Measure Voltage at the Brake Magnets
- Connect a voltmeter across the brake magnet leads.
- Evaluate controller output under the specified test conditions.
- Distinguish smooth modulation from excessive threshold output.
- Digital multimeter — Select DC voltage to measure across the magnet leads; do not use the current input connection.
- Insulated test leads with clips — Maintain secure contact with the magnet leads without bridging exposed conductors.
- Back-probe adapters — Access compatible connector terminals while keeping the magnet circuit connected and avoiding terminal damage.
- Wheel chocks — Secure the stationary vehicle and trailer before the engine-running check.
- Work light — Identify magnet leads and inspect connections before attaching the meter.
Measure brake-system voltage at the magnet by connecting a voltmeter to its lead wires. This is a different connection method from inserting an ammeter into a current path.
The towing vehicle engine should be running during the voltage check so a low battery does not distort the reading. The source describes checking the system with the trailer connected.
Dexter describes a controller-bar check in which voltage begins at 0 V and rises gradually to about 12 V as the bar is slowly actuated. These are source-specific reference values; values vary by maker and the applicable controller instructions govern evaluation.
Threshold voltage is the voltage first applied when the controller turns on. A lower threshold supports smoother braking, while an excessive threshold can produce grabbing and harshness; consult the controller manual if the output does not provide the described control.
How It Works
Measure across the magnet
A voltmeter connects across the magnet lead wires to measure the electrical potential available at the load. Unlike an ammeter inserted into a current path, it does not require opening the circuit. Voltage alone does not establish that the magnet draws the correct current or that the brake operates mechanically.
Establish valid test conditions
Keep the trailer connected and the tow vehicle engine running so a low battery does not distort the voltage check. Secure the vehicle and trailer, provide safe exhaust ventilation, and keep leads clear of moving parts. Review the applicable controller instructions before interpreting the reading.
Interpret modulation and threshold
Dexter describes output beginning at 0 V and rising gradually to about 12 V as the controller bar is slowly actuated; these reference values are source-specific, and the applicable controller instructions govern evaluation. Threshold voltage is the voltage first applied when the controller turns on, and a lower threshold supports smoother braking. An excessive threshold or immediate jump to high output can produce harsh braking and potential wheel lockup.
Step-by-Step Procedure
0/6 doneDexter controller-bar voltage check; values vary by maker
| Before actuation | 0 V |
| As the controller bar is slowly actuated | Gradual rise to about 12 V |
Interactive Demonstration
Planning practice only. Follow the applicable manufacturer's instructions, employer safety program and qualified supervision on real equipment.
Plan the first action. Compare the service steps with the bay mistakes.
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Voltage is present before the controller bar is actuated. | Confirm that the meter is across the magnet leads, then compare the resting output with the applicable controller testing instructions. | Dexter reference before actuation: 0 V; values vary by maker. | Output may not be returning to the expected inactive condition; controller-specific behavior must be checked before declaring a fault. Basis: LDServiceOnline |
| Voltage immediately jumps high and braking feels harsh. | Slowly actuate the controller bar and observe the initial output and subsequent modulation; consult the controller manual if control is incorrect. | Dexter reference: Gradual rise to about 12 V; an immediate jump to high output is not normal modulation. | An excessive threshold applies braking abruptly and can promote grabbing or wheel lockup. Basis: LDServiceOnline |
| Voltage appears low during a check performed with the engine stopped. | Repeat the check with the trailer connected and the tow vehicle engine running, then compare with the applicable controller instructions. | The trailer remains connected and the tow vehicle engine runs during the voltage check. | A low battery can distort the reading and lead to an incorrect assessment of controller output. Basis: LDServiceOnline |
| Voltage at the magnet is intermittent. | Verify secure meter contact, then inspect accessible wiring and connectors for looseness, corrosion, or damaged insulation. | Use the applicable controller instructions for acceptance criteria; no universal intermittency limit is established here. | Poor probe contact can create a false symptom, while loose or corroded connections can interrupt delivery to the magnet. Basis: General shop practice |
At the Bay: Diagnose the Case
CASE 1 / 4
Voltage is present before the controller bar is actuated.
- Test
- Compare
- Correct
Select the next check
Basis: LDServiceOnline. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Inserting the meter into the current path instead of measuring across the magnet leads.
- ✕ Evaluating output with the trailer disconnected or the engine stopped.
- ✕ Treating Dexter reference values as universal controller specifications.
- ✕ Accepting an immediate jump to high output as normal modulation.
Quick Check
Practice only — not scored.
Plan the Service Sequence
Planning practice only. Follow the applicable manufacturer's instructions, employer safety program and qualified supervision on real equipment.
Plan the first action. Compare the service steps with the bay mistakes.
Hands-on: Test Bench Practice
Symptom: Healthy trailer. Learn what good readings look like.
Tap a point to place the red probe.
Good system: ~12.6 V at pins, 3.2 Ω across a 12×2-inch magnet (unplugged), 3.0 A per magnet at 12 V full gain, and at least 10.5 V at the magnet — the chart values from the axle manuals.
Hands-on: Shop Test Equipment
IPA #9107B Electric Brake Force Meter
Plugs into the truck's 7-way socket on a 25 ft cable; simulates a trailer load and shows controller output on 10 blue LEDs
🛠 Equip your bay for this test →Symptom: Customer wants the truck checked before buying a trailer.
Spec limits used
- 1–10, each 1.0–1.5 V — Gain LEDs (IPA #9107B manual, p.5)
- 15 s — Max time holding brake pedal (IPA #9107B manual, p.4)
- set-up step 5 — Connect the 9-pin only after the engine is running (IPA #9107B manual, p.3)
- hold brake pedal during ECU detection — 2016–2017 'Trailer Connected' not shown (IPA #9107B manual, p.3)
- faulty brake control module or relay — ECU light never lights on a vehicle with a controller (IPA #9107B manual, p.5)
Simulated readings for practice. Always follow the manufacturer's procedure and values on a real trailer.
Hands-on: Wire-Trace Overlay
Pin 2 · Blue
Electric brakes
- Left and right brake magnets
- Breakaway switch output
Check points
- Two brakes draw about 6.0 A (3.0 A per magnet) at 12 V full gain.
- Unplugged: 3.2 Ω per magnet; at least 10.5 V at each magnet under load.
Common failures
- Open magnet: about half the expected current and a pull to one side.
- Shorted magnet: very low resistance and controller overload.
- Wire chafed where it drops to the axle.
Colors shown are the most common RV harness colors; makers vary — always verify against the trailer's wiring diagram.
Source: LDServiceOnline — 8, 21