Three measurements will tell you whether the fault is in the controller, the wiring or the magnets, and you can take all three with a cheap multimeter in about ten minutes. Every page answering this question lists the possible causes. Almost none of them tells you how to find out which one you have, so the usual outcome is replacing a working controller.
The measurements, in order: voltage at the connector’s brake pin, voltage at a magnet, and resistance across that magnet. Each one isolates a different section of the circuit, and the expected values are published by the company that makes most trailer axles.
Start with the fork that costs nothing
Before any meter comes out, do this, because it splits the problem in half:
Actuate the controller’s manual override with the trailer connected and the engine running. Does the trailer brake?
- Yes, but the brake pedal does nothing — the controller can drive the magnets and the trailer wiring is fine. What is missing is the signal telling the controller you are braking: the brake-light input, usually spliced into the vehicle’s stop-lamp circuit. Check the vehicle’s own brake lights first.
- No, nothing happens either way — the fault is in the output side, and the three measurements below will place it.
The three measurements
These are Dexter’s own procedures and figures, from its light-duty axle service manual. Dexter makes the axles under a large share of North American trailers, so unless yours says otherwise, these are the numbers.
1. At the connector’s brake pin. Trailer unplugged, engine running, meter between the brake pin on the vehicle’s 7-way and a good ground. Actuate the manual override slowly. Voltage should rise smoothly from 0 V to about 12 V.
- Rises correctly → the controller and its feed are fine. The fault is on the trailer.
- Nothing, or it jumps straight to full → the controller, its power feed or its ground.
2. At a magnet. Trailer connected, engine running, meter across the two magnet lead wires at any brake. Dexter: voltage should “begin at 0 volts and, as the controller bar is slowly actuated, should gradually increase to about 12 volts.”
- Present at the connector but absent here → an open circuit in the trailer wiring, or a bad ground.
- Present but noticeably lower than at the connector → voltage drop in the run, often corrosion or undersized cable.
3. Resistance across one magnet. Leads disconnected, meter on ohms.
| Brake size | Expected magnet resistance |
|---|---|
| 7 in × 1¼ in | 3.9 Ω |
| 10 in and 12 in (all sizes Dexter lists) | 3.2 Ω |
- Open circuit → that magnet is broken.
- Well below spec → that magnet is shorted, and it will be dragging the whole circuit down.
Replacing a magnet means the wheel comes off, and Dexter is as specific about putting it back as it is about the resistance above: torque the lug nuts in staged passes to the figure for your stud size, then re-torque at 10, 25 and 50 miles.
And the cross-check: total current. Measured in series at the controller’s blue output wire with all magnets energised, Dexter’s figures are 2.5 A per magnet on 7-inch brakes and 3.0 A on every 10- and 12-inch size — so 12 A for four 10-inch brakes, 18 A for six. A reading low by roughly one magnet’s worth tells you one brake is not participating, without testing each one.
7-way and brake circuit test card
Print this and take it to the trailer. Three measurements isolate the fault to the controller, the wiring or the magnets — which is the step every "trailer brakes not working" article skips.
Identify every wire by function, never by color. Two color conventions are in use for this connector and they disagree on four of the seven pins. California OES prints the warning on its own diagram: “Color coding is not standard among all manufacturers.” Both conventions are below so you can see the conflict — but test the function.
1 · At the connector
| Pin | Function | RV color | Auto / SAE color | What you should measure | Reading |
|---|---|---|---|---|---|
| 1 | Ground | White | White | Continuity to trailer frame — near 0 Ω | |
| 2 | Electric brakes | Blue | Blue | 0 V rising to ~12 V as the controller is actuated | |
| 3 | Tail / running lights | Green | Brown | ~12 V with the headlights on | |
| 4 | Battery / 12V feed | Black | Black | ~12 V constant, engine running | |
| 5 | Left stop & turn | Red | Yellow | ~12 V on left indicator or brake pedal | |
| 6 | Right stop & turn | Brown | Green | ~12 V on right indicator or brake pedal | |
| 7 | Auxiliary / reverse | Yellow | Purple | Varies — often reverse lights |
Shaded cells are where the two conventions disagree. Note that blue is the brake wire and white is ground in both — those two are the only ones you can take on trust.
