Weird issue with emergency brakes

Maple_Geek

I've been scratching my head on this one since I first noticed it a couple of weeks ago. Today I decided to take out the multimeter and get under the trailer to see what was going on.

In my junction box, this is what some of the terminals are, starting from the left:
(1) Brakes
(6) 12V
(7) Ground

Emergency pull pin has 2 wires:
One connected to (1) and one to (6)

2 wires from the brakes from what I can see
One connected to (1) and one connected to (7)

With my car disconnected, I see the following:
a) 13.X Volts between (6) and (7)
b) ~0 Volt between (1) and (7)
c) Pull the safey brake pin I see 12.X Volts between (1) and (7) so the brakeway switch appears to be working. Not sure why it's not 13.X Volts however
d) I didn't measure but I hear the brakes when I hit the brake button on the Autowbrake remote. I also feel the brakes engage when it's connected to the car so this works.
e) I also hear the brakes when the car is connected and the brake pedal is depressed so I'm assuming they work

So the brakes appear to be working but the wheels don't lockup and I don't hear the brakes when I pull the safety pin, even though the brakes get voltage when the pin is pulled. 

I'm missing something obvious....from the bit of research I did, shouldn't there be 3 wires from the brakes? (1), (6) and (7)?
Maybe I missed it in the wire mess but I believe I only saw 2.

Grumpy_G

Sounds all correct. The brakes use electromagnets that pull quite a bit of current which is why the voltage drops when they engage. Most brake controllers use pulse width modulation which makes the magnets hum which is what you hear. When you pull the pin the magnets get constant voltage so you won't hear much. The wheels should be pretty much locked though. The breakaway switch simply applies full voltage to the same wire that is normally used by the trailer brake controller. 

 

Maple_Geek

Thanks for confirming @Grumpy_G. My observations today also matched what I knew except for the no noise when the pin is out...thanks for the explanation!. The wheels do not lockup however when the pin is out and the car is not connected which is very unexpected and a bit worrisome in case of a catastrophic disconnect. Everything else appears to be correct except this. I'll take another look tomorrow...

Grumpy_G

Maybe I should have phrased that differently. These trailer brakes need the wheel to turn to move the shoes against the drum. If you just pull the pin you can rock the trailer back and forth a bit until the shoes reach the drum. This is different from typical car brakes or even bicycle brakes. 
If you can move the trailer more than a foot without the brakes engaging (with the breakaway active) there is something wrong. 

Maple_Geek

Ah, that is something I definitely did not do. I will try it tomorrow. Thank you so much for this!