Brake magnet size

Olsy wrote:
Thanks for the input everyone. I have a good meter as i am an electrician...from what i have learnt from reading i should be getting around 3.2 to 3.8 for a single brake magnet and half of that if all is well and they are properly paralleled...now i am getting the itch to get this sorted and get out camping :-)

Hi Olsy,

You have found a good bunch of knowledgeably helper folks on your post. To help the cause, I'll try to add some more not yet stated.

See this Dexter manual for in print specs on amps and current and a lot of other good stuff on new brake shoes. Like the need to burnish in the new brakes before towing a lot and the very important 1st 200 mile brake adjustment.

http://www.dexteraxle.com/i/u/6149609/f/600-8K_Service_Manual/Electric_Brakes.pdf

Next is the troubleshooting hunt. I agree with Gdetrailer, the ohms check depending on where you do it can be misleading as trailer wiring is in the middle of this. If you tap into the wiring right at the coil, you can eliminate a number of variables but still it can have issues affecting the check. Unless you split open the wires you may not get a good connection.

If you are an industrial maintenance electrician, you did not say what kind you where, many of them have a Fluke 87 meter which the average home owner does not. While I have one, it is because of work as why I do. Even then your only working with 3.2 ohms on 1 coil on a good day with a 100% positive connection. Getting the good connection is the problem. That said it can help you with an open circuit at the coil just not so good if the coil is worn through and 1/2 grounded out.

You can do a voltage drop check, now you increased the odds of what you have to work with over ohms. On a 100% fully charged battery you have 12.7 volts to start with. When plugged into the truck and running it might be up at the 14 volt area pending the battery charge state in the system. You can check the voltage drop starting at the brake controller all the way to the brake coil and see what drop you have.

Some campers have a junction box up at the A frame header which makes it easier to tap into the system. With the truck running and the 7 wire connected, have someone apply the manual lever on the controller full power and measure the voltage output on the blue brake wire right at the controller to gnd. This is your starting voltage. It should be good here. Then go to the A frame junction box and check there. If it is less by much, well you are loosing some of it between the controller, the 7 wire and the gnd at the trailer header.

Then if you really want to see it, go back to each brake coil and get your probes right on the coil wires. This one might open your eyes... to what you lost in voltage. Since you only have 12.7 to 14 volts pending the source to start with, you do not want to be loosing a lot due to bad RV trailer wiring or a high resistance 7 wire plug connection.

The next area is amps, pending the bag of tricks in tools you have there are a couple of ways to do this. Do an amp check, this may be the fastest way to get read on the system. With the 7 wire un plugged from the truck, (a must as you can back feed the truck in some case and mess up the brake controller pending your setup). Unhook the gnd wire at the camper battery and using your amp meter in series with the battery post and the camper gnd wire, pull the emergency break away pin and read the total current draw. Check that the battery voltage is up at 12.6 to 12.7 or above if you just had it charged. Each coil on a good day with no resistance in the wiring is in the 3.0 amps range. And obviously make sure your meter can handle at least 10 amps on 2 coils, 20 amps on 4 coils

Note: I said on a good day. Many break away switches have 14 awg wire. On us guys with 4 brake coils we have it worse then you on a 2 brake coils. That 14 awg wire in some cases acts like a resistor and you loose a little current but, you can get a quick read if you are in the ball park. They may even put it at 14 on purpose to not apply full power. If a coil is worn open, you will only get 1/2 amps. If you have wiring with bad connections and undersize wire, well the voltage drop will prevent the power from getting to the coil and that will affect the amps.

Point: You have multiple ways to sort out if your wiring is giving you part of your brake strength problems. If you have a high end meter or a Harbor Freight special, you can get a good read on the system with some know how and what to be looking for.

When I put new axles in, I redid the brake connections back at the wheels. See here

Independent Brake Wire Feed Upgrade There is also a link there to LAdams upgrade, he used 2 wire cable, I used single strand. As was said do not use the axle tube inside, this is a text book wire failure over time with shorts to gnd. Shows up more as the camper ages, the wire insulation gets brittle and mileage increases.

If your axles are 3,500# axles, I agree they should be 10" dia x 2 1/4" wide if they are Dexter or Alko. But check in case you have a odd ball. The newer campers, like approx the last 15 plus years only have certain size axle capacities on a brake axle regardless of GVWR on the camper.

I also agree, if you need new parts, get the entire brake plate. This gives you new magnets, new pivot arm bushings, new springs, brake adjuster and shoes. And many time cheaper than just the plain shoes. Here is where I get many of mine, http://www.easternmarine.com/Electric-Trailer-Brake-Assemblies/

Right now 10" x 2 1/4" Dexter's brake plate assembles are on sale for $24.99... These are Dexter original parts....in the original boxes. The mark up on these things must be off the charts how they can sell them this cheap and others can sell for $70 a plate.

If you are thinking about going self adjusters, lets talk first so I can give you a heads up on what to look for on your brake drums. I upgraded mine and glad I did, however my existing brake drums got me on 1 wheel. Another sad story of the RV industry on quality.

To the compass trick at the wheel, I understand why it should work and how, just I have never been able to get it to work right that I knew it was working or trust it. Had more faith from hearing the coil hum that the brake was at least getting some power.

Good luck and hope this helps. Let us know how you make out.

John

PS, to see what 10" x 2 1/4" Dexters look like, see mine here from my prior camper. Annual Brake Inspection and Axle Re - Lube (Pic's)