How to fix electric drum brakes?

[corksil]

We all know that drum brakes are a very archaic mechanism and they have been around for many years without any changes to the design.

With that said, I'm trying to diagnose and fix electric drum brakes. Electric drum brakes are basically normal drum brakes, with the addition of an electromagnet on a lever which causes the shoes to widen within the drum and create friction which slows the rotation of the axle.

The amount of voltage sent to the electromagnet corresponds to the sticking force of the magnet within the drum, which translates to the outward force applied to the shoes. In simple terms, 1 volt causes brake to barely engage. A full 12 volts causes the brake to engage with the maximum amount of stopping force capable.

I am not able to get these brakes to lock up the axle and cause the tires to skid. I need these brakes to fully lock up the axle as a matter of safety. If the brakes cannot fully stop the rotation of the axle, they are deemed unsafe because the stopping force required to slow the load will be solely the responsibility of the tow vehicle. The combination of trailer [with weak brakes] behind tow vehicle with inadequate braking system to stop the tow vehicle AND trailer could result in all sorts of horrendous things which I would rather not get into.

I have already re-wired the entire braking system of trailer and replaced most of the truck wiring between dash-mounted brake electric brake controller and the trailer connection plug at back of vehicle [power for lights/brakes/turn-signals.]

I'm getting 10 volts at the magnets on the trailer brakes. The brake controller is putting out 12.5 volts at the back of the truck. I don't believe the voltage drop is due to insufficient wire size because I re-wired the braking electrical circuit entirely with 12awg wire and the trailer manufacturer used 14awg when the trailer was built.

The brakes USED TO lock up both trailer axles but they have become weaker over time. I need to fix this issue.

Any ideas?

[Triple Jim]

What is the current when 12v is applied directly to the brakes?

[corksil]

The drum brake shoes are only half worn out, and the surface inside the drum that the shoes drag on is clean and shiny. The wheel hubs have not leaked grease into the brake drums, and the shoes have not been fouled -- nor do they appear glazed.

I have had the brake drums turned on a lathe to freshen them up once already. I don't know if there is enough drum material to have them turned again but I'm going to measure them tomorrow and see if it's possible to do again.

The surface of the drum that the electromagnet engages with has slight grooves and the magnets look slightly worn but I don't know how to tell if they are worn out of not.

All four electromagnets have ~1.4 ohms of resistance through them.

[corksil]

Originally Posted by Triple Jim

What is the current when 12v is applied directly to the brakes?

I don't know how to do that off the top of my head but I'm going to figure out how to test that now. Unless I am mistaken, you're asking for amperage being drawn through the brake wiring.

I did pull the breakaway switch, which should have sent full 12v from the trailer battery directly to all four electromagnets. I tested voltage at one of the electromagnets during this experiment and my helper believes the reading to be either 10.25v or 11.25v (he doesn't remember which.)

The brakes were stronger when the breakaway switch was removed, and one or two of them would lock up intermittently but I need all four wheels locked up solid and all at the same time. Enough brake power to fully lock all four wheels on pavement (lots of traction.)

[Triple Jim]

That's starting to sound like the breakaway switch or connections to it are adding resistance to the circuit.

[corksil]

The tow sub-harness on the back of the truck is new along with the plug on the truck side.

The plug on the trailer side is new, along with all of the wires coming out of it. The lighting circuit seems to be working fine. All turn/tail/stop lights are operating properly (although some of the small running lights dim slightly in conjunction with a flashing turn signal.)

I have spent entire days on this project. If I cannot figure out exactly what the problem is, my only other option is to start replacing components to the tune of several hundred dollars per component. This would really not be preferable.

[corksil]

The breakaway switch is also brand new. It wasn't cheap either.

[JohnnyBravo]

[Triple Jim]

Originally Posted by corksil

The brakes were stronger when the breakaway switch was removed...

So either the switch or the wiring to it are adding enough resistance to cause or add to the problem. That's why I asked about the current at 12v (really 14). 1/4 of an ohm of resistance at ten amps, for example, would cause a 2.5v drop, which from what you're saying is significant.

You can go through the circuit from the supply (the battery) to the brakes themselves, and write down the voltage at each point along the way, and maybe get an idea of where the worst voltage drops are. And if the frame is used as the ground wire, all it takes is a rusty joint somewhere that adds a little resistance and you could have your 1/4 ohm.

[corksil]

This is more complicated than it seems. The trailer has it's own battery to power separate and unrelated functions. The breakaway switch is wired in such a way that it sends direct power from the trailer battery to the brakes.

