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Tequila's avatar
Tequila
Explorer
Jun 29, 2014

Simple trailer brake test

I do this test before each trip:

I have a brand new trailer with all 4 brakes presumable working. Unless you have some sort of fancier brake controller all your controller will tell you is that at least one of the 4 brakes (as in the magnets) are working. On my old trailer I once had an open on the one side (single axle) and I could not tell except for the way it was towing.

With my new trailer I took an ohmmeter and measured the resistance at the main trailer plug in cord. If you look at it end on with the nub at the top, the trailer brake contact is at the 7 o'clock position. Take the resistance from that to a frame ground. In my case it was exactly 1 ohm. That means that each magnet has a resistance of 4 ohms since they are wired in parallel. They can be anywhere between 3 and 4 ohms each depending on type. If they are 3 ohm and all 4 are good, you would get a total reading of 0.75 ohms. It is a simple matter to occasionally check this. In my case with 4 ohm brakes, if I have one bad magnet, I will get a reading of (4)/3 or 1.3 ohms, if 2 are bad Ii will get 4/2 or 2 ohms, if 3 are bad I will get a reading of 4/1 or 4 ohms, if all are bad I will get 0 ohms.

The trick after that is finding out which ones are bad, of course.

Resistance will be higher if the brakes have been recently used and are warm or hot, so the readings need to be done cold.

9 Replies

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  • Thsldo's avatar
    Thsldo
    Explorer
    Jun 30, 2014
    Gsragtop wrote:
    What I have always done with trailer brakes is, come to a stop.. Manually engage the trailer brakes from the controller.. Release the vehicle brakes, if it holds were good. Do it at the start of every trip..


    I do the same thing.
  • John___Angela's avatar
    John___Angela
    Explorer
    Jun 30, 2014
    brulaz wrote:
    I use a clamp-on ammeter and measure with the emergency switch pulled.

    Also, after a hard, fast stop, I check the drum temps by hand. Once found a greased drum that way; it was cooler to the touch than the others, but the ammeter readings were fine.


    concur. I usually get 12 to 16 amps on a good set of brakes. Anything way out of line from that I start looking.
  • brulaz's avatar
    brulaz
    Explorer
    Jun 30, 2014
    I use a clamp-on ammeter and measure with the emergency switch pulled.

    Also, after a hard, fast stop, I check the drum temps by hand. Once found a greased drum that way; it was cooler to the touch than the others, but the ammeter readings were fine.
  • Gsragtop's avatar
    Gsragtop
    Explorer
    Jun 30, 2014
    What I have always done with trailer brakes is, come to a stop.. Manually engage the trailer brakes from the controller.. Release the vehicle brakes, if it holds were good. Do it at the start of every trip..
  • ScottG's avatar
    ScottG
    Nomad
    Jun 30, 2014
    Ya, you really need a 4 wire meter for something that low.
  • tvman44's avatar
    tvman44
    Explorer
    Jun 29, 2014
    A ohm meter is not very accurate at such low resistance values, besides the resistance of the coils can vary enough to mess up your readings. I have a P-3 controller and measure the current draw which I feel is a little better but still not the most accurate indication. :)
  • Gdetrailer's avatar
    Gdetrailer
    Explorer III
    Jun 29, 2014
    Tequila wrote:
    I use an analogue meter with an ohms adjust. I do not like digital meters for this sort of thing.


    Analog meters are not accurate at low Ohms either, you basically are simply zeroing out the "error".

    Kelvin or Wheatstone bridges are a better choice when dealing with a few ohms of resistance and will yield a repeatable and more accurate measurement.

    FROM HERE

    What is a Low Resistance Measurement?
    A low resistance measurement is typically a measurement
    below 1.000 ohm. At this level it is important to use test
    equipment that will minimize errors introduced by the test
    lead resistance and/or contact resistance between the probe
    and the material being tested. Also, at this level, standing
    voltages across the item being measured (e.g. thermal emfs
    at junctions between different metals) may cause errors,
    which need to be identified.

    To allow a measurement to compensate the errors, a four-
    terminal measurement method is employed with a reversible
    test current and a suitable Kelvin Bridge meter. Low
    resistance ohmmeters are designed specifically for these
    applications. In addition the upper span on a number of
    these meters will range into kilohms, which covers the lower
    ranges of a Wheatstone Bridge (please see the appendix for a
    discussion of the Wheatstone and Kelvin Bridge methods).
    The lower range on many low resistance ohmmeters will
    resolve 0.1 micro-ohms. This level of measurement is required
    to perform a number of low range resistance tests

    WHEATSTONE BRIDGE

    KELVIN BRIDGE

    Ah.. brings back memories of Tech school lab so many years ago.

    Something else missing is not ALL the magnets are going to be the SAME resistance.. It will vary according to the amount of windings, temperature and all the splices in between will fudge your "results"..

    Additionally modern brake controllers use PWM, you CAN have a shorted winding and NEVER know it if you are using DC resistance readings. A shorted winding however will mess with the brake controller by altering the ring back pulse the controller sends out to see if there is brakes attached..

    Your "test" really is not a valid one that I would bet the bank on, I would opt for using an AMMETER before resorting to an Ohm meter.

    Ammeter will show 3A for one brake, 6A for two brakes, 9A for three brakes, 12A for four brakes provided you have a fully charged battery. Pretty cut and dry, I miss my old Jordan Ultima 2020 controller since it has a AMMETER readout..

    Might some day retrofit a Ammeter on my IBC output line, very helpful to have..
  • Tequila's avatar
    Tequila
    Explorer
    Jun 29, 2014
    I use an analogue meter with an ohms adjust. I do not like digital meters for this sort of thing.
  • Gdetrailer's avatar
    Gdetrailer
    Explorer III
    Jun 29, 2014
    Pretty much not worth the effort.

    Very few "ohm" meters can actually read ohms less than say 10 "accurately", not even most Flukes can do this.

    I have one of the very first Radio shack DVMs which has a low ohms zero adjustment (25+ years old) and unless you have that adjustment your meter will be 1-3 ohms off. I also have a 40+ yr old analog Triplet meter which also has a low Ohms offset adjustment.

    Just short your leads together, pretty fair chance you WILL get a reading of 1-3 ohms which WILL be added to your reading (offset).

    In other words the reading you get you WILL need to SUBTRACT the offset resistance you got when shorting the leads together.

    Your BEST way to check is to take an AMMETER and connect it in series with your breakaway switch. Pull the breakway and take your reading and if all is well you should get around 11-12A draw..

    But really, that is over the top worry for nothing.

    If you are this concerned then I would suggest redoing all the factory splices at the brakes with weather proof connections then wire tie the wires to the axles to prevent movement. Chances are if you do that you will never experience any broken connections for the life of your trailer.

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