Use the correct controller test load, then compare the connector and magnet voltages before isolating a magnet for resistance testing. These checks help locate an electrical problem. They do not prove the drum mechanism is working.
This guide concerns electric drum brakes. Surge and electric-over-hydraulic systems need their own procedures; use the brake-system guide if the type is unclear.
Start with the fork that costs nothing
Read the controller’s fault codes and exact test procedure. Secure the rig, inspect the unplugged connector for damage or corrosion with power off, and confirm supply, ground, settings and brake-type compatibility.
Manual override and the pedal need not produce the same output while parked. A proportional controller responds to deceleration, and stationary hold behavior varies. The P3 manual, for example, describes a timed stationary hold. Do not diagnose a missing brake-signal input from a parked pedal test alone.
If override works but the approved pedal test fails, investigate the brake-signal input and configuration. Working brake lights are a useful clue, but do not prove the controller receives the correct signal. Do not splice into an unfamiliar vehicle circuit by colour or assumption.
The three measurements
Conceptual electric drum-brake circuit, with one magnet shown. This is not a connector face or pinout. Identify output and return from the exact vehicle, controller and trailer diagrams.
1 + 2 · Loaded voltage checks
Use the controller maker’s test procedure and required load. Keep the magnet connected for the magnet-voltage check.
Compare readings at the same setting and load. A difference helps locate loss in the wiring or return path. A meter on an empty socket alone may not trigger trailer detection; an unloaded reading does not prove the circuit can carry current.
3 · Isolated magnet resistance
Turn power off. Isolate every source, including the trailer’s breakaway battery, and verify zero voltage before switching to ohms.
Compare with the exact magnet specification. Connected magnets can form parallel paths and give a misleading reading. Restore and seal the wiring and reconnect the breakaway system before service.
Secure the stationary rig before testing. Voltage probes go across a circuit; a current meter does not. Total brake current can exceed a handheld meter’s rating. Use suitable equipment and a qualified technician where the procedure requires it. These checks do not prove the mechanical brakes work.
References: Dexter service manual, pp. 20–22, REDARC’s load-test requirements and Fluke’s resistance procedure. No regional pin numbers or wire colours are transferred into this sketch.
1. At the vehicle connector: check output against return with the required load. Use the controller manufacturer’s test setup. A multimeter alone may draw too little current for trailer detection. REDARC’s Elite V2/V3 test guide specifies a suitable incandescent load; its regional connector pin numbers must not be copied to a different connector. A plausible unloaded voltage does not prove the wiring can carry brake current.
2. At a connected magnet: measure across its two leads. Compare with the connector reading under the same command and load. Lower voltage can indicate loss in either the output or return path. Use suitable test access and restore weather sealing afterwards. Dexter’s voltage discussion uses an engine-running test; follow the controller and vehicle instructions for the actual setup.
3. Across an isolated magnet: switch to resistance only after removing power. Isolate vehicle and trailer sources, including the breakaway battery. Verify zero voltage, then disconnect both magnet leads from the trailer wiring before using ohms. Fluke’s resistance procedure requires power off; other connected paths can alter the result.
| Dexter brake size | Nominal magnet resistance in the cited table |
|---|---|
| 7 in × 1¼ in | 3.9 Ω |
| Listed 10 in, 12 in and 12¼ in sizes | 3.2 Ω |
These figures come from the Dexter service manual, pp. 20–22. They are not specifications for every magnet. Check the exact part, lead resistance and test conditions. An open or unexpectedly low reading needs investigation; a matching reading does not establish mechanical serviceability.
Optional current check: use equipment rated for the entire load. Four of Dexter’s listed 3 A magnets total about 12 A. A common handheld meter’s 10 A current input is insufficient. Current measurement requires the specified series connection or suitable DC clamp equipment around one conductor; it is not a voltage measurement across the circuit. Hand this test to a technician if the method or equipment rating is uncertain.
Reconnect and seal everything, restore breakaway protection and follow the manufacturer’s functional check before returning to service. If a wheel is removed, follow the matching wheel-fastener procedure.
7-way and brake circuit test card
Electric drum brakes. Record the exact controller, brake model, test load and command before comparing readings.
Date: __________ Controller/model: ____________________ Brake/magnet: ____________________
Test load and output setting: ____________________ Supply voltage: __________ V
Identify terminals by function, never colour alone. Use the correct connector side and viewing direction from its own diagram. This card contains no universal pin numbers. Secure the rig. Follow the controller's approved load test; a meter or LED tester alone may not simulate a trailer.
1 · Voltage at the vehicle connector
Measure brake output against return under the manufacturer's required load and command, using the approved test method.
Reading: V
An empty-socket reading alone cannot clear or condemn the controller. Check supply, ground, configuration and the required test load.
2 · Voltage at connected magnet leads
Measure across the magnet's two leads while it remains connected. Use the same output command and load conditions as test 1.
Reading: V
A lower reading can indicate loss in output wiring, connectors or the return path. Power-off continuity alone does not establish load-carrying ability.
3 · Resistance of one isolated magnet
Power off; isolate all sources, including the breakaway battery. Verify zero voltage. Disconnect both magnet leads from the trailer circuit, then measure across the magnet in ohms.
Expected for this part: Ω
Measured: Ω
Dexter's cited table lists nominal 3.9 Ω for 7 × 1¼ in and 3.2 Ω for its listed 10, 12 and 12¼ in brakes. Verify the exact part and test conditions. Other connected magnets give parallel paths and a misleading reading.
Before returning to service
Reconnect and weather-seal all leads, restore the breakaway system, and perform the manufacturer's functional check.