The brakes are not grounded to the chassis of the trailer -- each of the four trailer brakes has it's own signal and ground wire connected through the trailer plug at the back of the truck.

This is baffling me.

[corksil]

Okay how about this.

I'll hook up my jumper pack (a stand-alone 12v power supply) to each individual wheel, independent from the rest. With the jump pack hooked directly to each individual wheel, I will try to drag the trailer with truck. If I can repeat the test four times (with jump pack hooked to each of four different wheels) and have all four wheels lock up and skid, I think that's pretty conclusive that there is nothing wrong with the brakes themselves and then it would clearly be a wiring issue.

Am I doing this right?

I'm gonna perform that test now.

[empire00]

Originally Posted by corksil

Okay how about this.

I'll hook up my jumper pack (a stand-alone 12v power supply) to each individual wheel, independent from the rest. With the jump pack hooked directly to each individual wheel, I will try to drag the trailer with truck. If I can repeat the test four times (with jump pack hooked to each of four different wheels) and have all four wheels lock up and skid, I think that's pretty conclusive that there is nothing wrong with the brakes themselves and then it would clearly be a wiring issue.

Am I doing this right?

I'm gonna perform that test now.

Sounds like the right idea on the electrical end. But have you looked into the mechanical end? Some trailer brakes don't have particularly reliable auto adjusters on them. Especially if they set for very long between uses. I would make sure the adjuster is out enough. They shouldn't free wheel like a axle with no brakes but they shouldn't be tough to turn by hand either. I found a picture that might be helpful if you haven't messed with drum brake adjustments before.

https://www.etrailer.com/static/imag...128827_800.jpg

[corksil]

Good picture. It illustrates exactly what we are talking about.

Sadly, I've adjusted them quite perfectly and I think I'm dealing with a combination of issues.

Minor voltage drop from the back of the truck to the trailer brakes is one issue. The entire brake circuit has been re-wired with wire of higher gauge than specified by the manufacturer so hopefully this issue is negligible.

The fact that the drums have been turned on a lathe once by myself [and who knows how many times by the previous owner] is also a factor. When you turn a brake drum, the radius of the brake shoes to the drum changes in proportion. After the drum is turned, the internal circumference which the shoe drags on becomes bigger. Even with new brake shoes, the angles are incorrect and the shoe only makes contact to the drum at certain points. This is probably also a factor.

The electromagnets could also be worn. I have read that once a magnet is worn to the point that the 'windings' are exposed, a direct short to ground can occur when the brakes are applied. This could also be a factor as the magnets look worn but I don't see any exposed wires from the internal magnet armature.

Dangit I really hate to do this and hopefully it will resolve the issue but I think it's time to replace... everything. New drums [including new lug studs, wheel bearings], and new backer-plate assemblies [including magnets and shoes].

Hopefully this will resolve the issue. I can see after visual inspection that the adjusters are in almost fully threaded out. The brake shoes still have life left in them but they are probably only making contact to the drum in certain points due to the drums being widened so many times.

I don't believe "widened" is a word but hopefully I conveyed what I was trying to communicate.

[corksil]

Another relevant picture.

Blue/green arrows show electromagnet engagement surface.

Red arrows show brake shoe engagement surface.

Thinking about this visually helps me pretenderize all of it. Another word that you won't find in the merriam-webster for another few years.

Thanks guys!

[CynicalC]

The jump pack is not the first approach I'd take, but it will work. Sounds like a mechanical issue to me. Especially if you've cut the drums down to nothing. The slack adjusters may not be making up for it, and also you've increased the inside radius of the drums so you're not getting full pad contact until the pads wear in.

[empire00]

On the electrical end I would try disconnecting the wires from the drums and checking voltage with no load. If your still less than about 11.5V you have a wireing issue. You might be on to something with the worn coils if they are starting to leak to ground your current would be high enough to start pulling the voltage down.

[CynicalC]

Originally Posted by empire00

On the electrical end I would try disconnecting the wires from the drums and checking voltage with no load. If your still less than about 11.5V you have a wireing issue. You might be on to something with the worn coils if they are starting to leak to ground your current would be high enough to start pulling the voltage down.

This is where I would have started. If you're getting 12.5v at the controller output, you should be getting no less than 12.4 at each wheel with all the brakes detached.

[Triple Jim]

Once you disconnect the load, it would be hard to have a voltage drop through the wiring. The important thing to check is the voltage under load.