Brake Circuit Amperage Testing
- Measure brake circuit current draw using an ammeter in series with the blue brake wire.
- Calculate expected total current based on the number of magnets and per-magnet values.
- Diagnose shorted, open, or poorly grounded magnets through amperage and resistance testing.
- Ammeter — Measure current draw in series with the blue brake wire while the controller is set to full output.
- Digital Multimeter — Measure individual magnet resistance to compare against the axle manufacturer's specifications.
- Brake Controller — Provide full output signal to the brake circuit for current draw testing.
Measure brake current with an ammeter in series with the blue brake wire, controller set to full output.
Total current should roughly equal the per-magnet value times the number of magnets. High current suggests a shorted magnet; low or zero suggests an open magnet, bad ground or poor connection.
Each magnet can be checked individually by resistance as well as current. Compare against the axle maker's chart.
How It Works
Series Current Measurement
To accurately diagnose the health of the brake system, the ammeter must be placed in series with the blue brake wire. This allows the technician to measure the total current flowing to all magnets when the controller is at full output.
Proportional Current Logic
The total current measured in the circuit should roughly equal the per-magnet value multiplied by the total number of magnets on the trailer. Deviations from this calculated total indicate specific electrical faults within the magnets or wiring.
Magnet Fault Indicators
Electrical faults manifest as specific current readings: high current suggests a shorted magnet, while low or zero current indicates an open magnet, a bad ground connection, or a poor wiring connection.
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Total circuit current is significantly higher than the calculated total. | Measure current draw at full output and compare to the sum of per-magnet values. | Roughly equal to the per-magnet value times the number of magnets. | A shorted magnet is drawing excessive current. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Total circuit current is low or zero. | Inspect ground connections and verify circuit continuity to the magnets. | Per the applicable manual | An open magnet, bad ground, or poor connection is restricting current flow. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Individual magnet performance is suspect. | Perform a resistance check on the individual magnet and compare to the axle maker's chart. | Compare against the axle maker's chart. | Internal coil degradation or wiring failure within the specific magnet assembly. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
At the Bay: Diagnose the Case
CASE 1 / 3
Total circuit current is significantly higher than the calculated total.
- Test
- Compare
- Correct
Select the next check
Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014). A likely cause is a hypothesis; confirm it with the applicable service procedure before repair.
Common Bay Mistakes
- ✕ Failing to set the brake controller to full output during the ammeter test.
- ✕ Connecting the ammeter in parallel instead of in series with the blue wire.
- ✕ Neglecting to multiply the per-magnet current value by the number of magnets.
- ✕ Ignoring ground connections when diagnosing low current symptoms.
- ✕ Substituting generic resistance values instead of using the axle maker's chart.
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: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) — 10