this. I did check the voltage going into this brake and it was about 8 to 9 volts but nothing on the return wire. I'm hoping I can clean up the magnet and still use it. Trying to order a new bearing kit and brake replacement kit. May be stuck in GA a few days longer than planned. I'm keeping my fingers crossed that this was the problem all along.

They should still be under warranty Contact Dexter and GD customer service.