tandem trailer brakes weak on one axle, bearings and the DOT date codes on the tires

cold hub on one axle is a good clean clue, so you are already most of the way there. electric trailer brakes almost always come down to three things and you check them cheapest first.

• ground. the single most common electric brake fault there is. each axle grounds through the frame and the mounting, and a rusty ground makes that axle weak or dead while the other one works fine. run a dedicated ground wire from the brake back to a clean frame point and a lot of dead axles wake right up. this is a ten minute, two dollar check and it is where i would start on a cold rear axle.
• magnets. the flat round magnet in the drum wears out and gets weak, and a cheap meter across the brake wires will tell you if the magnet is pulling the right amps. a worn magnet grabs soft.
• adjustment. drum brakes have a star wheel and if the shoes are backed way off, the magnet has to work through slack before it does anything. adjust them up till there is a light drag and back off a couple clicks.

do the ground first before you buy a single part. i would bet a coffee that is your cold axle.

ground wire, every time, on a cold axle. i carry a spool of 12 gauge and butt connectors specifically for this because a trailer that grounds through rusty u-bolts will drive you crazy chasing it. i ground every brake straight back to the frame with its own wire now and i stopped having dead axles.

the thing i want to jump on is your bearings line. three years and never touched is not a crime by itself if you do not put many miles on it, but you tow it loaded in the mountains, and heat plus old grease is how a bearing lets go and dumps a wheel on the interstate. repack them while you are in there, inspect the races for pitting, and from now on put your hand on every hub at the first fuel stop. a hub that is warm is fine. a hub you cannot hold your hand on is a bearing about to come apart, and that is your five second check that you already stumbled into doing.

everybody is right about the ground. i will be the guy who brings up the tires since you said the trailer is three years to you but you did not say how old the tires are, and that matters more than people think on a trailer.

trailer tires age out before they wear out because the thing sits still for months at a time. the DOT date code on the sidewall is the four digit number at the end of the DOT string, week then year, so a code ending 2219 was built in the twenty-second week of 2019. a trailer tire past about six or seven years old is living on borrowed time no matter how much tread is on it, and a dry-rotted one lets go at highway speed with no warning and usually takes a fender and a wiring harness with it.

go read your date codes this week. if they are older than you thought, put new ones on before spring. there is good plain-language stuff on reading the date code and tire aging on the government site at nhtsa.gov, worth five minutes before you load a car on old rubber.

one more on the controller since you said it is an old cheap one. a time-delay controller ramps the brakes on a timer and you set the gain by feel, and it is the worst of both worlds on a tandem because it cannot tell a weak axle from a correct setting. a proportional controller reads your truck slowing down and applies the trailer brakes to match, and the first time you feel a good one you understand what you were missing. if your truck is a 2014 it may have the factory integrated one, so check before you buy, a lot of guys buy a controller they already have in the dash.

none of that fixes a cold axle though, that is still a ground or a magnet. the controller only changes how evenly and smoothly both working axles come on. fix the dead axle first, then worry about the controller.

textbook. cold axle equals bad ground nine times in ten, and this board just saved somebody a trip to a shop that would have charged him a diagnostic hour to run the same two dollars of wire.

the tire part is the one i want to underline for the next person reading this. a trailer tire with good tread and an old date code is the single most common thing i see let go on the side of the road, and it is pure age, not miles. read your date codes twice a year on anything that sits, and put your hand on your hubs at the first fuel stop. do those two free things and you avoid most of what strands a car hauler.