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Old 05-03-2005, 12:18 AM   #1
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Thumbs up Brake Magnet Resistance Spec.?

Does anyone here know the specification for the resistance of a single brake magnet?

The consensus seems to be that each magnet draws about 3 amps. If this is so, then the resistance should be about 4 ohms. I would like to know the actual allowable specification.

My brake controller, a BrakeSmart, occasionally reports, "Brake Short Detected." It does this only if I apply the brakes firmly. It will reset itself if I unplug the trailer and plug it back in again. I assume (I haven't asked BrakeSmart yet) that the reason for the message is that the controller is detecting too much current (too low a resistance) to the brakes.

Obviously, if I have a faulty brake I'd like to fix it, but with six brakes I don't want to go about this without a plan. I intend to disconnect each brake individually and read the resistance at each brake. I am hoping to find the one that is faulty, but I don't know what reading I should see.

I would very much appreciate any help.

Thanks,
Loren
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Old 05-03-2005, 07:19 AM   #2
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Quote:
Originally Posted by Loren
Does anyone here know the specification for the resistance of a single brake magnet? ...Obviously, if I have a faulty brake I'd like to fix it, but with six brakes I don't want to go about this without a plan...
A plan? That's no fun.

Just kidding. I have four old (round) magnets out in the shop that I was planning to measure the resistance of this morning, but ended up getting distracted. I can still do that if it would help, but Metro Centre Ltd. Trailer says the magnets should measure 3.2 ohms.

A plan.


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Old 05-03-2005, 09:23 AM   #3
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That's a plan?

I think your numbers are right. I recall mine were in that general jurisdiction.

I read somewhere that the magnets on 10" brakes are less than 12" brakes. Maybe the 10" are 3.2 ohm, and the 12" are 4 ohm?
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Old 05-03-2005, 10:21 AM   #4
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Thanks for the help. I wonder how much mine will read. If they are just a small amount out of specification then I may have a hard time finding the problem brake.

I won't be able to do this until we get home from our current trip. That will be next Monday, 9 May 2005. I should know something shortly after that, assuming that it's not raining when we get home. I want to get these fixed, but I'm not crazy enough to crawl around under the trailer in the rain.

My brakes are 12".

Loren
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Old 05-03-2005, 02:41 PM   #5
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Just in case somebody is looking at this post for 10" brakes, I have a new magnet by Tekonsha 5105 which is a small round magnet. It looks like pre 1987. Hayes. It reads 3 ohms on my meter.
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Old 05-03-2005, 02:53 PM   #6
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.. but I'm not crazy enough to crawl around under the trailer in the rain. Loren
Loren, there are of course many variations between models of Airstreams, but on my Excella, if you're under the trailer, you won't be in the rain.
A useful trouble shooting chart on the www.dexteraxle.com website is at http://i.b5z.net/i/u/1080235/f/6-8K%...rakes_3-04.pdf
When I had a similar problem, it was a brake wire rubbing on the frame. The insulation wore through and shorted out the circuit. Perhaps it would be useful to sheath the exposed wires in the corrugated plastic used on car and truck electrical systems. I will look at doing this. Good luck. Nick.
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Old 05-04-2005, 06:35 PM   #7
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I finally called BrakeSmart for some help in understanding what triggers the short message on the controller. I was hoping that they would tell me that if the controller didn't read the resistance it expected that it would report a short. That's not what they told me. Actually, I didn't get an exact answer as to what causes the controller to report a short. They did help a little bit by confirming some of the things that I thought.

Since the error message goes away after unplugging the trailer and re-connecting it, then that means the fault is in the trailer, not on the truck.

They also feel that the fact that I get the fault message only when the brakes are applied firmly means that it is most likely a fault with a magnet, not with the wiring. If it were a wiring fault, then it wouldn't matter how hard I stepped on the brake.

My current plan is this:
When I get home, I'll leave the trailer connected to the truck and try to duplicate the fault by applying the brakes firmly. If I can get the fault to appear with some degree of repeatability, I'll cut the wires to one brake and try again.

I'll continue to cut wires to brakes until I no longer get the fault. At that point I'll start restoring the wiring to the brakes in the same order I cut them, on the assumption that the last one cut is the problem.

I hope this works and allows me to narrow this down to a single brake magnet.

I just re-packed the bearings a couple of weeks ago and didn't notice any problems with the brakes at the time, but I wasn't checking for that.

We'll see how this goes. Finding an intermittant electrical short is one of the more annoying problems to solve on a vehicle. I hope it works out easily.

