Trailer Brake Wiring

jsb5717 · 07-04-2023, 09:05 AM

I'm a fairly decent backyard electrician but I'm currently confused by my trailer brake wiring. I really don't understand how it's supposed to work...unless there's something wrong in my current wiring.

When I apply the brakes the trailer responds. However...

There are 2 wires running to the trailer braking system at the wheels. When I put a continuity meter on those wires they are both common to ground and to each other. I thought they would be separate with one ground and one not until the brakes are applied and the circuit is closed by the trailer brake controller in the truck.

There are 2 wires from the trailer that are connected to the blue trailer brake wire from the truck. One wire (light blue below) shows a continuity to ground and the other (black wire) doesn't. Where do these wires go?

I've looked online for how trailer brakes are supposed to be wired and I mostly find a simple 2-wire system that works the way I think it should with hot and ground and the circuit being closed by the trailer brake controller in the truck causing the brakes in the trailer to apply. I assume that the variability of braking pressure is controlled by the controller itself and communicated by voltage to the trailer brakes.

I'm trying to inspect the trailer brake wiring to make sure all is well but finding my meter useless since all wires seem to share a commonality to ground. For now I've cut and isolated the blue brake wires to try to trace what goes where. The dark blue wire, when disconnected from the others, shows continuity to the correct pin on the umbilical cord plug. But when connected they all show continuity to ground.

I'm suspecting a short somewhere in the trailer. Is there something I'm missing?

Thanks

ChuckS · 07-04-2023, 09:22 AM

Since you have tandem axles all four brake magnets are wired in parallel. Common ground to frame.
One 12 volt feed from RV 7 way plug to all four brake magnets

This link has a nice diagram to follow.

I’ve also added two images of pin box wiring ( my 2014 Keystone Alpine ) along with Keystones standard wire color code for the brake and light system on that pin box receptacle

https://www.etrailer.com/faq-wiring.aspx

jsb5717 · 07-04-2023, 09:59 AM

Thanks, Chuck. That's some of the same stuff I've seen as well. It all makes sense on paper.

I see that you also have 2 wires from the trailer connecting to the blue wire from the plug so that seems consistent. But where does the 2nd wire go? Seems like you'd only need one as shown in the diagrams.

Where I'm confused, is why the blue wire show continuity to ground...especially when disconnected from the truck.

**chuckster57** · 07-04-2023, 10:14 AM

Quote:

Originally Posted by jsb5717

Thanks, Chuck. That's some of the same stuff I've seen as well. It all makes sense on paper.

I see that you also have 2 wires from the trailer connecting to the blue wire from the plug so that seems consistent. But where does the 2nd wire go? Seems like you'd only need one as shown in the diagrams.

Where I'm confused, is why the blue wire show continuity to ground...especially when disconnected from the truck.

I was responding when Chuck posted, I'm glad you got it figured out. If you look at the second wire going to the blue brake wire it's probably black and it comes from the break away switch. It will be "live" only if you pull the pin and have a battery installed.

ChuckS · 07-04-2023, 10:19 AM

If you measure the blue wire at the 7 way plug connection AND DONT DISCONNECT the BLUE WIRE FROM the backside of each of the four magnets you will read the resistance across the brake magnet coil..

You will read somewhere around 3.2 ohms per magnet.. since these four magnets are fed in parallel the total resistance reading of all four brake magnets if measured at the 7 way plug will be around

** the reading will of course be less than 3.2 when reading all four magnets at one time due to parallel connection

See images.. this is the reading of all four brake magnets at the RV side 7 way plug when measured from the blue wire to ground wire on the 7 way plug

If you removed all four blue wires from the backside of each brake assy backing plate and then measured at the RV 7 way plug you would read an open .. if not then you have an issue somewhere in the blue feed wire that parallel feeds all four magnets

I went out and took a quick measurement for you with all four magnets connected ..

jsb5717 · 07-04-2023, 10:50 AM

OK, making more sense now. I appreciate the help. In the meantime I've also removed hubs to inspect wires and have found a broken one in the right rear. I will inspect all wires and also plan to run new wires across the axles proactively since I can't see in there.

All brake pads look good so a bearing lube and reassembly and hopefully I'll be in good shape for a while.

Thanks, again, for the help

ChuckS · 07-04-2023, 10:53 AM

Glad you got it all in hand now and don’t forget to seat that axle nut to 50 ft lbs and then back it off finger tight before installing then spring clip for the axle nut

I am still running the OEM Dexter brakes, bearings and races with over 40K miles on them now

jsb5717 · 07-04-2023, 10:56 AM

Quote:

Originally Posted by ChuckS

Glad you got it all in hand now and don’t forget to seat that axle nut to 50 ft lbs and then back it off finger tight before installing then spring clip for the axle nut

I am still running the OEM Dexter brakes, bearings and races with over 40K miles on them now

Good to know. How often do you lube the bearings? I've lubed these a couple times since new. At about 8K miles now. So far so good.

