Testing a brushed DC motor takes just a few basic tools: a multimeter, a power source, and your eyes. Whether the motor came out of a power tool, treadmill, or hobby project, you can diagnose most problems in under ten minutes by checking continuity, inspecting the brushes, and running the motor under controlled power.
Check Resistance at the Terminals
The fastest first test is measuring resistance across the motor’s two terminals with a multimeter set to ohms. This tells you whether the windings inside the motor are intact. Touch one probe to each terminal and read the value. A healthy motor will show some measurable resistance, typically somewhere between a fraction of an ohm and several hundred ohms depending on the motor’s voltage rating and size. A small 24V coreless motor, for example, might read around 1 ohm at room temperature. Larger motors rated for higher voltages generally have higher resistance.
What you’re looking for are the two bad outcomes. An open reading (infinite resistance, or “OL” on a digital multimeter) means a winding is broken somewhere inside, and the motor won’t run. A reading of zero or near-zero ohms suggests a short circuit in the windings. Either result means the motor is likely beyond simple repair.
One thing to keep in mind: resistance changes with temperature. A motor that’s been running will read higher than one that’s been sitting on a shelf. That same 1 ohm motor can climb to around 1.4 ohms after sustained use, simply because the coils heat up. Always test at room temperature for the most accurate baseline, and compare your reading to the manufacturer’s spec sheet if you have one.
While you have the multimeter out, slowly rotate the motor shaft by hand and watch the resistance reading. It should stay relatively stable as you turn. If the value jumps erratically or drops to zero at certain positions, that points to a damaged commutator or a spot where the brush isn’t making clean contact.
Inspect the Brushes Visually
Carbon brushes are the most common failure point in these motors, and you can often diagnose the problem just by looking at them. Most brushed motors allow you to access the brushes by removing a cap, unscrewing a holder, or pulling the end bell off the motor housing. Once you can see the brushes, check three things:
- Length. Carbon brushes wear down with use. If a brush has worn to a fraction of its original length, or is approaching the minimum length marked on the brush or specified by the manufacturer, it needs replacement. Many brushes have a wear line molded into them for exactly this purpose.
- Surface condition. The face of the brush that contacts the commutator should be smooth and slightly shiny. Cracks, chips, burn marks, or a rough, pitted surface all indicate a problem. Overheated spring leads (the small wires attached to each brush) are another red flag.
- Even wear. Both brushes should be worn to roughly the same length and shape. Uneven wear suggests the brush isn’t sitting squarely against the commutator, which causes poor electrical contact and accelerates damage.
While the brushes are out, look at the commutator itself, the segmented copper cylinder the brushes ride against. It should be smooth, slightly darkened with an even patina, and free of deep grooves, scoring, or blackened segments. A single blackened segment usually means a shorted or open winding in the armature.
Apply Power and Observe
If the motor passes the resistance test and the brushes look healthy, the next step is a powered test. You’ll need a DC power source matched to the motor’s voltage rating. A bench power supply with adjustable voltage is ideal because you can start low and ramp up gradually, but a battery of the correct voltage works too.
Before connecting anything, make sure the motor shaft can spin freely. Connect the positive terminal of your power source to one motor lead and the negative to the other. DC motors typically label their terminals as “+” and “-” or “A” and “B.” Apply power and watch for smooth, steady rotation. A healthy motor spins up quickly and runs without grinding, stuttering, or excessive vibration.
If the motor doesn’t spin, double-check your connections and confirm the power source is delivering voltage. If everything checks out and the motor still won’t turn, the problem is internal, likely a failed winding, seized bearing, or brush that isn’t making contact.
Swapping the two leads reverses the direction of rotation. This is normal for brushed DC motors and a useful secondary check: a motor that runs smoothly in both directions is almost certainly healthy.
Listen and Feel for Problems
A running motor tells you a lot through sound and vibration. A healthy brushed motor produces a steady, even hum. Clicking or tapping at a regular interval often means a flat spot on the commutator or a chipped brush bouncing as it passes a segment. Grinding suggests worn bearings or debris inside the housing. Excessive sparking visible through ventilation slots, especially bright orange or white sparks rather than a faint blue glow, indicates poor brush-to-commutator contact or damaged windings.
Feel the motor housing after a minute or two of running. Some warmth is normal, but if it gets hot to the touch quickly, the motor is drawing too much current. This can mean shorted windings, dragging bearings, or brushes that are creating high-resistance contact points.
Measure No-Load Current
If you have a bench power supply with a current readout (or a multimeter in series), measuring how much current the motor draws while spinning freely with no load is one of the most telling diagnostic tests. A good rule of thumb: most healthy brushed motors draw roughly one-third of their rated full-load current when running unloaded at rated voltage.
So a motor rated for 3 amps at full load should pull around 1 amp with nothing attached to the shaft. If the no-load current is significantly higher than that, something is creating extra drag internally, whether that’s a bearing problem, brush friction, or partially shorted windings. If the current is extremely low and the motor barely turns, you may have an open winding that’s limiting current flow through only part of the armature.
The Short-Circuit Shaft Test
For motors you can’t easily power up, particularly treadmill motors or other large units, there’s a simple mechanical test. Disconnect the motor from any controller or circuit board, then use a short piece of wire or even a metal staple to jumper the two motor leads together. Now try to rotate the shaft by hand.
With the leads shorted together, a healthy motor becomes very difficult to turn. This is because spinning the shaft generates electricity in the windings, and with the terminals connected, that current flows in a loop and creates a braking force. Remove the jumper and the shaft should spin freely again. If the motor feels the same with or without the jumper, the windings are likely open or damaged.
Putting the Results Together
Each test targets a different failure mode. Resistance testing catches broken or shorted windings. Visual inspection catches worn brushes and commutator damage. Powered testing catches mechanical problems like bad bearings. Current measurement catches internal drag and partial shorts. The short-circuit test confirms the windings can generate a magnetic field.
A motor that passes all of these checks is almost certainly functional. If it’s still not performing correctly in your application, the problem is more likely in the controller, wiring, or power supply feeding it rather than the motor itself. A motor that fails any single test gives you a clear starting point: replace the brushes if they’re worn, and consider the motor beyond repair if the windings or commutator are damaged.