Performance Improvement Ideas

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"How to rebuild Electric Trailer Brakes."

October 16, 2009 by Mike Strawbridge
JC Whitney - Everything Automotive

This story begins when I offered to haul my friend's Aerostar van for him. He lives at the top of a very steep gravel hill.

We drove the van on the trailer, strapped it down tight, set the weight distributing hitch and I headed down the hill. It was all going well until I hit the brakes.

Basically there were none. Only the fronts on the Suburban were locking. I pushed the big red button on my home made trailer brake controller giving them the full 13.7 volts but nothing. Well almost nothing. I was still going down way faster than I wanted to!

The trailer brakes should have locked up solid when the red button is pushed making everything at least slide straight - instead of the trailer trying to pass the Suburban.  Talk about the cart before the horse!

The way the load was pushing around that Suburban, I was really glad I had not tried to tow with my little Jeep Cherokee.

Once that job was done I backed the trailer into the shop to take my first look at those trailer brakes.  I have owned this trailer for nearly twenty five years and I have never seen inside the brake drums on it.  I guess it was about time.

My inspection of the first drum showed me a little about how electric trailer brakes work.  There is an electromagnet that pulls against the inside of the drum.  The friction of the magnet against the drum moves one shoe that in turn moves the other shoe with a cam action.

Pretty simple.  But I did not see anything wrong.  The shoes looked fine, the drum was fine except for some scoring where the magnet rubbed.  I put power to the magnet and it seemed to pull OK.  But, the friction of the magnet against the drum was obviously not enough to work the shoes.

I could not figure out how a magnet could go bad, but that is what it seemed.  Time for some internet research.

I eventually found a site that said magnet failure was common on electric trailer brakes.  So I went shopping for trailer brake magnets.  Most places wanted more for the magnet than I could buy the whole backing assembly for. To get one at a local parts store I would have to pay more than I could buy two backing assemblies for.  And new magnets are included in the backing plate assemblies.

Just when I was about to give up and order new assemblies, I stumbled upon Auto Safety House.  They could use some work on their web site, but they have great prices.  

I found a pair of new drums on e bay and I was the only person who bid so I got both for the price of one.  Then the wait for Danny - my UPS driver - to bring them to me.  Naturally the drums and magnets arrived on different days so he got to visit with my dog Smash twice.

Upon closer inspection of the trailer brakes, I noticed that I still did not understand how they worked.  Something just did not look right about the way the cams worked.  It sure looked like they would be stronger in reverse. And this trailer has always been tough to back up when the brakes are working right. 

A quick look at the Dexter Brakes web site confirmed my suspicions.  The rear axle on my trailer was installed backwards.  

I decided the bearings did not really care if they were spinning backwards, so I elected to simply swap the brake assemblies left to right.

Having them off made installing the magnets and routing the not so little green wires somewhat easier.  I found that some lubricant made getting them through the grommets a lot easier.

Dexter Magnets are identified by the wire color.  The correct one for my 3500lb axles is the green wire.  Nice they have not changed that in 25 years.

The magnets are held in by spring clips that are really hard to get off.  I used a small screwdriver and a pry bar and bent them slightly to get them off.  The new magnets came with new clips anyway.

I loaded the new bearings with axle grease and installed the new brake drums.  They are not Dexter brand but the fit just fine.

The wiring is was pretty simple to reconnect since there is no polarity on the magnets.  Just connect one wire to the brake controller power lead and one to ground.  Dexter recommends running a separate ground wire but I just grounded it to the trailer frame like it had been done before.

Now my trailer has brakes again. See the pictures below.

The Dexter web site reads: The actuating lever should curve around the front of the spindle.  Note the front of the trailer is to the right in this picture.

A larger view so you can see which way is the front

New magnet installed. Note the lever now curves around the front of the spindle.

The old magnet was also missing the 0.25 layer of friction material.  So you can wear out a trailer brake magnet.

All done on this side with the new brake drum installed.  Just need to complete the wiring.

The old drums were scored up where the magnets rubbed.  Too deep to resurface.

The trailer does not fit on the lift but I still used it to safely support the trailer while I repaired the electric trailer brakes.

Mike Strawbridge October 16, 2009

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