Brake magnet size

I can't say what size, but I do suggest that if replacing the magnets, you buy the backer plate with new shoes and magnets as they get almost as much for the magnets as they do for the whole assembly. You can check prices on Etrailer

Etrailer:brakes
show 3500lb. axles to be 10"

Thanks, thats what i was thinking, i hope they dont need replacing. I am having problems with my brake controller and need to know size so i can deteine what resistance i should be reading. Thanks for the input

bob213 wrote:
Etrailer:brakes
show 3500lb. axles to be 10"

But with a gvwr of 3500, that may not be true as they use the tongue weight to reduce the size of axle needed and they try to use as light an axle as possible. That has been my experience.

I had an issue with my brakes, and after a bunch of inspections and checking I found out my magnets were shot! I replaced them and the ones I received were 20% larger and the trailer finally was able to stop! BTW my trailer has a GVWR of 11,200lbs!

dodge guy wrote:
I had an issue with my brakes, and after a bunch of inspections and checking I found out my magnets were shot! I replaced them and the ones I received were 20% larger and the trailer finally was able to stop! BTW my trailer has a GVWR of 11,200lbs!

They did improve the magnets on the 12" brakes, but the OP most likely has 10"

BTW, 7" is usually on trailers with very small wheels and those brakes are next to useless.

Olsy wrote:
Thanks, thats what i was thinking, i hope they dont need replacing. I am having problems with my brake controller and need to know size so i can deteine what resistance i should be reading. Thanks for the input

If your axles are 3500 lb. Dexter axles, they are likely equipped with 10"x2 1/4" brakes. I e-mailed Dexter a couple of months back regarding the axles on our 2013 WhiteHawk. Dexter's warranty service rep. e-mailed pdf copies of the factory build sheets for both axles. Very useful info when I need parts, especially if the tags on the axles with SN & Model #'s ever peel off.

Olsy wrote:
Thanks, thats what i was thinking, i hope they dont need replacing. I am having problems with my brake controller and need to know size so i can deteine what resistance i should be reading. Thanks for the input

Typically brake magnets will draw a max of 3A and as far as I know that is across the board for all drum brake magnets.

3A at 12V is equivalent to 4 Ohms per magnet (use Ohm's law to solve E (voltage)/I (current)=R (resistance) the problem).

If you have a single axle then you would have two magnets, the resistance would be 2 ohms in this case.

The problem is that most Ohm meters simply are not accurate at this low of resistance AND on top of this is the consideration that you are dealing with coils of wire which may or may not be 100% dead on with the 4 Ohms to start with.

Now toss in the shoddy wiring practices used on trailers which often have multiple spices using non sealed crimps, broken wires or even shorted wires inside the axle tube.

Personally, I would highly recommend saving a lot of time and frustration and scrapping the entire brake wiring on the trailer. It simply is not worth your time or aggravation.

Run a new set of wires from the tongue back to the axle, do not use the wire INSIDE the axle. Instead use a bunch of wire ties and run the wire across the top of the axle ON THE OUTSIDE (wires inside the axle tube tend to move about and eventually leads to chaffing causing intermittent shorting.

Use a bigger gauge of wire than what the manufacturer used, 10 ga is my favorite, use stranded wire do not use solid wire.

Make sure you use weather proof splices for the connections to the magnets.

Additionally inspect the magnet wires inside the drums for any signs of chaffing (if any chaffing found you can simply add some heatshrink tubing to cover the affected area).

I already have plans on a rewire, i read only 3.4 ohms so one magnet is shot or open circuited.

Olsy wrote:
I already have plans on a rewire, i read only 3.4 ohms so one magnet is shot or open circuited.

Most likely a broken WIRE or connection to one of the magnets or even badly corroded wires at the splices.

However, like I stated, reading resistance of a COIL may or may not be exact. Not to mention most ohm meters typically can not read this low of a resistance value ACCURATELY.

Most Ohm meters will be off 1 to 2 ohms since they typically do not have a calibration button to zero out the display.

Check your ohm meter by shorting the leads together, I would bet that it will read about 1.2 ohms when you do this. If it does show about 1 ohm then there is nothing wrong with the wiring or magnets. 1 ohm of the meter being off would be added to the reading giving you the 3.4 or so reading.