What Happens When....... Brake Controller Question

[Kanman]

So what happens if you run a brake controller rated for only 2 axles on a 3 axle trailer?

I have been running this way for a little while, and I just haven't gotten around to installing a new controller.

What I have noticed is it never seems like the same axles lock up twice in a row. Its almost a crap shoot as to which lock. And, they do LOCK, geeze it is annoying to smoke the tires every time I touch the pedal.

Curious as to why this happens, I would have figured all three axles would have gotten some voltage, just not enough. But that isn't the case, the current seems to pick two axles.

Curious as to why this happens?

[STurbo90]

I ruined an old controller which I am pretty sure was only a 2 axle controller, but I can't say it was because of too many axles, as I abused that controller.

While it still worked, it lost its ability to vary the braking force, it seemed like it was stuck at its last setting.

[danjreed]

A 2 axle controller is not made to deliver the current that 3 axles need to operate according to specification. Hence you don't know which ones will lockup.
The electric trailer brakes (excluding electric-over systems) can for our purposes be seen as resistors. They're actually electromagnets, and that creates some really strange things, so we'll just call them resistors for now.
Trailer brakes are wired in parallel. Meaning that the + wires are all wired together, and the - wires are all wired together. When 12V is applied to the + wire and the - of the magnets is on the - of the battery, all the magnets see 12V.
The problem is related to Ohm's law, V=I*R (Voltage in a circuit is equal to the current through that circuit mulitplied by the resistance as seen from the power supply).
When resistors are in parallel, the resistance that the brake controller sees goes down. In order for the controller to maintain 12V on the + wire of the magnets, it must allow more current to flow to the brakes.
Not having enough current will limit the maximum voltage that the brake controller can supply. Which is why it is unpredictable.
In order to supply enough current to the brakes, the controller needs to have components sized to handle the loads.

Unlike mechanical parts, electronics usually can't handle much more than what they are specified at. Obviously the controller manufacturer added a little security to their design since it still works, but I wouldn't keep doing that.

[Kanman]

but I wouldn't keep doing that

I hear ya. Beyond the safety factor its playing hell with tire wear. Did I mention triple axles suck?

[TrailerHauler]

I'd rather have tandem duals if you can put em on that specific trailer!

[larry_minn]

Something is wrong somewhere. I have NO idea where. I have one 3 axle trailer and pair of two axle. The two axle controller works fine I just have to set it (higher) for three axle (to get similar results) then for 2 axle. It lacks the power to lock up all three unless set at max.
From what you say it needs to be fixed. I would likely get correct controller and install it. (Or hook trailer to friends truck that you know the controller is working and see if problem is trailer/truck side.

[short_stuff]

Just thinking out loud, but what would it do if you just disabled the brakes on one axle? Seems like that would eliminate the apparent randomness of which axle is going to get the juice to lock up. I don't have any trailer with brakes, so I've never had to worry about it. But it is an interesting question.

short_stuff

[Cool_Canuck]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Kanman</div><div class="ubbcode-body">So what happens if you run a brake controller rated for only 2 axles on a 3 axle trailer?

I have been running this way for a little while, and I just haven't gotten around to installing a new controller.

What I have noticed is it never seems like the same axles lock up twice in a row. Its almost a crap shoot as to which lock. And, they do LOCK, geeze it is annoying to smoke the tires every time I touch the pedal.

Curious as to why this happens, I would have figured all three axles would have gotten some voltage, just not enough. But that isn't the case, the current seems to pick two axles.
Curious as to why this happens? </div></div>
From what I understand, a 2 axle unit is not equipped to supply the amperage require for 3 axles. I'm surprised that you can lock up a tire, but then again, that might be because the controller is damaged and can no longer proportion the power. I am assuming that you cannot adjust that out.

[jimmy]

locking up random axles on that trailer is very unlikely to have anything to do with the controller. There is no way the current would be unequal to all the magnets unles there is resistance in wiring and then it would always be that affected wheel brake set that does not lock.

The connection of three axles through the equalizer link (piece that ties the leaf springs together) is usually the culprit combined with unloaded trailer. The torque of back axle forces the front of spring down and that rotates the equalizer annd lifts the back of the spring in front and unloads that tire and makes it lock up easily.
Improperly adjusted brakes can make it worse.
The trailer is probably heavily sprung and that adds to the condition also.
When it is loaded does the random wheeel locking still occur?

Use of 2 axle controller on that three axle trailer would, if anything, decrease the chances of wheel lock.