tandem axle brake wiring... series between front and back?

[budwich]

I decided to chase an issue with brakes today. In the past, the trailer "pigtail" had an issue where it was "compromised" as it passed by the battery tray such that someone (previous owner) had spiced a few wires... one of course was the brake wire (ie. blue). I had corrected this work with new splices and better weather proofing.

I noticed this spring that the brake controller had to be increased to get an "equivalent response" to previous years (year three). On the Prodigy P2, it went from 9 to about 12 now.

Previous park / storage for last month, we noticed a loss of brake light on one side. Some playing with bulbs (swap) pointed to the TV / trailer connection. Some "light cleaning" with wd40 restore the lights but gave "notion" to perhaps a dirty plug connection that needed addressing. Hence, how I got here.

The trailer is a terry dakota 20 footer ... 2006 2007 depending on what "marketing" you believe... 🙂

So I start looking, thinking that perhap poor connection at the plug were to blame for all things happening now, ie. the brake "issue", lights. Measured the resistance at the trailer plug... 3.2 ohms... ? seems high. same after cleaning with small file.

look in the dexter book, says each brake is 3.2 ohms... what! (more explicit than that... 🙂 )

So I pull apart "surgical area" of the cable... well wrapped with tape and interior "cast" of bicyle tire tube. The area seems pretty good but get down the to crimp connectors and find the blue wire connector / crimp was loose and wires only lightly touching.... guess my attempt at a crimp was just as good as the previous owner... 🙂

Get a new crimp and re-crimp things... with a better crimper than my earlier flat stamped steel one. Great, so check out the resistance at the plug. Hmmm, resistance has dropped, as expected, now to 1.6 ohm. Kind of strange that it is "exactly half" of the 3.2 / brake resistance that the book indicates for one brake wheel.

I check out the rest of the cabling where visible along with jacking the wheels off the ground and spin / brake test each wheel. All work / lock depending on voltage setting, including breakaway switch pulls / test.

Looking at the wiring at the front axle, driver side, there is one pair of wiring coming from the frame of the trailer, I assume, feed.
The pair is then mated with a pair of wires that go to the brake drum on that side along with a second pair of wires going thru the axle to the far side drum / brake... basically 3 wires at each "splice pack".

Similar wiring shows up at the second axle on the driver's side.

I haven't traced thru the wiring channels on the frame of the trailer or the wiring box yet, but based on the measurements and the fact that there are only 3 pair of wires at each axle, it would almost appear that the front and rear axle are wired in series.

Not sure is this is true yet as it will take some work to "tear into" the frame wiring which I am not sure that I want to do at this point... kind of leave well enough alone sort of thing.

My question would be... Is series wiring of the brakes on tandem axles an "option" that people (builders) can use... the brake drum wiring at the axles and drums appears to be "original" based on crimps packs used (I have never seen that kind of pack available for general public...flat pack with three wires on one side).

Any comments of this setup. I would think it lowers braking current which might be useful such that the controller has a broader "control range" to prevent such that things like grabbing / locking may be reduced possibly.

thanks for any comments / guidance.

[budwich]

So brakes are back the way they were last year... working 🙂
some adjustment of the mechanical adjuster along with the voltage at the controller (now back at 9v). Recent 5 days trip was OK.
Thanks for the information and ideas.

[Chuck_thehammer]

well, this is a reminder to check EVERYTHING.

yesterday.. I decided to clean-up the emergency brake cable/switch.
removed it and disconnected the wiring...

2 more wire nuts hiding under the propane tank mounts... connecting the main 12 gauge wire to the smaller wires going into emergency box.
and nothing was there to protect them from the weather.

so more cutting/soldering/heat shrink/taping. then painted switch body and frame area.

[lenr]

Agree with a second 12 gauge--less voltage drop and one still works if the other opens (not shorts). However, I run one 12 gauge to each axle. That way I can split the connection at the umbilical cord junction box and measure the amps to each axle with my 10 amp meter.

[stevemorris]

to improve your system assuming existing wire is clean, just add a second 12 ga cable(2 wire) down the right side from the front terminal box

[Chuck_thehammer]

Thank You ALL, for talking about wiring of brakes... as last fall dealer replaced all brakes on my New to Me Trailer.. never gave it a thought on wiring.

went under trailer this morning... and to my surprise ...
there were 2 butt connectors for each wire of 4 brakes.. and 4 wire nuts from Main brake wire to smaller wires going to backing plates..

so I removed 8 butt connectors and 4 wire nuts... and soldered everything..
the Main brake wire is 12 gauge from 7 pin plug to left side axles.

will figure out what I need to change to the Star setup.. as main wires is fine .


so timing is great... as we depart on a 5,000 mile, 5 month trip soon.

[spike99]

I also recommend replacing factory linear brake wiring with thicker 10 Gauge "STAR design" as well. re: Individual 10 gauge wires (current and ground) to each wheel hub. I do this upgrade to all my trailers (RV, boat, utility) and its much better than factory. IMO, STAR design should be factory default.