2 · At a magnet
Resistance across one magnet's two leads
Disconnected, meter on ohms. Dexter publishes 3.2 Ω for 10 in and 12 in brakes, 3.9 Ω for 7 in.
Measured: Ω
Open circuit means a broken magnet. Well below spec means a shorted one. Either way that magnet is the fault, and both magnets on an axle get replaced together.
Voltage at the magnet leads
Engine running, trailer connected. Dexter: it should “begin at 0 volts and, as the controller bar is slowly actuated, should gradually increase to about 12 volts”.
At rest: V
Override full: V
Nothing at all means the signal is not arriving — go back to pin 2. A low ceiling means voltage drop in the run or a bad ground.
3 · Total system current
Measured in series at the controller's blue output wire, all magnets energised. Compare against what your axle count should draw:
| Brakes | 7 in brakes | 10 in / 12 in brakes |
|---|---|---|
| 2 brakes (1 axle) | 5.0 A | 6.0 A |
| 4 brakes (2 axles) | 10.0 A | 12.0 A |
| 6 brakes (3 axles) | 15.0 A | 18.0 A |
| Yours, measured | ||
Low by roughly the draw of one magnet means one brake is not working. Zero with voltage present at pin 2 means an open circuit between the connector and the magnets. Dexter's published table stops at six brakes; an eight-brake trailer works out at 24 A on the same per-magnet figures.
What it points at
Fault isolated to:
Signal missing at pin 2 → controller or its feed. Signal present at pin 2 but not at the magnet → trailer wiring or ground. Signal at the magnet but wrong resistance → that magnet. Everything in spec but braking is weak → adjustment, or linings that have never been burnished.
Magnet resistance, system current and the voltage procedure from Dexter's light-duty axle service manual (600–8,000 lb capacity). Pin functions from California OES's “Typical Wiring Diagram for 7-Pole RV Connectors”, which is also the source of the warning that color coding is not standard. Verified 28 August 2026. RVTrove has not tested any of this equipment — these are the manufacturers' own published figures.
Symptom to cause
| What you see | Likely cause | What to do | Who |
|---|---|---|---|
| No braking at all, controller reports no trailer | Open circuit — most often the ground or a corroded connector pin | Clean both halves of the 7-way, then check continuity from the ground pin to bare trailer frame | Easy |
| Manual override works, brake pedal does not | The controller is not seeing the brake-light signal | Check the vehicle's own brake lights, then the controller's brake-signal input wire | Easy |
| Braking is weak on a trailer that measures correctly | Brakes underadjusted, or new linings never burnished | Adjust the drums; on new shoes, burnish by braking 20–30 times from 40 mph to 20 mph, cooling between | Moderate |
| Current draw low by roughly one magnet | One magnet open, or one brake disconnected | Measure each magnet in turn — 3.2 Ω on 10 and 12 in, 3.9 Ω on 7 in. Replace in pairs across the axle | Moderate |
| Brakes grab or lock in gentle stops | Gain set too high, or a time-delay controller set for hard stops | Reset gain by road test at 25 mph to just below lock-up. If it cannot cover both cases, that is the control type, not a fault | Easy |
| Braking fades after repeated stops | Grease or oil on magnets or linings, or glazed linings | Inspect and replace contaminated shoes and magnets; reburnish glazed linings | Technician |
| Braking is uneven side to side | One brake underadjusted, or scored and grooved drums | Adjust both, then compare. Scored drums need machining or replacement | Technician |
| Works, but only intermittently and worse in the wet | Corrosion raising resistance in the ground path | Clean and remake the ground at the connector and at the trailer frame | Moderate |
The color trap that causes half of DIY faults
Do not identify wires by color on this connector. Two conventions are in circulation for the same 7-way plug — the RV standard and the auto/SAE standard — and they disagree on four of the seven pins. Green is tail lights in one and right stop-and-turn in the other. Brown is right stop-and-turn in one and tail lights in the other.
California OES prints the warning on its own published wiring diagram: “Color coding is not standard among all manufacturers.”
Two wires are safe to trust: blue is electric brakes and white is ground in both conventions. Everything else gets identified with a meter, by function. The test card above prints both conventions side by side with the conflicts marked, precisely so the difference is visible rather than assumed.
