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Open Roads Forum  >  General RVing Issues

 > Power voltage to brakes??

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wa8yxm

Davison Michigan (East of Flint)

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Posted: 09/21/22 02:49pm Link  |  Quote  |  Print  |  Notify Moderator

Measure the current (I's use a clamp on ammeter first to make sure as most in-lines are limited to around 10 amps.

Then look at the wire size.. And length.

Then google Wire Size Calculator


Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
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Gdetrailer

PA

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Posted: 09/21/22 03:36pm Link  |  Quote  |  Print  |  Notify Moderator

wa8yxm wrote:

Measure the current (I's use a clamp on ammeter first to make sure as most in-lines are limited to around 10 amps.

Then look at the wire size.. And length.

Then google Wire Size Calculator


[emoticon]

Good grief!

Reading the current will tell the OP zero, zilch, zero on WHERE the problem is.

OP already knows that they are not getting full voltage at the magnets, therefore they will never get full current draw by the magnets. OP already knows they are not getting full braking effort.

Now if you were wanting to check the "balance" of current being drawn between each magnet to make sure the magnets are evenly getting the same voltage then a current reading of each magnet will tell you quickly if it is balanced without the need to access the wires electrically.

This isn't "rocket science", it is Elementary level basic electronics.

As far as using wire size calculators, they are designed for high voltage applications like 120V/240V and are designed around the acceptable NEC voltage drops for 120/240 which is 3% or 5% depending on application.

5% loss at 120V is 6V and you would get 114V which is acceptable

5% of loss at 12.8V is huge at .64V meaning you only get 12.16V when your battery voltage range is 10.5V (dead) to 12.8V fully charge resting voltage.

No need to "look up" the wire gauges, Dexter has already done that work for you in their manual I linked above. In that manual they give the MINIMUM recommended wire gauge for the length of the RV and the quantity of wheels.

Using Dexters charts, most average length RVs with 4 wheels will fall into 14 ga, 12 ga or 10 ga wire as the absolute minimum wire size. Extremely long RVs with 6 wheels fall into 10 ga or 8 ga wire.

I highly doubt most RV manufacturers will pop for more than 14Ga wire.. Myself I have found 16 ga wire used on my TTs and my 18ft 10K flatbed trailer they used 12 ga wire (unlike the cheap RV manufacturers).

Once again, OP already knows they are not getting full BATTERY VOLTAGE AT THE MAGNETS. That means there is a high resistance somewhere BEFORE the axles.

Grit dog

Black Diamond, WA

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Posted: 09/21/22 04:16pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks for trying to keep the peanut gallery on topic GDE.


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ivbinconned

highway 16

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Posted: 09/21/22 08:34pm Link  |  Quote  |  Print  |  Notify Moderator

The trailer is a 34 footer and around 12,500 pound.
Couldn’t get at it today but tomorrow I will start by testing my battery voltage under the load of full braking.
If it is good I will then cut the wires where they first appear under the belly at the first axel and test the voltage there.

The brakes two wires are encased inside there own grey coating and appear to be in a loom right to where they emerge under the belly.
We will report tomorrow.
Thanks for all your comments


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GottaRunGottaCamp

East of Atlanta, Ga.

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Posted: 09/22/22 04:52am Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer, thanks for the great information, time to start checking things till I find the problem, What I meant about locking up the brakes is just for setting up the gain on them, and yes you don't want your trailer brakes to lock up.
Again thanks for the information!


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eHoefler

ozark mountains

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Posted: 09/22/22 05:41am Link  |  Quote  |  Print  |  Notify Moderator

As long as all 4 brake assemblies are connected, you will never see 12 volts at the magnets, each of them are a designed short to ground and, will drop the voltage. Every splice in the system adds resistance and will drop voltage. If the splice is not a solid connection and water tight, they will corrode, add resistance, and drop the voltage. The cross over wire are usually inside the axle tube, where the jump around, rub, and chafe, which will short to ground and drop voltage.

I can go on, but I digress.

To improve performance, each magnet should have equal wire length to the source. The way they wire them, using under sized wire, is to run one lead down one side or the other, splice at the first axle, then to the second axle, then splice each axle to the other side. Resulting in the farthest wheel from the source will be the weakest braking wheel while the first in line closest to the source being the strongest.

The best way to wire them for best performance is to run 2 wires, one down each frame rail, then splice them at the center of suspension to each wheel. That way you will have the same wire length to each wheel.

And while your at it, use 10ga wire down the frame rails and splice 12ga to each wheel.

When all done and said, and got tired of not having comfortable feeling braking, I replaced the whole system with disc brakes and have never looked back.

* This post was edited 09/22/22 05:48am by eHoefler *


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MFL

Midwest

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Posted: 09/22/22 07:01am Link  |  Quote  |  Print  |  Notify Moderator

^ eHoefler...good explanation! Some excellent points here, from a man that understands the trailer braking system!