**JRTJH** · 07-04-2023, 11:06 AM

Just an observation on your meter reading....

If you're using an ohmmeter with a needle scale rather than a digital scale, and the meter sensitivity set on a high meter setting (say 200 K ohms or 100 K ohms) then the needle will read "so close to zero that it looks like a short". If you change the meter sensitivity to 1 ohm or 10 ohms, then you'll get some meter deflection and will be easier to see a 1.38 ohm reading.....

You probably already realized this, but I've been "caught up in troubleshooting so deeply that I couldn't see (or forget to adjust the meter) and didn't realize my mistake.... So, just a comment, not a criticism.

ChuckS · 07-04-2023, 01:44 PM

Quote:

Originally Posted by jsb5717

Good to know. How often do you lube the bearings? I've lubed these a couple times since new. At about 8K miles now. So far so good.

I prefer to pull my bearings at the start of each season and inspect, clean, grease.,etc..

The Dexter Service Manual states every 12K miles .. so there are some variables in there regarding how often to clean, inspect and grease wheel bearings

Honestly the last two years when I’ve pulled the hubs it was a waste of time “regarding the bearings needing grease”.
**But.. more importantly it gave me the opportunity to verify condition of brakes, magnets, and of course the bearings and races..

** Note - John made an excellent point if yo7 are using an analog multimeter instead of a digital one chose the lowest ohm scale for measurement, such as X1, X10…

Any class of meter will measure what you want on your RV.. having it set to the proper scale ensures accurate and reliable readings

Grease.. I prefer to use Lucas Red n Tacky and have for decades.. I also use it on all suspension wet bolts, etc as well as my autos

https://lucasoil.com/pdf/TDS_Red-Tacky-Grease.pdf

Attached images are what my bearings looked like after a year of use and getting ready to clean and repack . As you can see the Red n Tacky holds up quite well under heat and weight loads..

Second image is what all four sets of inner and outer bearings look like after 10 seasons and over 40K miles.

** Dexter OEM bearings

Last two images are what my Dexter OEM brakes look like this year after 10 seasons .. I tow a lot on mountain roads in Idaho, Montana ..

skids · 07-05-2023, 05:38 AM

When measuring resistance from the trailer plug for a tandem axle, it should be 4 resistances in parallel. 1/Req = 1/R1 + 1/R2 + 1/R3 + 1/R4, where Req is the equivalent Resistance of the system. Since each brake resistance is 3.8 (3.0 to 3.8 for 10" to 12" brakes), then 1/Req = 4/3.8. This is simplified as Req = 3.8/4 or 0.95 ohms. This assumes that connections are all good, brake magnets are good, wire resistance is low.

If there is only one magnet with good conductivity, you would measure 3.8 ohms.
If there are only two magnets with good conductivity, you would measure 1.9 ohms.
If there are only three magnets with good conductivity, you would measure 1.3 ohms.
If all 4 magnets have good continuity, you would measure 0.95 ohms.

ChuckS · 07-05-2023, 02:24 PM

The resistance check is a meaningful check but as stated it doesn’t take into account all the variables. A much better electrical check… IMO is using a clamp on amp meter and measure the amount of amperage drawn by each brake magnet coil when all brakes are activated ..

This can be done from the backside of each brake backing assy and requires no wires to be removed.

Mathematically the four magnet coils should read as stated .95 ohms total resistance on the parallel circuit but in the real world rarely do..

The reading I posted is typical..the magnets on my four brakes are in good working order and draw about the same amperage draw on each magnet ..

But due to variations in coil to ground at each coil assy, wire splices in the parallel feed,etc the total resistance of course read higher then .95 ohms

**chuckster57** · 07-05-2023, 02:29 PM

We put a DC amp clamp on the break away, pull the pin and look for about 3 amps per wheel.

ChuckS · 07-05-2023, 02:54 PM

Quote:

Originally Posted by chuckster57

We put a DC amp clamp on the break away, pull the pin and look for about 3 amps per wheel.

Yup that is the easiest and accurate way to test all four magnets ..

But if you have an open or shorted circuit testing at the beginikg of the circuit gives you a relative idea ..

If I read 0.0 I know there is a short to ground on the brake 12 vdc feed wire ..

If I read infinity then I know there is a break in the feed wire to the brake magnets..

My previous post using my fluke meter showed 1.9 ohms.. the fifth wheel had been towed about 2200 miles prior

In this pic I am adding here .. same test, doffent meter and leads, trailer had been sitting since Oct last year and was prepping for first trip

The meter read 1.0 ohms

Again..a quick relative check of the entire brake feed wire from RV 7 way plug to all four brake magnets

I was just showing a quick way to verify the feed wire and ground to all four magnet coils.

My video measuring all four brake magnets prior to the first trip of the year again shows relative readings from a low cost clamp on meter.

https://youtu.be/kpO2KgtzXkE

Each test has its usefulness depending on what you are troubleshooting .. if I am troubleshooting a short or an open then the amp meter is basically useless.

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