Checking Electric Brakes

Here's a tip I used recently to verify that brakes on both wheels of my 2619 were connected electrically (the brakes seemed fairly weak, so I thought it might be that one brake was working but the other wasn't, due to a loose wire or something):

I used an ohm meter to measure the resistance between the electric brake terminal and the ground terminal on the TM's 7-way trailer plug. According to the Dexter Axle manual, resistance should be 3.2 ohms for each wheel brake, so it should read around 1.6 ohm (.8 ohm if you have 4 wheels) if the wiring is perfectly resistance-free. Of course it isn't, so my reading came out to be 1.7 ohm - but that's still close enough. You can look up the trailer plug wiring diagram online to see where to hook up the leads. If the reading was 3.2 ohms, I'd know one of the wheel brakes wasn't electrically connected anymore.

This might be a good check before a long trip or after a long time in storage (up here, the rats love to chew on wiring). You still need to test your brake stopping power (in my case, the weak brakes were due to the brakes being out of adjustment), but this can let you electrically verify they are connected.

[wbmiller3]

Thanks, that's a keeper.

Sounds like a simple test to make sure both brakes work.....thanks for the heads-up.

Chap

Another tip which I mentioned some time ago...crawl under the TM and make sure the wires to the brakes are connected!!! One of ours was not, and we had no trailer brakes. Since this was the way it was delivered, I didn't know the 'feel' of properly working brakes, so I just thought all was well. Wrong. Good thing the TM is light and our truck was strong. Now when we leave I will roll down the street and just apply the trailer brakes using the brake controller and not the TV. If we stop, they are working!

When installing my lift kit, I discovered a wire on 1 side that was wore down to a couple of strands, and the other wheel had the insulation wore down to the copper, also. Must both have been the ground wires, as my fuse didn't blow and the brake controller did not show a default. I routed the wires through some automotive electical wiring harness where it goes thought the axle bracket.

Chap

I use a compass.

[ShrimpBurrito]

I'm confused. I presume the brakes are in parallel with each other.....so if each wheel brake has a resistance of 3.2 ohms, shouldn't the reading at the pigtail be the same?

Dave

Two resistors (3.2 ohms in this case) in parallel will be 1/2 the resistance of either resistor (assuming equal resistance in both resistors).

See http://physics.bu.edu/py106/notes/Circuits.html

The equation is something like:
1/R = 1/(R1) + 1/(R2)

That is, 1/R = 1/3.2 + 1/3.2, which is 1/R = 2/3.2, which becomes R = 3.2/2, or R = 1.6.

That also makes sense because if you pull the emergency breakaway cable, and apply 12 volts to both brakes, your battery will drain twice as fast compared to if only one brake was connected. Since the voltage is 12 volts in this case, whether one or two brakes are connected, Ohm's law (Resistance equals Voltage divided by Amps) says that resistance is 1/2 when the battery drains twice as fast:

R = 12/I (for one brake, where R is resistance and I is amps drawn) for one brake (for 12 volts, "I" ends up being 3.75 amps, or R = 12/3.75). For two brakes, "I" would be double (twice the amps, twice the battery drain), or 7.5 amps at 12 volts. So R = 12/7.5, or 1.6 ohms.

Originally Posted by AstroBruce

I use a compass.

Does all the wheel and drum metal attract the compass before you apply the brake??

Another unique idea.

Chap