Loren
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Old 05-04-2005, 06:39 PM   #8
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Nick,

No, I wouldn't be in the rain, per se. I would just be in the water that runs along the ground under the trailer.

I do intend to install that plastic loom around those wires ASAP. I have heard of too many problems with brake wiring to leave it alone. I don't think this is the problem here, though. The previous owners (we're #3) told us that this trailer doesn't have many miles on it, so my guess is that the wires haven't moved around enough to wear through the insulation yet.

Loren
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Old 05-04-2005, 07:09 PM   #9
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..If I can get the fault to appear with some degree of repeatability, I'll cut the wires to one brake and try again...I'll continue to cut wires to brakes until I no longer get the fault...
I hold little hope in that approach because I think one of your magnets may be failing only slightly, and your controller might be just on it's design edge to run six magnets since the bulk of the world only has four or less. As soon as you cut the wires to one magnet, the resultant load will end up being within spec for your controller to indicate "all's well".

Applying the brakes firmly means more voltage is being applied to the brakes, and hence the magnet will produce more heat which, in turn, will force a mildly shorted winding (if a magnet is at fault) to act up. If you have access to an inductive DC ammeter, another approach, with due preparation, would be to have an assistant actuate the manual override lever whilst you measure the current each magnet is drawing when the "brakes bad" indication displays itself.

If you are not fortunate to have access to this type of ammeter, you may want to consider clipping the wires each magnet, and then applying 12 vdc to each individual magnet for a short amount of time & then checking the resistance. The theory here is that a robust charge will make a bad winding reveal itself.

In spite of my earlier levity, I do appreciate a good troubleshooting plan, and would be more than happy to fill in details on anything I have outlined above.

Good luck,
Tom
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Old 05-04-2005, 07:54 PM   #10
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ah-Ha, truth revealed. Tom is the consummate planner, despite his prior protestation.

And an outstanding plan it is!
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Old 05-04-2005, 08:25 PM   #11
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the smoke test

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The theory here is that a robust charge will make a bad winding reveal itself.
you could take that idea to an extreme and do it like we do at the power company!

apply enough power to a suspect circuit and the problem will become self evident!

just hook a battery directly to the brake circuit and see which wheel starts smoking!

>>>>>>>>>just kidding of course!!!<<<<<<<<<<<<<<

seriously, disconnect ALL of your brake magnets and see if the wiring in the trailer has any resistance with an ohm meter. if that checks ok then start looking for individual magnets with a problem.

once that is resolved, consider replacing all of your brake backing plates with new ones, fully loaded they are cheaper than just the magnets! around here they run in the 40 to 50 dollar range for 12 inch brakes.

john
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Old 05-05-2005, 07:36 PM   #12
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Tom,

Excellent advice. Thanks.

I do have a DC inductive ammeter. I agree that one magnet is probably on the ragged edge of specifications, and checking current instead of resistance would be better, since the brakes will be applied at the time, which is the only time I'm getting the problem.

The BrakeSmart will control from one to four axles, so I don't think the problem is with the controller's error detection. If you are towing a trailer with anything other than two axles, you run through a two-second calibration routine before starting out. I suspect that this lets the controller know how much resistance it is working with, and therefore, how much current (voltage) to send to the brakes.

Thanks again for the new, improved plan.

Loren
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Old 05-05-2005, 07:47 PM   #13
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John,

I also like the idea of smoke-checking the brakes, but I believe I'll follow Tom's plan first.

I realize that the problem could be in the trailer wiring and not at an individual brake, but my gut feeling tells me that it's a brake. Working under that assumption, if Tom's plan doesn't reveal the problem magnet, I'll start cutting them from the wiring and measuring resistance. I don't expect that method to reveal the problem, however, because it shows up only when the brakes are applied firmly.

I do know that buying a complete backing plate is almost as cheap as just the shoes, and you don't have to risk your life hooking up springs. I think I paid about $75 each for them for our previous trailer, which has identical brakes. I probably won't buy six of them just to replace all the brakes. I might buy one and replace individual brakes with the hope that I will eventually replace the bad one. That's not a bad back-up plan.

Thanks.

Loren
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Old 05-05-2005, 08:03 PM   #14
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Just when you think you have seen it all..... BAMB !!! ya get hit with something new.
Resistance is a good test but under real world conditions when the brakes are drawing current a coil could fail and check good. You can test them to death and never find out the faulty brake. Save your time and replace the brake magnets and while at it make sure your wiring is in top shape, that is if your still having trouble.
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