Brush Motor Overcurrent Faults: Why the Scrub Deck Keeps Tripping and How to Find the Cause

Key takeaways

• A brush overcurrent fault means the brush circuit drew more current than the controller allows, not that the motor is bad.
• Too much down pressure for the floor, pad or brush is the most common trigger.
• Measure brush amp draw with a DC clamp meter and compare it to the rated value in the manual.
• A brush motor that draws high current with the deck raised has a mechanical or electrical problem of its own.
• Repeated resets cook motors and connectors, so diagnose after the second trip.

A brush motor overcurrent fault means the controller measured more current in the brush motor circuit than its limit allows, for longer than its allowed time, and switched the brush off to protect the motor and wiring. The usual cause is too much load on the brush: excessive down pressure, an aggressive brush on a high-friction floor, debris wrapped around the hub, or a worn motor.

The current a brush motor draws rises with the work it is doing. A pad gliding over wet sealed concrete takes much less current than a grit brush grinding on dry, textured tile under heavy pressure. The controller cannot tell those situations apart; it only sees the number. That is why this fault so often appears with a new pad type, a new floor, or a new operator who turns pressure to maximum.

What the controller is detecting

Brush motor current is measured either by a current sensor in the controller output or by a separate brush relay or module. Controllers typically have two limits:

• Overload (time-based): current above the rated value for a sustained period, often seconds to tens of seconds. This protects against heat.
• Overcurrent or short (instant): current far above normal, such as a locked rotor or a short to ground. This trips almost immediately.

Many machines also respond to sustained overload by automatically reducing down pressure before tripping, so operators may notice the deck lifting slightly or the pressure indicator dropping first. On machines with a brush motor thermal switch, an over-temperature fault can appear separately.

Typical full-load brush motor current varies widely by machine size and voltage. On a 24 V walk-behind, brush current in the range of roughly 15 to 40 A is common; larger ride-ons with two or three brush motors draw more in total. Your service manual or motor data plate gives the actual rated value; that number is what you compare against.

Typical display wording

Look for messages referring to brush overload, brush current, brush motor overcurrent, scrub motor fault, deck overload or brush thermal. LED machines may flash the brush indicator. Some machines report open circuit in the same family (brush motor not connected or brush open). Look up the exact code in your manual by serial number range.

Common triggers, ranked by likelihood

1. Down pressure too high for the floor, pad and soil level. See down pressure.
2. Brush or pad too aggressive for the surface, especially a grit brush or stripping pad on rubber, textured tile or dry floors. Use the pad selector.
3. Running dry. No solution flow raises friction sharply. Check the no solution flow guide.
4. Debris wrapped around the hub or drive (string, shrink wrap, banding straps, hair).
5. Wrong or damaged pad driver, or a pad driver with bristles worn so the plate rubs.
6. Worn motor carbon brushes or commutator, which raises current and causes arcing.
7. Seized or dragging bearings in the motor or a gearbox, or (on cylindrical decks) a binding idler or belt.
8. Deck actuator not lifting fully, so the brush drags during transport.
9. Low battery voltage, which forces the motor to draw more current for the same work and can push it over the limit.
10. Chafed wiring or a short to ground, usually producing an instant trip rather than a delayed one.

Step-by-step diagnosis

Safety: key off and disconnect the battery before removing brushes or touching the deck. For live current tests, keep hands and loose clothing clear of the brush.

1. Note when it trips. Instantly at brush start points to a short or locked rotor. After several minutes points to overload or heat. Only on one floor area points to friction.
2. Inspect the brush and deck. Remove the brush or pad driver. Cut away wrapped debris. Spin the motor hub by hand (battery disconnected). It should turn smoothly with no grinding or tight spots. A gearbox will have more resistance, but it should be even.
3. Reduce load and retest. Set the lowest down pressure, fit the brush or pad you normally use, ensure solution is flowing, and scrub a smooth test area. If the fault disappears, load is the cause.
4. Measure amp draw. Clamp a DC clamp meter around one brush motor lead and take three readings: brush spinning freely with the deck raised (if your machine allows it), low pressure on a wet floor, and your normal pressure. Compare them to the rated current. A free-spinning motor should draw a small fraction of its full-load current. High no-load current means the motor or gearbox is the problem, not the floor.
5. Check voltage at the motor under load. Measure across the motor terminals while scrubbing. If it is much lower than battery voltage (more than about 1 V on a 24 V system), there is voltage drop in the wiring, relay or connectors.
6. Check continuity and insulation. With the battery disconnected and the motor unplugged, measure resistance across the motor leads: a very low reading is normal for a DC motor, but an open reading (OL) means an open armature or worn-out carbon brushes. Then measure from each lead to the motor frame: it should read open. Any continuity to frame is a short to ground.
7. Inspect carbon brushes on brushed DC motors. Replace if worn to the service limit in the manual, and look for scoring or burning on the commutator.
8. Check the deck lift. Raise the deck and confirm the brush clears the floor. A weak actuator or worn linkage can leave it dragging.

Fixes

When replacing brushes and pads, see when to replace brushes and pads. If you smell hot insulation, read burning smell before running again.

Reset behavior

Most brush overcurrent faults latch until the brush switch is cycled or the key is turned off and on. Some controllers enforce a cooldown before allowing restart. If the fault returns within a few minutes, do not keep resetting it: each reset runs the motor hot again, and the damage (burned commutator, softened insulation, melted connector housings) accumulates. Thermal faults clear only once the motor cools, which can take 15 to 30 minutes.

Fault record worksheet

• Exact code or message and whether it trips instantly or after running
• Hour meter reading and minutes into the run when it tripped
• Floor type and condition (wet, dry, textured, sealed)
• Brush or pad type and color, pad driver type, age of brush
• Down pressure setting
• Solution flow setting and whether solution was reaching the floor
• Amp draw: free spinning, low pressure, normal pressure
• Voltage at the motor terminals under load and battery voltage at the same moment
• Motor lead resistance and lead-to-frame reading
• Carbon brush length (if checked)

If the brush simply does not turn and there is no code, start with brushes not turning.

Frequently asked questions