Simple trailer brake test

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..