When it really is the controller
Most of the time it is not. But these point at the unit itself:
- No output at the connector’s brake pin with a good feed and ground, on manual override. That is the controller failing to drive its output.
- Diagnostic codes on the display. Most modern controllers report shorts, open grounds and overloads directly — the Primus IQ, for instance, reports error codes PL, OG, SH, OL and nc.
- It never worked after installation, which usually means the harness rather than the controller — and on some vehicles a factory 7-way is not powered until tow/haul mode is enabled.
- A plug-in controller conflicting with an existing one. CURT states that “an existing aftermarket or factory brake controller will not function if Echo brake controller is installed”, so on those the two do not coexist.
If you do end up replacing it, size the replacement before you shop: do I need a brake controller, and which ones fit checks your axle count and brake current against every controller we hold a record for, and best trailer brake controllers writes up all five with the ratings that decide which are candidates. Which type to buy is a separate question — proportional vs time-delay covers it, and the two most cross-shopped units are compared in Tekonsha P3 vs REDARC Tow-Pro Elite.
Questions worth answering
Why is my trailer brake controller saying no connection?
Nine times in ten it is the ground or the connector, not the controller. A controller detects the trailer by looking for the brake magnets on its output — so a broken ground, a corroded pin or a failed magnet all present as "no trailer". Before anything else, unplug the 7-way, look into both halves for green or white corrosion, clean them, and check continuity from the connector's ground pin to bare metal on the trailer frame.
The manual override works but the brake pedal does nothing. What is wrong?
That is a useful result, because it splits the fault cleanly. The override proves the controller can drive the magnets and the trailer wiring is intact — so the missing part is the signal that tells the controller you are braking. That is the brake-light input to the controller, usually a red wire spliced into the vehicle's stop-lamp circuit. Check the vehicle's own brake lights work, then check that input.
How do I test a trailer brake magnet?
With a multimeter on ohms, across the magnet's two leads with them disconnected. Dexter publishes 3.2 Ω for its 10-inch and 12-inch brakes and 3.9 Ω for 7-inch. An open circuit means a broken magnet; a reading well below spec means a shorted one. Replace magnets in pairs across an axle so braking stays balanced.
How many amps should my trailer brakes draw?
Dexter publishes it per magnet — 2.5 A on 7-inch brakes, 3.0 A on every 10-inch and 12-inch size it lists — measured in series at the controller's blue output wire with all magnets energised. So four 10-inch brakes should draw about 12 A and six about 18 A. A reading low by roughly one magnet's worth points at one dead brake; zero with voltage present at the connector points at an open circuit between the two.
Can a bad ground stop trailer brakes working?
It is one of the most common causes, and it produces confusing symptoms rather than a clean failure — weak braking, brakes that work intermittently, or a controller that reports no trailer. The brake circuit's return path is the white ground wire and the trailer frame, so corrosion anywhere in it raises resistance and drops the voltage reaching the magnets. Check continuity from the connector's ground pin to bare frame metal.
My trailer brakes are weak but everything tests fine. Why?
Two causes worth ruling out before you suspect the electrics. First, adjustment — drum brakes need periodic manual adjustment, and an underadjusted brake feels weak while measuring perfectly. Second, burnishing: new shoes and linings have to be bedded in, and Dexter specifies doing it by applying the brakes 20 to 30 times from 40 mph down to 20 mph, allowing them to cool between applications. New brakes that feel disappointing are often just unburnished.
Is it the controller or the trailer?
Test at the connector before you buy anything. With the trailer unplugged, engine running, put a meter on the brake pin of the vehicle's 7-way and actuate the controller's manual override. If voltage rises smoothly from 0 to around 12 V, the controller and its wiring are working and the fault is on the trailer. If nothing happens, it is the controller, its power feed or its ground.
Which wire is the brake wire on a 7-way connector?
Blue, and it is one of only two you can trust by color. Two color conventions are in circulation — RV and auto/SAE — and they disagree on four of the seven pins, which is why California OES prints "color coding is not standard among all manufacturers" on its own wiring diagram. Blue for electric brakes and white for ground are the same in both. Identify everything else by function with a meter.