Testing a scooter controller before replacing it: the workshop method

The controller is often the first component blamed when a scooter stops working. However, it is rarely the culprit, and it's one of the most expensive parts. Before replacing it, you need to answer two questions: does it receive everything it needs to function, and is its power stage intact? In most cases, a few measurements with a multimeter are sufficient. Here's the method we follow.

What a controller needs

The controller receives power from the battery and delivers it to the motor according to the rider's input. To make the wheel turn, it requires:

• the battery voltage on its thick power cables;
• a ignition signal, originating from the display, the key, or a button;
• a throttle command, which can be direct or transmitted by the display;
• "**No brake signal:** An active brake contact will prevent acceleration."
• a healthy motor, with its three phases and position sensors.

If any of these conditions are not met, the controller may appear to be dead, even though it is simply responding to the signals it is receiving. Our controller guide explains this behavior.

Step 1: Read what the controller says

Many controllers report their faults: a code displayed on the screen, a light that flashes at a certain rate, or a message in the application. Note this above all: a Hall sensor fault, an overcurrent error, or a voltage issue immediately points to a specific problem. On an adjustable controller, the error history is valuable, as shown in our guide on VESC faults. A communication fault with the display usually indicates a problem with the cable or the display itself.

Step 2: Verify the inputs

Scooter on a stand, wheel raised. These measurements are taken while the scooter is powered on: ensure the probes are well-insulated, use slow movements, and make sure no probe touches two pins simultaneously. A powerful scooter battery can generate an arc and cause burns.

1. Battery voltage at the controller input: Measure the DC voltage on the main wires, as close as possible to the controller. This value should be close to the voltage measured at the battery terminals. Any significant difference indicates a problem with a connector, a fuse, or a cable.
2. Power-up: When the scooter is turned on, the power wire should carry the voltage specified by the manufacturer. If it remains at zero, the problem is located upstream: display, button, or column cable.
3. Throttle (Accelerator): The throttle should be powered, and its signal should increase when you press it. Our guide on "throttles that are no longer responding" details how to measure this.
4. Brakes: Disconnect the brake contacts to eliminate any unwanted braking signals, for testing purposes only.

If the throttle or Hall sensor power supply is absent while the controller is powered on, this is a strong indication of a problem with the controller itself: its internal low-voltage power supply may have failed, often due to a short circuit in one of these wires.

Step 3: Remove the motor

A motor with a shorted phase, an insulation fault, or a dead Hall sensor can cause a healthy controller to appear faulty. Test the motor separately, using the method described in our guide for testing a motor with a multimeter. A motor where a phase is shorted to the casing can also damage the controller: repair the motor before installing a new controller, otherwise it will suffer the same fate.

Step 4: Test the power stage

The controller generates the motor current using six power transistors, two per phase: one connected to the positive terminal of the battery, and one connected to the negative terminal. When a transistor fails, it often shorts out, which can be detected with a multimeter while the controller is powered off.

1. Disconnect the battery and motor from the controller. Wait a few minutes: the internal capacitors may still be charged. Before proceeding, check the DC voltage between the positive and negative input terminals of the controller to ensure that the voltage has dropped close to zero.
2. Using a multimeter in diode test mode: Connect the red probe to the negative input of the controller, and the black probe successively to each of the three phases. You should read three similar values, which are typical of a diode that is conducting.
3. Reverse the polarity: connect the black wire to the negative terminal, and the red wire to each phase. The reading should indicate an open circuit, or a value that slowly increases due to the capacitors.
4. Restart the process between the input and each phase, in both directions.

The principle is as follows: each transistor contains a diode, and the three phases must behave in the same way. A phase that displays a value close to zero, while the others are not conducting, indicates a short-circuited transistor. A phase that differs significantly from the other two is suspect. This test does not detect all types of failures, such as a faulty gate driver, but it identifies the most common ones.

Step 5: Confirm through communication

The safest way to test is to swap the suspect controller with an identical one that is known to be working, connecting it in its place. If the scooter starts working again, the diagnosis is confirmed. On a scooter with two motors, it's sometimes possible to swap the two controllers to see if the problem follows the controller or remains with the motor.

Summary: