Controller Symptoms
- Differentiate between electrical causes and mechanical component failures in brake locking scenarios.
- Identify the specific controller-related symptoms associated with harsh and surging brake performance.
- Correlate improper synchronization and under-adjustment with trailer braking malfunctions according to Dexter guidelines.
- Brake system resistor — Adjust wheel load impact during locking brake scenarios to mitigate insufficient load issues.
- Drum micrometer — Measure drums to identify out-of-round or cracked conditions contributing to surging or locking.
- Multimeter — Test for open or short circuits when no braking action is detected.
- Brake adjustment tool — Address severe under-adjustment conditions that lead to no brakes or harsh braking.
No brakes: open circuits, severe under-adjustment, faulty controller, or short circuits.
Locking brakes: under-adjustment, improper synchronization, faulty controller, broken components, out-of-round drums, or insufficient wheel load (adjust system resistor).
Harsh brakes: under-adjustment, improper synchronization, improper controller, faulty controller.
Surging brakes: grease/oil on linings, out-of-round or cracked drums, faulty controller.
How It Works
Controller Functionality and Synchronization
The brake controller is a central component in brake performance; a faulty controller or improper synchronization between the tow vehicle and trailer can result in locking, harsh, or surging brake behaviors. Proper timing and voltage output are required for smooth deceleration.
Mechanical Integrity of Drums and Linings
Brake performance is not solely dependent on electrical signals. Physical defects such as out-of-round or cracked drums, or the presence of contaminants like grease and oil on the linings, will manifest as surging or locking regardless of controller input.
Circuit Continuity and Load Balance
A total loss of braking is often tied to electrical interruptions, such as open or short circuits. Conversely, locking brakes may be caused by insufficient wheel load, which requires the adjustment of the system resistor to balance the braking force.
Diagnostic Chart
| Symptom | Check | Spec / Expected | Likely cause & fix |
|---|---|---|---|
| No brakes | Inspect for open circuits, short circuits, and verify controller functionality and adjustment levels. | Per the applicable manual | Open circuits, severe under-adjustment, faulty controller, or short circuits. Basis: Dexter 600-8K Axle Service Manual |
| Locking brakes | Check synchronization, drum roundness, wheel load, and inspect for broken components. | Adjust system resistor if wheel load is insufficient. | Under-adjustment, improper synchronization, faulty controller, broken components, out-of-round drums, or insufficient wheel load. Basis: Dexter 600-8K Axle Service Manual |
| Harsh brakes | Evaluate the controller type, synchronization settings, and mechanical adjustment. | Per the applicable manual | Under-adjustment, improper synchronization, improper controller, or faulty controller. Basis: Dexter 600-8K Axle Service Manual |
| Surging brakes | Inspect brake linings for contamination and drums for cracks or roundness. | Per the applicable manual | Grease/oil on linings, out-of-round or cracked drums, or faulty controller. Basis: Dexter 600-8K Axle Service Manual |
At the Bay: Diagnose the Case
CASE 1 / 4
No brakes
- 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
- ✕ Assuming all braking issues are electrical without checking for grease or oil on linings.
- ✕ Replacing a controller for 'harsh brakes' before verifying synchronization settings.
- ✕ Overlooking out-of-round drums as a cause for locking brakes.
- ✕ Failing to adjust the system resistor when locking occurs under light wheel loads.
- ✕ Mistaking a severe under-adjustment for a complete circuit failure.
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: 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 — Troubleshooting chart