Magnets, Shoes & Amperage
- Evaluate brake shoe lining condition and contamination limits for replacement.
- Determine magnet replacement criteria based on coil exposure and axle pairing.
- Perform a system-wide amperage test to identify short circuits and verify normal current flow.
- Digital Multimeter (DMM) — Measure amperage and voltage across the brake system to detect shorts or low voltage.
- Ammeter — Specifically used to measure the current draw in Amps (A) across individual magnets or the entire system.
- Depth Gauge or Scale — Measure brake lining thickness to ensure it is not worn to 1/16" or less.
- Flashlight — Inspect the magnet coil for exposure and check shoes for scoring, pitting, or gouging.
Replace linings worn to 1/16" or less, or contaminated with grease or oil, or scored/pitted/gouged. Heat cracks are normal.
Replace a magnet if its coil is exposed. Replace magnets in pairs across an axle.
Low or no voltage is the most common brake problem. High amperage indicates a short circuit.
How It Works
Shoe Replacement Criteria
Linings must be replaced if worn to 1/16" or less, or if they show signs of contamination from grease or oil. Physical damage such as scoring, pitting, or gouging also requires replacement, though heat cracks are considered normal.
Magnet Service Life
A magnet is considered failed and must be replaced if the internal coil becomes exposed. To ensure balanced braking performance, magnets must always be replaced in pairs across an axle.
Electrical Symptom Analysis
Brake problems typically manifest in two ways: low or no voltage, which is the most common issue, or high amperage, which serves as a primary indicator of a short circuit in the system.
Normal amperage
| Per magnet | 3.0 A |
| Two brakes | 6.0 A |
| Four brakes | 12.0 A |
| Six brakes | 18.0 A |
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| Low or no braking force. | Perform a voltage test at the brakes and check for magnet coil exposure. | Replace if coil is exposed. | Low voltage is the most common brake problem; exposed coils indicate magnet failure. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| High amperage reading during system test. | Measure total system amperage using an ammeter. | 3.0 A per magnet. | A short circuit within the wiring or magnets is causing excessive current draw. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Brake linings appear cracked but thickness is sufficient. | Visual inspection of the lining surface for heat cracks versus deep gouges. | Replace if 1/16" or less. | Heat cracks are normal and do not necessarily require replacement unless other wear limits are met. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Brake pull or uneven braking across an axle. | Inspect magnets for wear and verify if they were replaced individually. | Replace magnets in pairs across an axle. | Uneven magnet strength or wear due to failing to replace magnets in pairs. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
| Total system amperage for a dual-axle trailer is significantly above 12.0 A. | Compare total amperage against the four-brake specification. | 12.0 A (Four brakes). | High amperage indicates a short circuit in the four-brake configuration. Basis: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) |
At the Bay: Diagnose the Case
CASE 1 / 5
Low or no braking force.
- 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
- ✕ Replacing shoes due to normal heat cracks.
- ✕ Replacing only one magnet on an axle instead of in pairs.
- ✕ Ignoring grease or oil contamination on linings.
- ✕ Assuming high amperage is a voltage supply issue rather than a short circuit.
- ✕ Failing to replace linings worn to 1/16".
- ✕ Neglecting to check for exposed coils on magnets.
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.
Source: Lippert Trailer Axle Owner's Manual (Rev 03.18.2014) — 11, 20–21