Electric trailer brakes: A simple device driving me to madness

On the way home with my new to me camp trailer I noticed that the trailer brakes didn't seem to do much braking. Today I jacked up each wheel and tested them out individually. 3 of the 4 just hum, but do not slow down their wheel.

I pulled the drum off of one and everything looks OK. The assembly moves and the magnet magnetized when the brakes are applied. They are manually adjusted and seemed to be on the loose side, so it put it back together and adjusted the shoes so they just barely drag, but still no joy.

Here are some pictures of the assembly. The front of the trailer is to the left.

The magnet sticks to the inside of the drum and slides backwards moving the lever it is attached to and pushes the top of the front pad forward. I'm not sure how the rear pad is applied.

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It seems that the magnet is not grabbing the drum as it should, as even that would slow the free spinning wheel. Any thoughts on what I should try?

Check how many amps the magnets draw and also how many ohms resistance through the coils. You may need new magnets.

The rear shoe gets pushed through the lower adjuster as the front one tries to force itself into the drum. Think of both shoes as one circle being pushed against the inside of another circle.

Make sure you have a good power supply, and a good ground through to the tow vehicle.  Make sure your brake controller is adjusted properly, and working properly.  If that's OK, then buy new brake assemblies for far, far less money than seems reasonable to me.

The vehicle makes the brakes on other trailers work just fine so I think it's the trailer that's the issue.

What sort of current and resistance should I be looking for. Also how much magnetic force should they generate? At full application it held onto a screwdriver pretty well.

If I replace is the bolt pattern on the backing plate universal? Would I reuse the old drum?

Thanks guys!

They are really universal. Take some measurements, or take a sample to the nearest trailer shop.  Last time I bought, they were under $50cdn each.

The rear SHOE is applied because the front shoe follows the drum rotation and pushes the rear shoe into contact through the adjuster at the bottom.

The whole shoe assembly (front and rear) is stopped from rotating by the anchor pin. The rotation helps to push the shoes into the drum. This is why they used to be called "self servo" brakes. The rotation of the drum helps to apply the shoes.

When you adjust drum brakes, you don't "adjust them until they barely drag"

You adjust them until they're locked up tight, then back the adjuster off until they barely drag.

This ensures that there is no free play in the assembly and that the shoes are out as tight as they can be. If there's slack in the assembly and you adjust them until they barely drag, there's still slack in the assembly.

Don't start swapping parts, adjust them correctly first.

Also, if you have the shoes off, check the condition of the pads on the backing plate that the shoes float on. If there's grooves in the pads, grind the pad down until the groove is gone or, if they're really bad, weld up the groove and grind it smooth.

The grooves will catch the metal shoe and make it hang up and not apply properly.

Setting up drums isn't difficult, it's just that nobody is taught how to do it properly anymore.

Magnet testing procedure:

https://www.etrailer.com/faq-testing-trailer-brake-magnets-for-proper-function.aspx

Have you adjusted the trailer gain on the tow vehicle?

i tow a variety of trailers and some seem to take higher gain in order to operate correctly.

it does seem odd that they won’t stop a wheel you spin by hand.

I can't tell for sure from the angle the pictures are taken but it looks like the magnets might be warn to the point that they no longer make good contact with the drum.

I'm not normally a fan of firing the parts cannon but, I'd start with loaded backing plated all the way around and probably a brake rewire.  It's been my experience that new to me trailers need that to be reliable. 

Thanks. I'll try that adjustment procedure and then check my magnets. It's been 20 years since I messed around with drum brakes, so I'm definitely rusty.

I tried adjusting them to as tight as I could and then backing off, but no change in operation. I took each drum off and they all have shoes that move freely have a good amount of friction material left and don't have any backing plate wear.

My multimeter has kicked the bucket, so I couldn't test the electromagnets. While I wait for a new multimeter I think that I will also fire up the parts cannon and buy new magnets. They all look a bit worn down. This was the one working break, but they all seem to be worn down about the same amount.

If that's the case, you may find that complete new assemblies are around $75.00 to $150.00 each.

Usually I just change the whole thing out for peace of mind. I figure by the time the shoes have worn out, everything else is on the way too.

I recommended adjustment because your shoes look brand new.

If you call your FLAPS and give them the drum diameter and shoe width, they should be able to supply the assemblies. Dexter makes them and they're universal.

I put brand new assemblies on my wife's old horse trailer from the 1970s a few years back and they bolted right on. Stuff hasn't changed in a long time.

In reply to ShawnG :

Thanks for the help!

In reply to bgkast :

Shawn did an excellent job with the analysis and adjustment procedure. But the folks on here tend to like to spend the money on fully loaded backing plates. And they are correct; it IS the cc easy button. But IF it is the magnets, individual parts cost A LOT less. If it needs shoes and hardware and magnets, of course, go loaded. But the magnets can be had cheap, if ya shop around. 
There is a spring behind the magnet... is anything preventing it mechanically from moving out enough to contact the drum properly?

The magnets float freely on their springs.

I stopped by NAPA today to try and find some parts. Apparently there is a national trailer brake shortage (along with everything else). No magnets, and only left side loaded plates available. They said rights have been on order for 3 weeks. At least I got to look at the loaded plates and my pads only have about half of the friction material that they did.

eTrailer said they had loaded plates in stock. Luckily I looked at the delivery date before I clicked "order".

I found some on another website that should be here next week.

Agri-Supply has a horrible web site, but if there happens to be one near you they were the best price by far, the last time I was in one. Two years may have changed, and never ordered from them, but worth a look. 

Did you try energizing the magnet with a battery just sitting by each wheel? Mine grab so hard it's violent if the controller is not adjusted below @ half way.

In reply to bgkast :

See if you have a trailer repair/supply store in the area.  When I bought my trailer's brakes back in June, I found one not far away, and they had a lot in stock.  I also wanted to see the parts in person, as mine had lost much of the original parts way before I got the trailer.

I'm betting you have an electrical issue not a brake issue. 

Start with your grounds. 

Camper companies also frequently run the power wires through the axle tube. Rust in thr tube rubs through the insulation and cause shorts that keep the magnets from grabbing at full power.

New loaded backing plates went on this morning and they all work great now! The old magnets were the issue, but I'm glad I went with the full assembly because it looks like the break shoes were about half worn out.

Thanks for the help