For my previous post about STAR design wiring, surf: - click here -

[stevemorris]

being able to measure the brake current flow when fully energized is critical in assessing the condition of the wiring. other measurements like resistance and voltage at the 7 pin connector are valuble too but don't tell the whole story

the culprit is voltage drop. voltage and therefore current drops with length of wire, size of the wire and condition of the wire and all of the connections

when I was working on our tt brakes I measured the voltage at the "last" wheel, it was less than 5 volts but still 11 volts at the 7 pin.

adjusting the shoes at the backing plate will have zero effect on the electrical side of things, each magnet will still draw about 3 or 4 amps each depending on the size of the brakes

one magnet should show a resistance of about 4 ohms. if you have 4 brakes, then the resistance will be 1 ohm, remember they are in parallel not series. this resistance also varies slightly depending on brake size

moving the magnet and thus actuating the shoes with the drum off will only move one shoe. the other shoe is moved by the first shoe but that requires that the drum be in place and the drum rotating. best thing here is to make sure that they can move freely, drum brake lubricant should be used at the contact points on the backing plate and any other moving parts. just don't get it onto the shoe material or drum surfaces

I did do an entire thread on checking and rewiring brakes a few months ago, I learned a lot, more than I ever wanted to know about electric brakes!!

btw, 4 brake magnets with 3.2 ohms in parallel like your brakes will have a resistance of 0.8 ohms

1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4

1/Rt = 4 x 1/R1

1/Rt = 4 x 1/3.2

Rt = 1 / (4 x 1/3.2) = 0.8

[budwich]

tracked down the splice / coupler along the route going to the axles. It was tucked in behind the cable runs / plastic sleeving for all the wiring running along the trailer frame.

So basically, the main run from plug gets to the front axle where is it split in two between the front and rear axles. At each axle there is a second splitter that splits the wiring for each side. All look original so without redoing all the connections and probably wiring (as suggested) maybe the 1.0 ohm is it.

Based on Steve's posts, it would appear that's where he ended up at.... maybe his is a bit better depending on what the voltage was at the battery.

[budwich]

took down the wiring box. There is just one brake connection in there. Its where the main brake run is connected to the breakaway switch (ie. three wires, one from the main brake wire, one from the switch and one continuing on towards the brakes. I measured the resistance there and got the 1.0 at that location so that at least means that my "splicing" at the plug pigtail isn't more resistive than it should be.

Perhaps the "nominal" 1 ohm resistance is about as good as it gets given old wiring and magnets. Perhaps moving adjusters a bit... they haven't been done for about 8k miles might improve things.

[budwich]

So up at it today. Some of my issue was "operator error"... or maybe test equipment maintenance. 🙂 The battery on the meter was a little old / weak which causes poor readings. After replacing and checking "0" along with comparison with another meter, the resistance is down to 1.0 ohms which is getting better to "expected". I am going to crack open the wiring box today where hopefully I get access to splices where the main run is "split" between the two axles so that I can measure each axle seperately. Hopefully the "split" isn't along the trailer "rail" which would mean lots of "disturbing". The good news is the underside of the trailer is open "construction".

The resistance is getting close... perhaps some small adjustment at the brake adjusters maybe need. The shoes themselves appear reasonable at about 3/16 in thick.

One question on the brake / shoe operation. When I move the magnet assembly by hand, in one direction, only one shoe moves out while the other stays put. If I move the magnet in the other direction, the other shoe moves out while the previous one stays put. To me, that would mean that you only get about "half" the brake pad action happening for a given travelling direction. This seems to be quite different from the "traditional" automotive / car setups that I have seen / worked on in the past where the piston pushes out both sides to cause the brakes shoes to expand on both sides. I would have thought that the trailer system would have an equivalent operation mechanically in some form.
Is this the right "read" on trailer brake "principles"???

Thanks again for the suggestions.

PS. stevemorris, you are correct about "current testing" which is a great test as it tests the whole system, wiring, brake magnets along with the power output of the controller. Alas, my current meters only do 10 amps on their "special input" terminal.

[stevemorris]

we were having braking issues on our 4000lb(loaded) tandem axle tt. ohm reading was around 3 ohms, good voltage at the 7 pin.

but the most important check of electrical performance is the current draw at maximum braking. it should be around 12-16 amps. resistances and voltages are meaningless numbers, you need to measure the current

a decent vom that reads up to 20 amps will do the trick



shown in the pic, I'm applying the trailer brakes from the trailer battery directly "through" the vom and the meter is reading 9 amps. this was part way into the rehab of the brake wiring, I went from 4 amps to 12.5 amps.

I replaced every wire, from the truck battery to the controller, to the truck 7 pin, new trailer 7 pin whip, new wire down both sides of the trailer.

I ran a new 10-3 cable down the length of the truck, used it for 12volt supply to the tt battery, brake and ground. I also ran 10-2 to the brake controller(power and brake) at the tt I used a new whip, new battery wires and new 12-2 to the brakes, one cable down each side. I did not cut the cable at the front brake, I stripped the wire enough to wrap the magnet wires around the cable then soldered and insulated everything. I used "liquid" electrical tape for decent weather proofing

[westend]

Your OEM wiring is a daisy-chained parallel configuration. The wires are fed to one brake location and then proceed to each following location. The best remediation is to rewire the brakes in a Star configuration, a pair of wires or at least one power wire to each brake location. You can terminate the star wiring at a terminal strip in the front of the trailer and lead your umbilical wire to this terminal strip. If you install a pair of wires to each brake, and install the terminal strip, diagnosing an electrical brake problem becomes very easy. It can all be done from that forward terminal strip.

[Boon_Docker]

Only one axle connected is what I thought at first also, but couldn't be possible after reading this comment by the OP.


I check out the rest of the cabling where visible along with jacking the wheels off the ground and spin / brake test each wheel. All work / lock depending on voltage setting, including breakaway switch pulls / test.

[lenr]

The brake coils are wired in parallel, period. Your 1.6 ohm reading possibly indicates that only axle is connected. I have totally (every single wire replaced) rewired 2 trailers of the OP's vintage with dramatic results. Increasing the wire gauge never hurts either. I usually run 12 gauge with one wire from each axle to the connection with the 7 lead umbilical cord. If OP still has the original wire inside the axle tube it's time to replace it with new wire tie wrapped to the outside of the tube to prevent future shorts.