Jerry





ivbinconned

highway 16

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Posted: 09/22/22 08:57am Link  |  Quote  |  Print  |  Notify Moderator

Ok. Just Cut the power wires to the first axle there I had 12 1/2 V. As soon as I hook up the brakes to the front axle only it dropped to 10 1/2 when I hook up the rear axle to the furthest away wheel I’m still at 9 V.

I’ll do a road test next and see if all new brake assembly’s has improved my breaking. So glad I have an engine brake on the ram.

Gdetrailer

PA

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Posted: 09/22/22 10:46am Link  |  Quote  |  Print  |  Notify Moderator

Glad folks have found some of my information helpful so far!

For a bit more advanced information I have done some "back of the napkin" noodling.. Bear with me, this got a bit long..

Wire line resistance has a profound effect on low voltage and low resistance items so to demonstrate how much this can affect your electric brakes braking efficiency I have made some measurements to get some known information of what voltage is needed to get a brake magnet for 3,500 lb axle family to draw 3A.. From the voltage an current measurements I can calculate the DC resistance of the brake magnet.

To get 3A draw from a brake magnet I set my adjustable lab power supply to 12.21V at the brake magnet wire connections, measuring only one magnet (my adjustable Lab supply only can handle 6A per output).

Brake magnet measurement of 3A at 12.21V = 4.06 Ohms

4 magnets at 4.06 Ohm in parallel = 1.015 Ohms (resistance in parallel is divided by the quantity)

Assuming 25’ from tongue to brakes, we must double the length since we must account for pos and negative wires = 50’ of wire.

So for our calculations we are assuming 50ft of wire round trip.

For different lengths of wire or different ga of wire refer to a wire resistance calculator, I used
THIS ONE HERE

16 ga = .201 Ohms add that to 1.015 Ohms = 1.216 Ohms.
Divide 12.21V by 1.216 Ohms = 10.04A
Divide 10.04A by 12A = .87 or 87% of max braking, basically 13% loss of potential braking.

14 ga = .126 Ohms add that to 1.015 Ohms = 1.141 Ohms
Divide 12.21V by 1.141 Ohms = 10.7A
Divide 10.7A by 12A = .89 or 89% of max braking, basically 11% loss of potential braking

12 ga =.079 Ohms add that to 1.015 Ohms = 1.094 Ohms
Divide 12.21V by 1.094 Ohms = 11.16A
Divide 11.16A by 12A = .93 or 93% of max braking effort, basically 7% loss of potential braking

10 ga = .05 Ohms add that to 1.015 Ohms = 1.065 Ohms
Divide 12.21V by 1.065 Ohms = 11.46A
Divide 11.46A by 12A = .96 or 96% of max braking effort, basically 4% loss of braking potential

8 ga = .031 Ohms add that to 1.015 Ohms = 1.046 Ohms
Divide 12.21V by 1.046 Ohms = 11.7A
Divide 11.7A by 12A = .98 or 98% of max braking effort, basically 2% loss of braking potential

6 ga = .02 Ohms add that to 1.015 Ohms = 1.035 Ohms
Divide 12.21V by 1.035 Ohms = 11.8A
Divide 11.8A by 12A = .983 or 98.3% of max braking effort, basically 1.7% loss of braking potential

To be clear, every single splice or wire connection will add some resistance, absolutely no splice is 100% resistance free and that resistance adds to the overall wire resistance. Moisture from the air and even direct moisture getting into unprotected splices over time creates a oxide coating on the wires in the unprotected splice, that coating affects the resistance of the splice and with repeated exposures over time weakens the wire to the breaking point adding further resistance. Damaged or compromised insulation can allow moisture to seep into the wire and degrade the wire.

It is critical to use a good moisture proof connectors if the connector is going to be exposed to the weather (yes, even the connections under the trailer get exposed to moisture, driving in rain splashes water and soaks the underside of the trailer).

For the ones advocating using Amp meters to trouble shoot this, generally most DVMs with built in 5A or 10A amp reading will insert too much resistance and that resistance will severely affect your overall circuit amperage draw and reduce the voltage, severely skewing your measurements and results.

Gdetrailer

PA

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Posted: 09/22/22 11:48am Link  |  Quote  |  Print  |  Notify Moderator

ivbinconned wrote:

Ok. Just Cut the power wires to the first axle there I had 12 1/2 V. As soon as I hook up the brakes to the front axle only it dropped to 10 1/2 when I hook up the rear axle to the furthest away wheel I’m still at 9 V.

I’ll do a road test next and see if all new brake assembly’s has improved my breaking. So glad I have an engine brake on the ram.


There is still some seriously high resistance in the wiring, I would have expected no more than .5V drop and even that would be high.

If you have some 14 or 12 gauge wire hanging around, might be easier and faster to bypass the wire from tongue to the axle as a temporary test. You could feed the test wire through a window at the front and then out a window nearest to the front axle. Doesn't have to be super fancy for a test. Heck even a 14ga or 12ga extension cord could be employed and if you didn't want to cut the connectors of the extension cord you can make some adapters to connect it temporally.

Sometimes it is not worth the damaged brain cells to fret over the junk wire used by RV manufacturers and abandon it.

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