How many amps at controller and brakes magnets?

I had a broken wire at the strain relief of the brake drum and could measure 12V right up to it, but zero amps.

So measure the amps.
As stated, you should get ~3A per magnet when ~12V applied.

Vaguely recall people replacing magnets because they were weak with age? Perhaps that's the issue? Pretty cheap fix as I recall.

old guy wrote:
I didn't' think stainless steel was even magnetic

Depends on the composition of the SS.

I didn't' think stainless steel was even magnetic

DrewE wrote:
If your wiring is good (i.e. not shorted), you must see the same current at the magnets as at the brake controller. The electrons don't have anywhere else to go; current into a wire equals current out of said wire.

And yes, thanks to Ohm's law, it is impossible to have voltage and current controlled independently of each other for a resistive load. The laws of electromagnetism are most directly stated in terms of current, so the brake controller is eventually controlling the average current...but the average voltage has to likewise vary. Measuring the voltage at the controller and at the brakes would tell you what the voltage drop is between them, a very useful thing to check.

Do make sure that the ground return path is properly functional too.

I think I have spent too much time in the sun to read this correctly, something isn't right with the statement..

See if I can explain it better..

The sum of the RESISTANCE of the wire AND the brake magnets AND the voltage applied to the circuit will determine the exact amount of current drawn by the brake magnets..

If you were to connect a 12V battery directly to a brake magnet it should draw at least 3A at about 12.6V DC..

Four brake magnets should draw about 12A max..

Now, add in the wire from the vehicle to the brake magnets and you will get LESS than 12A for four magnets..

To further complicate things the current drawn by each magnet often is no equal due to the wiring resistance.. The magnets with the most wire will draw less than the magnets with the least wire.

To put it bluntly, if your brakes have gotten weaker, chances are the connections at each magnet have gotten corroded.

Pull apart each electrical connection and cut back the wire unitl you find shiny copper..

Many folk have often changed out the wire,upgrading the wire reduces the resistance of the wire. This allows more voltage to reach the magnets. This increases the current which increases the magnets strength.

Increasing the magnet strength will increase the braking strength.

If your wiring is good (i.e. not shorted), you must see the same current at the magnets as at the brake controller. The electrons don't have anywhere else to go; current into a wire equals current out of said wire.

And yes, thanks to Ohm's law, it is impossible to have voltage and current controlled independently of each other for a resistive load. The laws of electromagnetism are most directly stated in terms of current, so the brake controller is eventually controlling the average current...but the average voltage has to likewise vary. Measuring the voltage at the controller and at the brakes would tell you what the voltage drop is between them, a very useful thing to check.

Do make sure that the ground return path is properly functional too.

I use a 1/2" 18-8 stainless steel nut to check magnetism. Not nearly as magnetic as pure steel or iron but if it doesn't adhere tightly, the magnet ain't magneting.

Ummmm, errrr, Ohms law, folks. High voltage but no amperage through a supposed "fixed" resistance that is for some reason too high or just plain open circuit? I've seen shorted electromagnets, then I have seen high resistance wire to magnet lead connections. The 18-8 stainless magnetic test plus amperage measurement at full voltage is a viable test. Each magnet's conductivity should be tested individually. I started using JB Quick Weld as a strain relief at the wire lead magnet interface. Beats me why the wire strain reliefs at the backing plate didn't do the job.

Typically, the controllers output a Pulse Width Modulation Voltage as opposed to a steady volts and resulting amps.

Good thinking.
I'd suggest to do a cursory inspection, first. There is a possibility, if your brake wires are daisy-chained through the axle tube that a wire might have chafed. This will measure as full voltage and amperage if a bare wire is against metal.

(Op here...)

OK thanks folks. Voltage.

Do you think I'm starting trouble shooting right way or is there something else I should do or check first? My initial thinking was to make sure I'm getting enough juice out of controller. And then make check to see if that juice was making it to brakes.

Is my thinking right or wrong?

dougrainer wrote:

Old-Biscuit wrote:
At full battery voltage........3 amps per magnet

To expand. Trailer brakes work off VOLTAGE, not amps. You activate the amount of braking by the amount of voltage to the brake Magnets.
1 volt--minimum. 12 volts or higher--- locked up. That is what the Brake controller controls. Doug

No amps no stop period. Volts decide how many amps.......