Measuring Voltage & Amperage
- Measure voltage at the magnet leads while the controller is applied.
- Perform an amperage test by placing the meter in series within the blue brake wire circuit.
- Evaluate circuit integrity based on amperage consumption per magnet.
- Digital Multimeter (Ammeter Mode) — Measure amperage in series within the blue brake wire to detect shorts or open circuits.
- Digital Multimeter (Voltmeter Mode) — Measure voltage at the magnet leads with the controller applied to diagnose low or no voltage complaints.
- Wire Strippers/Crimpers — Facilitate circuit isolation by disconnecting sections to locate shorts.
Voltage: measure at the magnet leads with the controller applied. Low or no voltage is the most common brake complaint.
Amperage: put the ammeter in series in the blue brake wire. Normal is about 3.0 A per magnet (6.0 A for 2 brakes, 12.0 A for 4).
High amperage means a short (shorted magnet or bare wire to ground). Low amperage means a poor connection, open circuit or undersized wire.
Isolate a short by disconnecting one section at a time.
How It Works
Voltage at the Source of Action
Voltage must be measured directly at the magnet leads while the controller is actively applied. Low or no voltage is identified as the most common cause of brake complaints in electric systems.
Amperage as a Diagnostic Indicator
Amperage measurements require placing the meter in series with the blue brake wire. Normal operation consumes approximately 3.0 A per magnet. Total expected amperage scales with the number of brakes: 6.0 A for 2 brakes and 12.0 A for 4 brakes.
Interpreting Deviation from Normal Amperage
High amperage readings indicate a short circuit, typically caused by a shorted magnet or a bare wire touching ground. Conversely, low amperage indicates high resistance from poor connections, open circuits, or undersized wiring.
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Brake performance is poor or non-existent. | Measure voltage at the magnet leads with the controller applied. | Per the applicable manual | Low or no voltage at the magnet leads. Basis: Dexter 600-8K Axle Service Manual |
| High amperage reading on the blue brake wire. | Isolate the short by disconnecting one section of the circuit at a time. | Above 3.0 A per magnet (e.g., >6.0 A for 2 brakes or >12.0 A for 4 brakes) | A shorted magnet or a bare wire touching ground. Basis: Dexter 600-8K Axle Service Manual |
| Low amperage reading on the blue brake wire. | Inspect wiring size and connection points for integrity. | Below 3.0 A per magnet (e.g., <6.0 A for 2 brakes or <12.0 A for 4 brakes) | Poor connection, open circuit, or undersized wire. Basis: Dexter 600-8K Axle Service Manual |
| Total amperage for a 4-brake system is significantly less than 12.0 A. | Measure amperage in series in the blue brake wire. | 12.0 A for 4 brakes | A poor connection or open circuit is preventing full current flow to all magnets. Basis: Dexter 600-8K Axle Service Manual |
At the Bay: Diagnose the Case
CASE 1 / 4
Brake performance is poor or non-existent.
- Test
- Compare
- Correct
Select the next check
Basis: Dexter 600-8K Axle Service Manual. A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Measuring voltage without the controller applied.
- ✕ Measuring amperage in parallel instead of in series with the blue wire.
- ✕ Failing to account for the number of brakes when calculating expected amperage.
- ✕ Overlooking undersized wiring as a cause for low amperage.
- ✕ Attempting to find a short without disconnecting sections to isolate the fault.
Quick Check
Practice only — not scored.
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: Dexter 600-8K Axle Service Manual — Electric brakes — How to Measure Voltage/Amperage