How is a device that does not work at all diagnosed step by step?
Follow the order from source to load: battery voltage, fuse, the value of the positive line at the device input, the negative/chassis path, then the device output. Measure voltage under load at every step. If the supply at the device input is adequate and the drop on the negative path is below 0.2 V, the problem is not on the supply side but on the command or the device side.
Instead of changing parts at random, verify the path the current follows in order.
- Source: measure the voltage at the battery terminals. If it is low, solve the battery and charging system first.
- Protection: check the fuse by measuring, not by eye. If you read a voltage between the two ends of the fuse while the circuit is energised, the fuse has blown. Do not replace a blown fuse with a larger one; the fuse protects the cable.
- Positive line: measure the positive at the device input with respect to the chassis, then measure again while drawing load. A value that collapses under load points to a cross-section, lug or fuse holder problem.
- Negative path: measure between the battery negative and the device negative terminal under load. A drop above 0.2 V is an earth fault; above 0.5 V it is definitely a fault.
- Command: if the supply is sound, check the on/off switch, button, sensor or communication signal.
- Output: measure the device output. If there is voltage at the output but the load does not turn, the problem is in the load or the load return line.
Make the measurement under load at every step. Measurements taken with no load hide a bad connection and send the diagnosis in the wrong direction for hours.
The full topic: DC motor and driver fault diagnosis matrix