Testing brake controller

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mm3670
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Testing brake controller

So we aren't getting our Scamp Deluxe till late fall, but I've installed a brake controller in our Tahoe in anticipation. From the reading I've done it looks like a typical electric brake electromagnet looks like about 4 ohms so for two brakes the current draw for a full 12V brake application would be 12/4 = 3 or 6 amps total. I'm thinking to get a 2ohm power resistor and connect it to the trailer jack and check the voltage applied. I'm assuming the brake controller will think there is a trailer connected and apply a voltage, at least with the manual brake controller lever.

Anyone ever done this kind of check?

 

Thanks.

mm3670
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By the way, I am an electrical engineer

I am an electrical engineer (I know, an Enginerd....) but I'd be happy to answer any electrical theory questions (note; i did not say how to wire a particular trailer... ). smiley

Greg A
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Easier Brake Check

Get one of these:

https://amzn.to/3eZ8AcE

Not only will it trick the tow into thinking the trailer is attached it will check your brake installation is working as well as all the other functions of the 7 pin. A good tool to put in your tow bucket as well for field issues.

Our Trailers:
2015 19 Escape

Buying or Selling Molded FG Trailers:

Fiberglass-RV-4Sale

ac0gv
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Not what you asked.....

....but I'll toss it in the mix anyway. My brakes draw about 7 amps.

salukispeed
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resistor

Resistor is good. Anything that uses power across ( even a old school incandecent test light )  the blue to White wire will fool the test LED on the controller and verify  you have a connection to the receptacle. I used a old school head light that draws nearly 5 amps as my test load. It shows power and ground are intact and that you can modulate the power with the control by a brightness change

2019 Scamp 13 standard

mm3670
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tekkonsha controller doesn't like 2 ohms....

So I built a test fixture to test the 7 pin trailer socket on my 07 Tahoe. All the light lines and charge lines are right. I put a 2ohm 50W resistor between the brake controller line and ground. The Tekonsha P3 controller says "brake shorted". Huh? 12V across 2 ohms would be 6 amps so about the max brake current. ac0vg confirmed he saw 7 amps on the brake circuit so 6 amps shouldn't tilt. ? I will double the load resistance to 4 ohms and see if the controller is happier. I seriously doubt that the controller can tell that a brake electromagnet is inductive versus an resistive load so I'm not sure what's going on. A tandom axle trailer would likely but a near 12 amp load on the brake controller so what gives? Any ideas? Thanks.

ELongest
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Dexter Book

Just checked the Magnet Amperage Chart in my Dexter axle book. Dexter #10, 10"x2 1/4" brakes, one magnet 3 Amps, two brakes 6 amps. Magnet 3.2 ohms. The internal brake wires should be green. Tandem, 4 brakes should pull 12 amps by the chart in the book.

Eddie

mm3670
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Right; tandem would be near 12 amps

Thanks for the confirmation. I can think of a simple circuit that the brake controller could use to see if the load was maybe a short or partial short, or if it was the inductive electromagnets. I wonder if they would really do that? I assuming that with a "real" brake load the controller would be happy.

mm3670
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Testing brake controller; SOLVED!

Turns out the Tekonsha P3 brake controller is way smarter than I thought; it can tell the difference between an equivalent resistor and the actual electric brake load. I assumed it did this by making a crude measurement of the inductance looking into the brake circuit. To prove this I happen to have an audio output transformer that had a secondary side with 1 ohm resistance and who-knows-what inductance, but definitely more than the brake wiring inductance. When I use this as the load the controller is happy and when I actuate the manual brake control on the controller the voltage on the  transformer ramps right up. Didn't leave the voltage there very long as it would probably burn up the transformer. I also measure about 75mVrms of 40Hz on the brake circuit when no load attached. Didn't put a scope on it so it could be a sine wave or some PWM signal, but definitely an AC signal to measure the  inductance. 

This is a good feature as it can tell if there is a short in the brake circuit (low inductance) or if ok (high inductance). Trying to tell the difference in resistance is not doable as the resistance can vary from sub ohm to a few ohms. 

Interesting.....