Expected electrical readings do not verify brake adjustment, lining condition or mechanical operation. Have unexplained weak, uneven or early-locking brakes assessed.
Optional · Total current, using suitably rated equipment
Dexter's nominal chart values below depend on the prescribed test conditions and applied voltage. Four larger magnets draw about 12 A, beyond a common 10 A multimeter input. Do not place a meter in current mode across output and return. A technician can use the specified series method or suitable DC clamp equipment around one conductor.
| Installed brakes | 7 × 1¼ in | Listed larger sizes |
|---|---|---|
| 2 brakes | 5.0 A | 6.0 A |
| 4 brakes | 10.0 A | 12.0 A |
| 6 brakes | 15.0 A | 18.0 A |
Actual current: A · Applied voltage: V
Findings and next action
Compare like-for-like loaded readings. Confirm suspected faults against the exact manuals; do not replace a controller solely because an unloaded meter test failed.
Guidance checked 6 November 2026: Dexter light-duty service manual, pp. 20–22; REDARC load-test requirements; Fluke resistance procedure. REDARC's linked regional pin numbers are not reproduced. RVTrove has not tested this equipment.
Symptom to cause
| What you see | Likely cause | What to do | Who |
|---|---|---|---|
| Controller reports no trailer | Open brake/return circuit, connector damage, incompatible configuration or unsuitable test load | Inspect with power off, then use the maker’s approved load test | Moderate |
| Override works; parked pedal test seems inactive | Possibly normal proportional behavior; otherwise brake-signal input or setup | Check the model’s stationary behavior and approved test before tracing the input | Moderate |
| Magnet voltage lower than connector voltage under the same load | Loss in output wiring, connector or return path | Locate the loaded voltage drop; repair and seal affected connections | Technician |
| One isolated magnet differs substantially from its specification | Magnet, leads or test-contact problem | Confirm the part and measurement; follow axle-maker replacement instructions | Technician |
| Braking weak despite expected electrical readings | Adjustment, lining/drum condition, contamination or mechanical fault | Inspect the brake mechanism before an approved functional or burnishing test | Technician |
| Brakes grab or lock early | Gain, boost, setup, loading or mechanical condition | Use the controller-specific setup procedure and inspect unexplained early lockup | Technician |
| Intermittent fault, especially in wet conditions | Damaged or corroded wiring, connectors or return connections | Inspect both output and return paths; repair and reseal faults | Moderate |
The color trap
Identify every wire by function, including brake output and return. Common blue/white conventions are not proof of how a particular harness was built or repaired. A connector drawing must match the vehicle or trailer side and the viewing direction. This page deliberately shows a circuit path without a connector-face drawing.
When it really is the controller
A controller becomes a stronger suspect only after its supply and return, brake signal, configuration, connector wiring and approved load test have been checked. Fault codes may describe an external short or overload rather than a failed controller. Vehicle-specific tow-package activation and harness requirements come from the vehicle manual.
Use the controller fitment checker before replacing a unit, and the model-specific brake-gain worksheet for setup. The controller roundup lists the relevant axle and brake-type ratings.
Questions worth answering
Why is my trailer brake controller saying no connection?
A damaged connection, return path or brake circuit can prevent trailer detection, as can the wrong test load. Inspect the connector with power off, then use the controller maker's detection and load-test procedure. A multimeter on an empty socket may not simulate a trailer.
The manual override works but the brake pedal does nothing. What is wrong?
A stationary pedal check alone does not establish a fault on a proportional controller: output depends on motion, settings and model-specific hold behavior. If the symptom persists during the manufacturer's approved test, check the brake-signal input and setup. Manual override operating some brakes does not prove every magnet or connection is sound.
How do I test a trailer brake magnet?
For resistance, isolate all power sources, including breakaway power, verify zero voltage, and disconnect both magnet leads from the trailer circuit before measuring across the magnet. Compare with the exact magnet specification. Dexter lists nominal 3.9 ohms for its 7-inch by 1¼-inch brake and 3.2 ohms for the 10-inch, 12-inch and 12¼-inch sizes in its table. A resistance check does not prove braking performance.
How many amps should my trailer brakes draw?
Dexter's cited chart lists 2.5 A per 7-inch magnet and 3.0 A per magnet for its listed larger sizes under the specified test conditions. Four larger magnets total about 12 A, beyond a common 10 A multimeter range. Current varies with applied voltage. Use suitably rated equipment and the manufacturer's procedure; never connect a current-input meter across the output and return.
Can a bad ground stop trailer brakes working?
Yes. Check the actual return path specified by the trailer manufacturer, including connectors, wires and any frame bonds. Power-off continuity can reveal an open path, but a low resistance reading alone does not prove that a joint carries brake current. A loaded voltage-drop test can reveal resistance that a continuity check misses.
My trailer brakes are weak but everything tests fine. Why?
Electrical readings do not verify drum adjustment, lining condition, contamination or mechanical operation. Identify whether the brakes are manually or self-adjusting and follow the exact brake manual. New linings may need the manufacturer's burnishing procedure after correct installation; unexplained weak braking needs inspection before road testing.
Is it the controller or the trailer?
Use the controller manufacturer's approved load test first, then compare output/return voltage at the connector and at connected magnet leads under the same command. An unloaded socket reading cannot by itself clear or condemn the controller. Check its supply, ground, brake signal, configuration and fault codes as well.
Which wire is the brake wire on a 7-way connector?
Identify the brake-output terminal by function using the correct connector diagram and viewing direction. Blue is a common brake-wire convention, but no colour is proof after repairs or across different harnesses. Confirm the return terminal too; this article's circuit sketch is not a connector pinout.